Effects of Clear Felling and Residue Management on Nutrient Pools, Productivity and Sustainability in a Clonal Eucalypt Stand in South Africa

Transcription

1 Effects of Cler Felling nd Residue Mngement on Nutrient Pools, Productivity nd Sustinbility in Clonl Euclypt Stnd in South Afric by Steven Bryn Dovey Disserttion presented for the degree of Doctor of Philosophy (Forestry) t the University of Stellenbosch Promoter: Dr. Ben du Toit Fculty of AgriSciences Deprtment of Forest nd Wood Science Co-promoter: Dr. Willem de Clercq Fculty of AgriSciences Deprtment of Soil Science December 2012

2 DECLARATION I declre tht the reserch reported in this thesis, submitted for the degree of doctor of philosophy t the University of Stellenbosch, is the result of my own originl reserch, except where otherwise indicted. This thesis hs not been submitted for ny degree or exmintion t ny other university. September 2012 Steven Bryn Dovey Copyright 2012 Stellenbosch University All rights reserved ii

3 ABSTRACT The subtropicl ecosystem of the Zululnd costl plin is prized by the South Africn commercil plnttion forestry industry for its rpid clonl Euclyptus growth, short rottions (6 to 7 yers) nd high yields. This region is typified by sndy soils tht re low in cly nd orgnic mtter, hve smll nutrient reserves nd re poorly buffered ginst nutrient loss. The subtropicl climte induces rpid decomposition of residues nd tree litter resulting in smll litter nutrient pools nd rpid nutrient relese into the soil, prticulrly fter clerfelling. A combintion of lrge nutrient demnds through rpid growth, rpid nutrient turnover nd smll soil nutrient reserves implies tht sites in this region re sensitive nd my be t risk of nutrient decline under intensive mngement. The work in this study set out to determine the risk of nutrient depletion through hrvesting nd residue mngement on site within the Zululnd region, to ssess nutritionl sustinbility nd the risk of yield decline in successive rottions. Some bulk biogeochemicl cycling processes of mcro-nutrients nitrogen (N), phosphorus (P), potssium (K), clcium (C) nd mgnesium (Mg) were ssessed, nd ssessments lso included sodium (N). An existing Euclyptus stnd ws clerfelled nd tretments were imposed on the residues fter brodcsting to simulte vrious levels of nutrient loss through levels of hrvesting intensity nd residue mngement. These included residue burning (Burn), residue retention (No-Burn), fertilistion (stem wood nutrient replcement), whole tree hrvesting nd residue doubling. Outer blocks of the stnd were not felled, but included s replictes of n undisturbed stnding crop tretment. Biogeochemicl nutrient cycling processes were ssessed primrily in the stnding crop, Burn nd No-Burn tretments, in the ssumption tht these represented the furthest extremes of nutrient loss. Dt collection commenced yer prior to clerfelling nd continued to two yers nd six months fter plnting with key dt collection over 20.1 month period from clerfelling to cnopy closure (one yer fter plnting). Wter relted nutrient pools nd fluxes were ssessed s tmospheric deposition (bulk rinfll, throughfll nd stemflow) nd grvittionl leching to 1 m soil depth. Dringe fluxes were predicted using the Hydrus model nd rel-time soil moisture dt. Zero tension lysimeters collected soil solution for chemicl nlysis. Sequentil coring in the 0 to 30 cm soil lyer ws used to determine in situ soil N minerlistion. Soil chemicl nd physicl properties were ssessed over the first meter of soil t clerfelling nd new crop cnopy closure to determine soil nutrient pools sizes. Biomss nutrient fluxes were ssessed from litterfll, residue nd litter decomposition, nd bove ground ccretion into the tree biomss. Leching nd N minerlistion were monitored in the No-Burn, Burn nd stnding crop tretments only. iii

4 Atmospheric deposition, while vrible, ws shown to be responsible for lrge quntities of nutrients dded to the Euclyptus stnd. Nitrogen nd K dditions were reltively high, but within rnges reported in previous studies. Rpid tree cnopy expnsion nd subsequent soil wter utilistion in the stnding crop permitted little wter to drin beyond 1 m resulting in smll leching losses despite sndy well drined soil. Further leching beyond this depth ws unlikely under the conditions during the study period. Minerlistion nd immobilistion of N lso remined low with net immobilistion occurring. The stnding crop ws shown to be reltively stble system tht, outside of extreme climtic events, hd reltively blnced or positive nutrient budget (i.e. nutrient inputs minus outputs). Lrge quntities of nutrients were removed with stem-wood-only hrvesting in the No-Burn tretment leving substntil mounts on the soil surfce in the hrvest residues. Whole tree removl incresed losses of ll nutrients resulting in the lrgest losses of P nd bse ctions compred to ll other tretments. This ws mostly due to high nutrient concentrtions in the removed brk. Loss of N in the Burn tretment exceeded whole tree N losses through combustion of N held in the hrvest residues nd litter lyer. The mjority of K leched from the residues prior to burning nd reltively smll frction of the bse ctions were lost from the prtilly decomposed residues during burning. Ash contining substntil mounts of C nd reltively lrge mounts of N nd Mg remined fter burning. Surfce soil C nd Mg ws significntly incresed by the sh which moved into the soil with rinfll directly fter burning. Rpid soil moisture rechrge occurred within few months fter clerfelling, incresing leching from the upper 50 cm of soil. Clerfelling incresed net N minerlistion rtes, incresing mobile NO - 3 -N ions in the soil surfce lyers. Nitrte concentrtion peked nd K concentrtion dipped in the upper soil lyers of the Burn tretment directly fter burning. Deep dringe nd leching (beyond 1m depth) over the 20.1 month period ws, however, not significntly different between the Burn nd No-Burn tretments. Rpid soil moisture depletion nd nutrient uptke with new crop growth reduced leching fluxes to levels similr to the stnding crop by six months fter plnting. Tking the full rottion into ccount, clerfelling induced short-lived spike in N nd ction leching compred with the low leching losses in the undisturbed stnding crop. Soil N minerlistion over the 20.1 month period in the burnt tretment ws hlf tht of the No-Burn tretment. Growth nd nutrient ccumultion ws significntly higher in the fertilised tretment thn in other tretments up to 2.5 yers of ge. Growth in the Burn tretment ws gretest compred to iv

5 other tretments during the first few months, but slowed therefter. No significnt growth differences were found between ll other tretments from yer to 2.5 yers fter plnting. Erly growth ws therefore pprently not limited by N supply despite lrge differences in N minerlistion between Burn nd No-Burn. Folir vector nlysis indicted tht fertilistion improved growth initilly through incresed folir N nd P t six months fter plnting followed by Mg nd C t one yer. The Burn tretment ws not nutrient limited. These growth results contrsted with similr interntionl reserch on sndy tropicl sites where growth ws reduced fter residue removl nd incresed fter residue doubling. The combined nutrients relesed from pools in the litter lyer or sh nd soil in ddition to tmospheric inputs were sufficient to provide most nutrients required to mintin similr growth rtes cross ll tretments. This demonstrted the importnce of residue derived nutrients to erly growth nutrient supply. Reduced N minerlistion through lck of substrte my limit N supply lter in the rottion where residue hd been removed. Construction of nutrient budget for the system reveled tht high levels of tmospheric inputs hve the potentil to prtilly replenish lrge proportion N, K, nd C lost during clerfelling, provided losses re constrined to stemwood removl only. However, loss of Mg tht occurred primrily through leching my not be replced under the low Mg inputs recorded in this study. Lrger nutrient removls (i.e. stemwood plus other plnt prts) plced hevier relince on the smll soil nutrient pools t this site which cn limit future productivity. More intense hrvesting nd residue mngement prctices drmticlly incresed the risk of nutrient depletion. Losses of specific nutrients depended on combintion of clerfelling biomss removl, residue burning nd subsequent leching. Nitrogen losses due to hrvesting nd burning were more substntil thn those due to leching. Mg nd K losses depended most strongly on the time fter clerfelling before re-estblishment of the new crop nd rinfll ptterns, while C nd P losses depended directly on the mount of biomss removed. Depletion risk ws the gretest for Mg nd K through rpid leching, even fter stem wood only removl. Deep root uptke nd deep dringe with ssocited ction loss needs to be investigted further to quntify ecosystem losses nd recovery of ctions displced beyond 1 m. Atmospheric deposition is one of mjor fctors countering nutrient losses. However, tmospheric inputs my not be relible s these my lessen in future through pollution control legisltion nd climte chnge. Chnges in growth rte under poor nutrient mngement prctices re smll nd difficult to detect reltive to the lrge impcts of chnging wether ptterns (drought), wildfire nd pest nd disese. This mkes it difficult to prove nutrient relted growth decline. It my be possible tht improvements in genetics, silviculturl technologies nd v

6 tmospheric inputs my lso be msking site decline (in generl) nd in prt explin the lck of evidence of growth reduction in the region. As the poorly buffered sndy soils on the Zululnd Cost re t risk of nutrient depletion under the short rottion, high productivity stnds, it my be necessry to stipulte more conservtive hrvesting nd residue mngement prctices. A more conservtive stem-wood only hrvesting regime is recommended, retining ll residues on site. Residue burning should be voided if N losses become concern. The length of the inter-rottion period must be kept short to reduce ction leching losses. Site nutrient pools need to be monitored nd ctions my eventully need to be replenished through ppliction of fertilisers or sh residues from pulp mills. Mngement prctices therefore need to be chosen bsed on the specific high risk nutrients in order to mintin sustinble nutrient supply to current nd future plnttion grown Euclyptus. vi

7 DEDICATION To my lte grndmother Mvis Mwmn who lwys encourged nd inspired me to study. Pointing to her hed she would lwys tell me: it s wht is in here tht counts A fmily of elephnts pssing the experimentl re en route to nerby indigenous forest vii

8 ACKNOWLEDGEMENTS I would like to thnk number of people nd orgnistions whose support nd fith hve mde the completion of this Thesis possible: Dr. Ben du Toit nd Dr. Willem de Clercq for stimulting my thoughts, giving guidnce nd hving genuine interest in this project beyond cdemic requirements nd for encourging me to publish work outside of my field of expertise. I would in prticulr like to give specil thnks to Dr. Ben du Toit for his friendship, fith nd confidence. Hving morl courge nd inspiring students nd scientists, young nd old to persevere in chieving beyond their cpbilities. My grtitude goes to Professor Colin Dyer (ICFR Director) for his support nd understnding. I would like to thnk the ICFR member compnies who funded the core project (Mondi, Sppi, SiyQhubek, NCT, TWK), SiyQhubek, stff nd contrctors for the time nd effort they plced in supporting this study. The stff of the ICFR for their support nd encourgement. Dr. Colin Smith for his project supervision nd support during the erly stges, giving me the opportunity to undertke this thesis nd for ssisting me with initil thinking nd plnning. Dr. Louis Titshll for review nd support during the finl stges. Specil thnks to Denis Oscroft nd the Zululnd technicl tem for their excellent mngement of the field experiment his relibility nd dherence to qulity in mintining the smpling equipment, smple collection nd storge, Mr. Nkosinthi Kptein for smple nd dt mngement. for his excellence in smple preprtion nd insightful lbortory nlysis nd for his keen interest in the project work. Ms. Mry Gilbrith for her excellent lbortory work, without which this Thesis would not hve been possible. Dr. Din Rietz for her encourgement nd helping me rtionlise my thinking. Jen-Pul Lclu (Univ. of So Pulo (ESALQ), Brzil) for his insight nd inspirtion during the plnning stges. viii

9 I would like to thnk the nonymous referees for their generous nd encourging contributions to the peer reviewed ppers tht form prt of thesis. My prents who gve me this opportunity despite never hving it themselves nd for teching me the vlue of hrd work nd persevernce. To my beloved wife Keigwyn for putting side her own life drems so tht I my pursue my own nd to my children (Timothy, Rebecc nd Emily) for giving me purpose nd joy. I would most of ll like to thnk God for ll the wonders of His cretion nd for lwys being present. ix

10 TABLE OF CONTENTS DECLARATION... II ABSTRACT... III DEDICATION... VII ACKNOWLEDGEMENTS... VIII LIST OF FIGURES... XVI LIST OF TABLES... XXI LIST OF APPENDICES... XXIV LIST OF TERMS AND ABBREVIATIONS... XXIX CHAPTER 1: Generl introduction nd study objectives INTRODUCTION COMMERCIAL FORESTRY AND LONG-TERM SITE PRODUCTIVITY A NEED FOR PROCESS-BASED SUSTAINABILITY RESEARCH EARLY INTENSIVE SILVICULTURE DURING THE INTER-ROTATION ZULULAND COASTAL ECOSYSTEM STUDY APPROACH HYPOTHESIS THESIS STRUCTURE... 5 CHAPTER 2: Generl literture review INTRODUCTION SUSTAINABILITY PRODUCTIVITY AND PLANT GROWTH RESOURCES PLANTATION MANAGEMENT AND SUSTAINABILITY NUTRIENT POOLS AND FLUXES NUTRIENT FLUXES NUTRIENT POOLS IN THE SOIL NITROGEN PHOSPHORUS BASE CATIONS NUTRIENT POOLS IN THE BIOMASS NUTRIENT BUDGETS CONCLUSIONS FROM THE GENERAL LITERATURE REVIEW x

11 CHAPTER 3: Site description nd experimentl tretments STUDY SITE STANDING CROP PRIOR TO CLEARFELLING SETUP OF EXPERIMENTAL AREA TREATMENT IMPLEMENTATION DATA COLLECTION WEATHER DATA WATER FLUXES RAINFALL THROUGHFALL AND STEMFLOW EQUIPMENT LYSIMETER INSTALLATION WATER SAMPLE COLLECTION AND ANALYSIS SOIL MOISTURE MONITORING SOIL CHEMICAL PROPERTIES WATER REPELLENCY SAMPLES GROWTH AND BIOMASS SAMPLING STAND BIOMASS AND NUTRIENT MEASUREMENTS LITTERFALL AND RESIDUE STATISTICAL ANALYSIS APPENDIX OF PHOTOGRAPHIC IMAGES CHAPTER 4: A comprison of soil moisture reltions between stnding nd clerfelled plots with burnt nd unburnt hrvest residue tretments ABSTRACT INTRODUCTION MATERIALS AND METHODS MODEL PREDICTION OF WATER MOISTURE AND DRAINAGE RESULTS RAINFALL AND CANOPY DRAINAGE CHANGE IN SOIL MOISTURE AFTER HARVESTING WATER COLLECTED BY PLATE LYSIMETERS PREDICTED WATER CONTENT USING HYDRUS MODEL DISCUSSION CONCLUSION APPENDICES xi

12 CHAPTER 5: Nutrient fluxes in Rinfll, Throughfll nd Stemflow ABSTRACT INTRODUCTION MATERIALS AND METHODS SECOND STUDY SITES CALCULATION OF DEPOSITION COMPONENTS RESULTS PH CATION CONCENTRATIONS IN THROUGHFALL AND RAINFALL NITROGEN CONCENTRATIONS IN THROUGHFALL AND RAINFALL CATION MASS FLUXES NITROGEN MASS FLUXES DISCUSSION SIGNIFICANCE OF THE DATA FURTHER COMMENTS CONCLUSION APPENDICES CHAPTER 6: Effects of clerfelling nd residue mngement on nutrient pools, erly growth nd bove ground nutrient ccumultion ABSTRACT INTRODUCTION MATERIALS AND METHODS NUTRIENT AUDIT RESULTS STANDING CROP BIOMASS AND NUTRIENTS LITTERFALL RESULTS (NOT INCLUDED IN PUBLICATION) POST FELLING RESIDUE NUTRIENT FLUXES RESIDUE AND LITTER RESULTS (NOT INCLUDED IN PUBLICATION) NEW CROP GROWTH ABOVE GROUND BIOMASS AND NUTRIENT ACCUMULATION FOLIAR NUTRIENT DIAGNOSTICS NUTRIENT AUDIT DISCUSSION xii

13 STANDING CROP NUTRIENT FLUXES TREATMENT EFFECTS ON NUTRIENT CAPITAL AND EARLY GROWTH COMPARISON WITH SIMILAR INTERNATIONAL STUDIES TREATMENTS USED AND MANAGEMENT CONCLUSION APPENDICES CHAPTER 7: Nitrogen minerlistion in clonl Euclyptus plnttion on snd s ffected by clerfelling nd residue mngement ABSTRACT INTRODUCTION MATERIALS AND METHODS SEQUENTIAL CORING AND ANALYSIS ATMOSPHERIC DEPOSITION IN FIELD MEASUREMENTS STATISTICAL ANALYSES RESULTS CHANGES IN SYSTEM N POOLS DURING THE STUDY PERIOD SOIL AND AIR TEMPERATURE SOIL MOISTURE MINERALISATION, NITRIFICATION AND IMMOBILISATION LEACHING AND ATMOSPHERIC INPUTS ROOT UPTAKE ATMOSPHERIC DEPOSITION CUMULATIVE NET NITROGEN FLUXES DISCUSSION STANDING CROP N FLUXES FELLED CROP N FLUXES CONSEQUENCE OF ATMOSPHERIC N INPUTS CONCLUSION APPENDIX CHAPTER 8: Contribution of grvittionl leching to inter-rottion nutrient fluxes ABSTRACT INTRODUCTION xiii

14 8.3. MATERIALS AND METHODS RESULTS RAINFALL AND DRAINAGE VOLUMES NUTRIENT CONCENTRATIONS SOIL SOLUTION MASS FLUX SOIL NUTRIENT POOLS AND FLUXES DISCUSSION LEACHING IN AN UNDISTURBED STANDING CROP POST FELLING ATMOSPHERIC INPUTS SOIL CATION ACCUMULATION STUDY OUTCOME CONCLUSION APPENDIX CHAPTER 9: Generl Discussion nd conclusion ABSTRACT INTRODUCTION THE STUDY METHODS OF DATA SUMMARY: SHORT-TERM NUTRIENT BALANCE PROJECTED LONG-TERM NUTRIENT BUDGET RESULTS AND DISCUSSION MAIN FINDINGS: THE UNDISTURBED STANDING CROP MAIN FINDINGS: CLEARFELLED AND REPLANTED CROP NUTRIENT BALANCE PROJECTED WHOLE ROTATION NUTRIENT BUDGET SOIL NUTRIENT ANALYTICAL METHODS RESIDUE BURNING WHOLE TREE REMOVAL LEACHING NUTRIENT BUDGETS MANAGEMENT OPTIONS NATURAL INPUTS EXPANSION OF WORK xiv

15 9.5. CONCLUSION References xv

16 LIST OF FIGURES Figure 2.1: Representtion of some mjor nutrient pools, dditions, losses nd cycling fluxes within mture plnttion ecosystem Figure 3.1: Mp showing the loction of the plnttion re in southern Afric nd the study site loction within KwZulu-Ntl commercil plnttion forestry res Figure 3.2: 50 yer men monthly rinfll observed from nerby wether sttion. Single stndrd devition is shown s I-brs Figure 3.3: 50 yer men monthly mximum (Mx) nd minimum (Min) tempertures observed from nerby wether sttion. Stndrd errors shown s I-brs Figure 3.4: Experimentl lyout of tretment plots showing the stnding crop zones nd internl felled Burn nd No-Burn tretment plots with dimensions given in metres. Additionl tretment codes pply to further components of this study not reported here (Whole- Tree s whole tree hrvest, Fert. No-Burn plus fertiliser replcement, Double - double residue lyer) Figure 3.5: Lyout of funnel cluster nd storge continer reltive to tree positions Figure 3.6: Side view of nrrowly slit stemflow collection tube ttched to tree stem Figure 3.7: Top view lyout of trench nd storge pit reltive to tree positions, showing lysimeter pltes tht re lterntively buried 15, 50 nd 100 cm. Profile probe positions shown reltive to trees Figure 4.1(A-C): Volumetric soil moisture content s mens of the stnding crop (SC), residue burned (Burn) nd residue retined (No-Burn) tretments t 15, 50 nd 100cm from 200 dys before hrvesting to 18 months fter plnting. Dshed lines represent () felling; (b) residue burning; (c) plnting nd (d) cnopy closure. Rinfll events re shown s verticl lines scling to n inverted secondry Y xis in 5. Single stndrd devitions re given s verticl I brs Figure 4.2: Demonstrtion of the time dely between wetting front detection t 15 cm nd 100 cm s percentge chnge in soil moisture content fter onset of rinfll in the stnding crop (SC), residue burned (Burn) nd residue retined (No-Burn) tretments. xvi

17 Clculted s difference from pre-rinfll soil moisture content, (zero vlues not shown) Figure 4.3(A-C): Men volumes (± SD) of wter collected t 15, 50 nd 100 cm depth with plte lysimeters in the stnding crop (SC), residue burned (Burn) nd residue retined (No- Burn) tretments. Dshed lines represent () felling; (b) residue burning; (c) plnting nd (d) cnopy closure. I brs represent single stndrd devitions. Rinfll events re shown s verticl lines scling to n inverted secondry Y xis in 6(A) Figure 4.4: Totl volume of wter collected t 15, 50 nd 100 cm depth with plte lysimeters in the stnding crop (SC), residue burned (Burn) nd residue retined (No-Burn) tretments from felling to 18 months fter plnting. Stcked brs represent time periods from felling to burning, nd six monthly time periods from plnting to cnopy closure onwrds. I brs represent 5% lest significnt differences for tretment nd collection period Figure 4.5: Predicted dringe volumes t 15, 50 nd 100 cm for felled nd stnding crop (SC) res of the study site. Verticl dshed lines represent () felling; (b) residue burning; (c) plnting nd (d) cnopy closure Figure 4.6: Rinfll nd Predicted dringe volumes t 15, 50 nd 100 cm for felled nd stnding crop (SC) res of the study site. Ech portion of the brs represents dringe predicted for ech period in Figure Figure 5.1: Monthly rinfll nd cnopy for ech month t the northern Dukuduku site nd southern KwMbonmbi site. Verticl dshed lines represent the strt nd end of ech six month wetter summer (S) nd dryer winter (W) periods for ech yer Figure 5.2: ph of rinfll nd throughfll solutions volume weighted for ech month t the northern Dukuduku (Duku) site nd southern KwMbonmbi (KwMbo) site. Verticl dshed lines represent the strt nd end of ech six month wetter summer (S) nd dryer winter (W) periods for ech yer. (Volume weighting ws clculted using ph derived H + concentrtion) Figure 5.3: Concentrtions of () K, (b) C, (c) Mg nd (d) N (mmol c l -1 ) in rinfll nd throughfll solutions volume weighted for ech month t the northern Dukuduku (Duku) site nd southern KwMbonmbi (KwMbo) site. Verticl dshed lines xvii

18 represent the strt nd end of ech six month wetter summer (S) nd dryer winter (W) periods for ech yer Figure 5.4: Concentrtions of () Orgnic-N, (b) NH4 N nd (c) NO3-N (mg l -1 ) in rinfll nd throughfll solutions volume weighted for ech month t the northern Dukuduku (Duku) site nd southern KwMbonmbi (KwMbo) site. Verticl dshed lines represent the strt nd end of ech six month wetter summer (S) nd dryer winter (W) periods for ech yer. Note the different scle for the Y xes Figure 6.1: Forest floor litter lyer nd residue biomss from prior to felling, through felling nd up to cnopy closure for ech tretment. Error brs represent single stndrd devition between smpling points. Dshed lines represent () felling; (b) residue burning; (c) plnting nd (d) cnopy closure Figure 6.2: Bsl re of ech tretment t six monthly intervls fter plnting. 0.5 yer bsl re dt clculted using gld, others clculted using dbh. Different, b, c superscripts denote significnt differences between tretments t ech ge in the tretment order of the legend (LSD 5% ) Figure 6.3: Men heights of trees in ech tretment t six monthly intervls fter plnting. Different, b superscripts denote significnt differences between tretments t ech ge in the tretment order of the legend (LSD 5% ) Figure 6.4: Biomss ccumulted in the bove ground tree components between the plnting to 0.5 yers, nd plnting to cnopy closure (1.0 yers). Vlues shown on brs with LAI (m² m - ²) on the top. (stnding crop is the sum of growth nd litterfll) Different, b, c superscripts denote significnt differences between tretments; lowercse for 0.5 yers nd uppercse for 1.0 yers (LSD 5% ) Figure 7.1: Men dily ir nd soil (10 cm depth) tempertures for the stnding crop (SC), single residue (No-Burn) nd burned residue (Burn) tretments. Dshed lines represent () clerfelling; (b) residue burning; (c) plnting nd (d) cnopy closure.125 Figure 7.2: Weekly volumetric soil moisture content t 15 cm depth in the stnding crop (SC), single residue (No-Burn) nd burned residue (Burn) tretments. Dshed lines represent () clerfelling; (b) residue burning; (c) plnting nd (d) cnopy closure. Rinfll events re shown s verticl lines scling to n inverted secondry Y xis. Single stndrd devitions re given s verticl I brs xviii

19 Figure 7.3(A-C): N minerlistion fluxes in the 0-30 cm soil lyer fter in situ incubtion of closed cores: (A) minerlistion to NH 4 -N (B) nitrifiction to NO 3 -N nd (C) net N minerlistion. Tretments re stnding crop (SC), residue retention (No-Burn) nd burned residue (Burn). Dshed lines represent the times of () clerfelling; (b) residue burning; (c) plnting nd (d) cnopy closure. I-brs represent lest significnt differences (LSD 5% ) Figure 7.4(A-C): Minerl N dded to the 0-30 cm soil lyer through tmospheric deposition (positive vlues) or lost through leching (negtive vlues), estimted by the sequentil soil coring method for (A) NH 4 -N, (B) NO 3 -N nd (C) net N. Tretments re stnding crop (SC), residue retention (No-Burn) nd burned residue (Burn). Dshed lines represent the times of () clerfelling; (b) residue burning; (c) plnting nd (d) cnopy closure. I-brs represent lest significnt differences (LSD 5% ) Figure 7.5: Chnges in minerl N in the 0-30 cm soil lyer through root ctivity (negtive vlues) nd residue/litter dditions (positive vlues). Tretments re stnding crop (SC), residue retention (No-Burn) nd burned residue (Burn). Dshed lines represent the times of () clerfelling; (b) residue burning; (c) plnting nd (d) cnopy closure. I-brs represent lest significnt differences (LSD 5% ) Figure 7.6: NO 3 -N, NH 4 -N nd Orgnic-N dded throughfll (weekly). Dshed lines represent the times of () clerfelling; (b) residue burning; (c) plnting nd (d) cnopy closure using dt from CHAPTER Figure 8.1(A-F): Soil solution nd volume weighted rinfll nd cnopy dringe NH 4 -N (), NO 3 -N (c) nd orgnic-n (e) concentrtions mesured t 15 cm depth nd NH 4 -N (b), NO 3 -N (d) nd orgnic-n (f) concentrtions mesured t 50 cm nd 100 cm depths for felled nd stnding crop (SC) res of the study site. Dshed verticl lines represent () felling; (b) residue burning; (c) plnting nd (d) cnopy closure Figure 8.2(A-H): Soil solution nd volume weighted rinfll nd cnopy dringe K + (), C 2+ (c), Mg 2+ (e) nd N + (g) concentrtions recorded t 15 cm depth nd K + (b), C 2+ (d), Mg 2+ (f) nd N + (h) concentrtions recorded t 50 nd 100 cm depths for felled nd stnding crop (SC) res of the study site. Dshed verticl lines represent () felling; (b) residue burning; (c) plnting nd (d) cnopy closure xix

20 Figure 8.3: A hypotheticl projection of nnul leching nd tmospheric inputs to rottion end using mture crop dt for projected yers (bsed on K dt) Figure 9.1: Nitrogen nd phosphorus fluxes (kg h -1 ) between clerfelling nd new crop cnopy closure, nd pools t cnopy closure s defined in section (nd = not determined) Figure 9.2: Ction fluxes (kg h -1 ) between clerfelling nd new crop cnopy closure, nd pools t cnopy closure s defined in section (nd = not determined)... Error! Bookmrk not defined. xx

21 LIST OF TABLES Tble 3-1: Bsic soil physicl nd chemicl properties of the top 100 cm (in 20 cm increments) t the strt of the study Tble 4-1: Men vlues of soil orgnic crbon, texture, nd bulk density tken t incrementl depths nd soil moisture retention nd conductivity prmeters Tble 4-2: Reltionships between rinfll collected in n open re nd tht collected with throughfll nd stemflow derived through liner regression Tble 4-3: Reltionships between wter content predicted with the Hydrus 1D model nd mesured with the profile probe t ech depth for the in the stnding crop (SC) tretments Tble 5-1: Generl chrcteristics of the smpling sites selected for tmospheric deposition monitoring Tble 5-2: Addition of N species, C 2+, Mg 2+ nd K + by wet deposition, cnopy dringe, nd dry deposition inferred from cnopy exchnge clcultions for six month wetter summer (S) nd dryer winter (W) periods for ech yer t the northern Dukuduku site nd southern KwMbonmbi sites Tble 5-3: Mens nd rnges in nutrient dditions with bulk rinfll derived from numerous globl studies (kg h -1 yr -1 ). Rnges re shown in round brckets nd numbers of studies re shown in squre brckets Tble 5-4: A hypotheticl clcultion of nutrient loss by removl of vrious tree components ssuming 150 Mg h -1 of stemwood nd potentil ddition of nutrients over seven yer rottion (Minerl nd orgnic N shown seprtely) Tble 6-1: Biomss nd nutrients contined in the litter lyer nd residues for tretments t times of felling, burning, plnting nd cnopy closure. Residue nd litterfll re clculted from felling to pre-burning, nd from felling to plnting nd cnopy closure, respectively Tble 6-2: Annulised mss of folir nd cnopy mcro-nutrients deposited s litterfll in the stnding crop during the period between felling nd cnopy closure of the new crop. xxi

22 Fresh folige mss is the litterfll equivlent mss of nutrients using folir concentrtions nd indictes potentil retrnsloction Tble 6-3: Mcro-nutrients ccumulted in the bove ground tree biomss t 0.5 nd 1.0 yers fter plnting (cnopy closure). Superscript nd subscript vlues re folir nd woody component msses respectively. Letters tht re different indicte significnt difference between tretment men bove ground mss t ech ge (LSD 5% ) Tble 6-4: An udit of boveground nutrient loss, relese (s relese of nutrients with decomposition nd fertiliser ddition) nd bove ground uptke for ech tretment between felling nd cnopy closure (kg h -1 ). It Includes clcultion of the difference between relese nd uptke Tble 7-1: Nitrogen pool sizes nd ccretion t nd between the times of clerfelling, burning, nd cnopy closure for stnding crop, Burn nd No-Burn tretments. Chnge in forest floor/ residues pools given in prenthesis Tble 7-2: Cumultive (kg h -1 ) nd nnulised (kg h -1 yer -1 ) net nitrogen fluxes in-field cores of 0 30 cm soil from clerfelling to cnopy closure of the new crop Tble 8-1: Leching (kg h -1 ) beyond 100 cm depth for felled nd stnding crop (SC) res of the study site over the felling to 0.5 yers fter plnting period (15-months), nd the 0.5 to 1.5 yers fter plnting period (12-months, fter cnopy closure). Different, b, c superscripts denote significnce difference between tretments t LSD 5% Tble 8-2: Above -ground nutrient pools t felling nd cnopy closure nd litterfll, burning loss nd tree growth nutrient uptke occurring between felling nd cnopy closure of the new crop Tble 8-3: Soil nutrient pools t incrementl depths between 0 nd 100 cm soil depth t the strt nd end of the study (kg h -1 ). Significnt differences (LSD 5% ) re shown between totl pool sizes (bold text) t the end of the study Tble 9-1: Short term fluxes of nutrients into the soil. Supply is tmospheric deposition, decomposition, post-burn sh nd fertiliser less leching losses. Uptke is boveground ccretion. See section for detiled explntion of clcultions.180 xxii

23 Tble 9-2: Projected seven yer dditions, losses nd budget (kg h -1 yr -1 ) with number of rottions to residue nd soil nutrient pool depletion. Pool sizes re defined in section xxiii

24 LIST OF APPENDICES Appendix 3.4: Appendix of photogrphic imges Imge 3-1: The sndy soil nd litter lyer Imge 3-2: Wood structure preventing soil collpsing into pit Imge 3-3: Pltes nd suction cups inserted into pit fce Imge 3-4: Slotted stemflow collector Imge 3-5: Automtic wether sttion in nerby clering Imge 3-6: Wood disk smples collected for mss, nutrient nd density determintion Imge 3-7: Stnding crop litter pre-clerfelling Imge 3-8: Clerfelling of the buffer strips Imge 3-9: Clerfelled comprtment showing Stnding crop in bckground Imge 3-10: Opening soil pit cover Imge 3-11: Whole-Tree plot surfce fter clerfelling Imge 3-12: Residue burning Imge 3-13: TDR probes inserted into the Burn tretment the dy fter burning Imge 3-14: Soil N-min cores in Burn tretment Imge 3-15: Ash lyer in soil t four weeks fter burning Imge 3-16: A Burn tretment plot t six months fter plnting Imge 3-17: A No-Burn tretment plot t six months fter plnting Imge 3-18: A Whole-Tree tretment plot t six months fter plnting xxiv

25 Appendix 4-1: Further Comments bout tree root Systems Appendix 4-2: Further Comments bout wter use efficiency Appendix 4-3(A-F): Volumetric soil moisture content s mens of ech depth in the stnding crop (SC), residue burned (Burn) nd residue retined (No-Burn) tretments for the durtion of the study. Dshed lines represent () felling; (b) residue burning; (c) plnting nd (d) cnopy closure. Single stndrd devitions re given s verticl I brs Appendix 5-1: Dukuduku rinfll, throughfll nd stemflow solution concentrtion mens (Men), stndrd devition (SD) nd 5% confidence intervls (CI). Totl N is the sum of mmonium, nitrte nd orgnic N Appendix 5-2: Kwmbonmbi rinfll, throughfll nd stemflow solution concentrtion mens (Men), stndrd devition (SD) nd 5% confidence intervls (CI). Totl N is the sum of mmonium, nitrte nd orgnic N Appendix 5-3: Rinfll, ph nd nutrient ddition with bulk rinfll t Dukuduku Appendix 5-4: Throughfll, ph nd nutrient ddition with throughfll t Dukuduku Appendix 5-5: Stem flow, ph nd nutrient ddition with stem flow t Dukuduku Appendix 6-1: Totl monthly rinfll (solid brs) nd long term mens (dotted line on br); men monthly temperture (line) nd long term mens (fine dotted line) recorded from felling to post cnopy closure. Verticl dshed lines represent the times of () felling; (b) residue burning; (c) plnting nd (d) cnopy closure Appendix 6-2: Annul folir, brnch nd brk litterfll rtes per litter trp collection, verged cross the stnding crop plots. Error-brs represent the single stndrd devition between ll litter trps. Verticl dshed lines represent the times of () felling; (b) residue burning; (c) plnting nd (d) cnopy closure Appendix 6-3(A-F): Concentrtions of N, P, K, C, Mg nd N in the litter nd forest floor of the stnding crop, Whole-Tree nd No-Burn tretments. Verticl dshed lines represent the times of () felling; (b) residue burning; (c) plnting nd (d) cnopy closure xxv

26 Appendix 6-4: Concentrtions of K, Mg nd N in the litterfll collected under the stnding crop. Verticl dshed lines represent the times of () felling; (b) residue burning; (c) plnting nd (d) cnopy closure Appendix 6-5: Concentrtions of N, C nd P in the litterfll collected under the stnding crop. Verticl dshed lines represent the times of () felling; (b) residue burning; (c) plnting nd (d) cnopy closure Appendix 6-6: Reltive growth rtes s the difference between bsl re increment of ech tretment nd bsl re increment of No-Burn tretment t six monthly intervls fter plnting Appendix 6-7 (A-E): Nutrients ccumulted in the bove ground tree components between the plnting to 0.5 yers, nd plnting to cnopy closure (1.0 yers). Vlues shown on brs with LAI (m² m - ²) on the top. Stnding crop (SC) is the sum of growth nd litterfll) Different, b, c superscripts denote significnt differences between tretments; lowercse for 0.5 yers nd uppercse for 1.0 yers (LSD5%) Appendix 6-8(A-J): Vector nlysis figures for ech mcro-nutrient element t 0.5 nd 1.0 yers fter plnting using No-Burn s the (100%) control nd interprettions fter Slifu nd Timmer (2001) (units re s percentge of the control). (FS =fertilised; W = whole tree; 2S=double). Vector tble is given below ech figure showing folir mss, nutrient concentrtion (conc) nd nutrient content reltive to No-Burn (set t 100) nd vector length Appendix 6-9: Equtions used to predict tree component msses nd lef re (LA) t 6 using gld (cm) nd 12 months fter plnting using dbh (cm) Appendix 6-10: Equtions using dbh to predict tree component msses nd lef re (LA) of the stnding crop Appendix 6-11:: Biomss nd mcro nutrients in the tree biomss nd litter lyer t felling117 Appendix 6-12: Bsl re nd men heights of ech tretment t six monthly intervls fter plnting Appendix 7-1: Men dily ir nd soil (2.5 cm depth) tempertures for the stnding crop (SC), single residue (No-Burn) nd burned residue (Burn) tretments. Dshed lines represent () clerfelling; (b) residue burning; (c) plnting nd (d) cnopy closure.141 xxvi

27 Appendix 7-2: Men dily ir nd soil (22.5 cm depth) tempertures for the stnding crop (SC), single residue (No-Burn) nd burned residue (Burn) tretments. Dshed lines represent () clerfelling; (b) residue burning; (c) plnting nd (d) cnopy closure.141 Appendix 7-3: Minerl NH 4 -N in the 0-30 cm soil lyer. Tretments re stnding crop (SC), residue retention (No-Burn) nd burned residue (Burn). Dshed lines represent the times of () clerfelling; (b) residue burning; (c) plnting nd (d) cnopy closure. I- brs represent lest significnt differences (LSD 5% ) Appendix 7-4: Minerl NO 3 -N in the 0-30 cm soil lyer. Tretments re stnding crop (SC), residue retention (No-Burn) nd burned residue (Burn). Dshed lines represent the times of () clerfelling; (b) residue burning; (c) plnting nd (d) cnopy closure. I- brs represent lest significnt differences (LSD 5% ) Appendix 7-5: Cumultive (kg h -1 ) mmoni, nitrte nd net nitrogen fluxes in-field cores t 0-5, 5-15 nd cm soil depths from clerfelling to cnopy closure of the new crop Appendix 7-6: Cumultive (kg h -1 ) mmoni nd nitrte fluxes in-field cores t 0-30 cm soil depth from clerfelling to cnopy closure of the new crop Appendix 7-7: Ammoni fluxes (kg h -1 ) in-field cores t 0-30 cm soil depth for ech mesured dte Appendix 7-8: Nitrte fluxes (kg h -1 ) in-field cores t 0-30 cm soil depth for ech mesured dte Appendix 8-1: Rinfll, cnopy dringe nd Hydrus predicted dringe volumes t 15, 50 nd 100 cm for felled nd stnding crop (SC) res of the study site. Ech portion of the brs represents dringe predicted for ech period Appendix 8-2: Totl N leched during ech time period t 15, 50 nd 100 cm nd dded with rinfll nd cnopy dringe for felled nd stnding crop (SC) res of the study site. Different, b, c superscripts denote significnce difference between tretments t ech depth for ech time period t LSD 5%. A LSD 5% stcked br is given to show significnt differences Appendix 8-3: () K, (b) C, nd (c) Mg leched during ech time period t 15, 50 nd 100 cm nd dded with rinfll nd cnopy dringe for felled nd stnding crop (SC) res of xxvii

28 the study site. Different, b, c superscripts denote significnce difference between tretments t ech depth for ech time period t LSD 5%. A LSD 5% stcked br is given to show significnt differences Appendix 8-4: Ammonium, nitrte nd orgnic nitrogen deposition nd leching t 15, 50 nd 100 cm soil depths cumulted for ech key period. Lest significnt difference (LSD) nd ANOVA F probbility given for leching differences between tretments Appendix 8-5: Potssium, clcium nd mgnesium deposition nd leching t 15, 50 nd 100 cm soil depths cumulted for ech key period xxviii

29 LIST OF TERMS AND ABBREVIATIONS Chemicl terms TKN Totl Kjeldhl nitrogen O-N Orgniclly bound nitrogen CEC Ction exchnge cpcity OC Orgnic crbon EC Electricl conductivity Hydrus model Qr Residul soil moisture content Qs Sturted soil moisture content Alph Prmeter in the soil moisture retention function n Prmeter in the soil moisture retention function Ks Sturted hydrulic conductivity Sttisticl terms ANOVA Anlysis of Vrince CV Coefficient of Vrition MSE Men Squre Error RCB Rndomized Complete Block R 2 SD LSD CI SE ns p * 0.01<p<0.05 ** 0.001<p<0.01 *** p<0.001 R-squre; coefficient of determintion Stndrd Devition Lest significnt difference Confidence intervl stndrd error Not significnt Probbility Tretment bbrevitions Stnding crop (SC) Undisturbed crop no hrvest xxix

30 Whole-Tree Burn No-Burn Fertilised (Fert) Double Removl of stem wood, brk, brnches nd folige t clerfelling Residues brodcst nd burned Residues brodcst nd retined No-Burn nd replcement of stem wood mcronutrients No-Burn nd residues dded from Whole-Tree plots Plnt/tree relted terms GPP Gross primry productivity NPP Net primry productivity Supply Refers to the vilble supply of tree growth resources RCE Resource cpture efficiency RUE Resource uptke efficiency dbh Dimeter of mture tree stem t 1.3 m bove ground level gld Ground-line dimeter of juvenile tree stem LAI Lef re index. SLA Specific lef re. SI 5 Site index - 80th percentile of tree heights projected to 5 yers of ge LOI Loss on ignition k Decy constnt Climte terms MAP Men nnul precipittion MAT Men nnul temperture APAN Refers to Clss A Pn Model of Evportion Other HDPE CIFOR LAN TDR High-density polyethylene Centre for Interntionl Forestry Reserch Limestone mmonium nitrte Time domin reflectometry xxx

31 CHAPTER 1: GENERAL INTRODUCTION AND STUDY OBJECTIVES 1.1. INTRODUCTION The development of science nd mngement strtegies in commercil plnttion forestry requires the merging of two seemingly contrdictory pproches: tht of plnttion forest s n griculturl production system nd s mnged ecosystem. The griculturl pproch (historiclly populr in some plnttion forestry circles in South Afric) clssiclly considers the use of vilble tools nd technology to mximise biologicl production through incresed or optimised resource use. A mnged ecosystem pproch considers the plnttion forest s n integrtion of biotic nd biotic components tht re mnged to ensure sustinble supply of products. Ech pproch hs merit, lthough the mnged ecosystem pproch is usully preferred by forest scientists nd mngers like s it considers brod-sense, holistic view of plnttion forestry. The lck of scientific dt describing mny spects of South Africn plnttion forests s ecosystems hs however, led to mngement decisions bsed more on empiricl evidence or experience-bsed intuition thn on scientific evidence or process understnding. Plnttion forest mngers hve therefore expressed need to integrte this intuitive pproch with quntittive, evidence-bsed understnding to ensure the mintennce or improvement of present nd future plnttion productivity without compromising site resources, neighbouring ecosystems, humn ctivities nd profitbility. Scientists nd mngers therefore need to gther dt nd evidence tht documents the impcts of vrious mngement prctices on sustinble plnttion forest productivity cross ll forest plnttion sites while not forgoing the welth of experience held by the South Africn nd interntionl forestry industry COMMERCIAL FORESTRY AND LONG-TERM SITE PRODUCTIVITY Commercil plnttion forestry, consisting of 51% Pinus spp., 40% Euclyptus spp., nd 8% Acci sp., presently covers 1.3 million hectres or 1.1% of South Africn lnd re (FSA 2009b). The plnttions, originlly estblished minly into grsslnd ecosystems (Acocks 1953; Mucin nd Rutherford 2006), re spred cross reltively lrge rnge of site types nd soil forms, ech hving unique site history with respect to previous lnd mngement. Plying vitl role in supplying South Africn timber nd fibre needs (pproximtely 17 million ir dry tons per nnum of which 63% is short rottion pulpwood) the commercil forestry industry 1

32 contributes 1.27% to the GDP nd employs 0.5 million people. A constnt supply of commercilly produced wood decreses the pressure on ntive forests for wood production (nd other forest products) while creting much wreness round socil nd environmentl concerns. The Euclyptus species supplies round 60% of South Africn pulp fibre (FSA 2009b) from plnttions nd is minly concentrted in the KwZulu-Ntl nd Mpumlng forestry regions, covering pproximtely 0.5 million hectres of lnd (FSA 2009b). In response to much criticism tht hs been received over the yers, prticulrly round wter nd environmentl concerns, the commercil forestry industry hs become one of the most regulted (self nd government) griculturl prctices in South Afric (Dye nd Versfeld 2007). These regultions, n incresed demnd for wood products, nd numerous other politicl, economic nd environmentl fctors hve pressurised the industry to seek innovtions to enble lrger quntities of timber nd biomss to be produced from reltively smll nd shrinking lnd-bse (FSA 2009b). This shrinkge of lnd re (9% between 1999 nd 2009 (FSA 2009b)) hs occurred through inter li: wetlnd delinetion, successful lnd-clims tht hve subsequently been converted to non-forestry ctivities nd the estblishment of wildlife corridors through plnttion forest res. However, increses in productivity of 19% through the sme (bove) ten yer period hve been relised through mngement improvements tht include: the selection of fster growing trees in tree breeding progrms, improved mtching of tree species nd hybrids to sites for optiml timber growth, plnting of disese resistnt clonl vrieties, shorter rottion lengths, more intensive silviculturl prctices t estblishment, mechnistion nd integrted pest nd disese mngement progrms (Schutz 1982; Schönu 1984; Burger 1996; Buhus et l. 2002; du Toit et l. 2010). In ddition to swtimber, pulpwood, mining timber nd poles, plnttion forestry hs in recent yers been considered source of renewble bio-energy, s the hrvest residues re seen s n esily vilble biomss source. This hs lso substntilly incresed the rte nd quntity of biomss removl per unit lnd re of plnttion forests nd consequently incresed pressure on wter nd nutrient resources on the remining lnd re (Nmbir 1996,1999). It is therefore crucil tht the negtive impct of greter demnd on wter nd nutrients be understood nd quntified so tht long-term sustinble productivity cn be mintined nd promoted. 2

33 1.3. A NEED FOR PROCESS-BASED SUSTAINABILITY RESEARCH Monitoring chnges in plnttion productivity over numerous rottions using mesures of tree growth nd forest product output (nrrow sense sustinbility, (Evns 1999)) my be poor indiction of ecologicl processes nd functioning within the plnttion forest when used lone. This is due to the effects tht climtic vribility, genetic improvements, pests nd diseses outbreks, wildfires nd non-uniform silviculturl prctices hve on tree growth bsed productivity determinnts of site. It is therefore necessry to understnd the fctors tht influence the complex biotic nd biotic functioning of ecosystem processes tht drive sustinble productivity, while excluding, or ccounting for s mny of the confounding effects s possible. The principles nd processes driving sustinble productivity re not fully understood or quntified in commercil plnttion forestry, prticulrly round fctors tht dictte site fertility (Blnco et l. 2005; Lclu et l. 2005; Wtt et l. 2005). Reserch therefore needs to become more focused on understnding the key processes tht determine the nutritionl sustinbility of productive forest plnttions, by quntifying the effects of intensive mngement prctices on key nutrient processes in productive plnttion forests grown on sensitive sites EARLY INTENSIVE SILVICULTURE DURING THE INTER-ROTATION The erly stge of short-rottion pulpwood plnttion growth cycle, from felling to post cnopy closure of the subsequent crop represents the most intensive silviculturl mngement period of plnttion forest. The opertions during this period often determine the growth nd productivity of the reminder of tht rottion (Nmbir 2008; du Toit et l. 2010; Smethurst 2010), while hving the potentil to dmge or improve ecosystem productivity, which cn ffect the growth of subsequent rottions (Schönu 1984; Bris et l. 2002). Although the tree growth responses to mngement prctices my vry widely cross sites, the impct tht mngement prctices hve on sustinble nutrient supply nd growth my become more importnt on infertile sites, i.e. sites tht hve smll nutrient reserves. Sites chrcterised by rpid growth nd fst nutrient turnover will be fr more sensitive to mngement interventions thn cooler, dryer, slow growth sites. 3

34 1.5. ZULULAND COASTAL ECOSYSTEM The Zululnd costl plins (northern KwZulu-Ntl, South Afric) consist of extensive res of sndy lbic renosols with single-grin structure (FAO 2006) nd sub-tropicl climte tht results in res of low nutrient reserves (Hrtemink nd Hutting 2005; Fey 2010), but high productivity potentil (Smith nd du Toit 2005). These soils re unble to buffer lrge chemicl, physicl nd orgnic (biologicl) chnges, plcing them t risk of degrdtion, ultimtely limiting tree productivity under poor mngement prctices. This hs been confirmed through tree growth responses to pplictions of N fertiliser t plnting (du Toit et l. 2001; du Toit nd Oscroft 2003). Clonl Euclyptus is grown primrily s short rottion (6-7 yers) pulpwood crop cross the re. Site mngement includes residue brodcsting, windrowing nd, where necessry, residue burning. Stndrd fertilistion prctices recommend the ddition of round 70 kg h -1 of N s limestone mmonium nitrte (LAN). Plnttions grown in this region hve given little conclusive evidence towrds nutrient relted productivity decline despite hving undergone numerous rottions of intensive silviculturl mngement prctices nd frequent wildfires (de Ronde 1992; Oscroft nd Little 2008) STUDY APPROACH This thesis describes reserch tht ws crried out to determine the impct of clerfelling nd two opposing extremes in stndrd mngement prctices (residue retention nd residue burning) undertken during the fllow inter-rottion period fter hrvesting, on pools, losses nd gins of selected elements. This ws lso done to describe the impct of site mngement on long-term sustinble productivity of clonl Euclyptus, with respect to nturl nd mngement induced nutrient losses nd gins. The study primrily compres the fluxes of nitrogen (N) within the wter, soil nd orgnic components of mture clonl Euclyptus stnd with those fter felling upon which residue burn nd no-burn mngement prctices where imposed. The elements potssium (K), clcium (C), mgnesium (Mg), sodium (N) nd phosphorus (P) re included in vrious components of the reserch. Dt collection begn prior to clerfelling nd continued to cnopy closure of the new crop growth, though some spects of the dt collection continued beyond cnopy closure. Additionl tretments were included tht involved mnipultion of residues nd fertiliser ddition so tht the interprettion of the results could be improved. The min objective of the study ws to: 4

35 Compre mjor nutrient pools nd fluxes in high productivity, clonl Euclyptus stnd grown on nutrient sensitive site tht ws left undisturbed with clerfelled stnd subjected either to residue retention or residue burning, through plnting to cnopy closure; relting this to current nd future nutrient supply nd productivity HYPOTHESIS Nutrient flux processes nd losses on productive, but sensitive site re ccelerted by clerfelling nd subsequent residue burning resulting in significnt reduction in soil nd biomss nutrient pools tht re not replenished through nturl dditions. This study will be chieved through number of component studies with the following objectives: To trck chnges in soil moisture fluxes, prmeterise soil moisture model nd predict grvittionl dringe fluxes within the first meter of soil. Determine the order of mgnitude of tmospheric nutrient dditions to the site nd the region. Describe the fte of nutrients held in tree, litter nd residue biomss components s consequence of clerfelling nd residue mngement by compring decomposition, litterfll, growth, nutrient ccretion in the undisturbed stnd with the newly plnted stnd. Determine chnges in top-soil N minerlistion nd how this contributes to soil minerl N fluxes throughout the study between clerfelling nd cnopy closure. To use predicted dringe fluxes nd collected soil solution dt to clculte nutrient leching within the first meter of soil. To determine net nutrient gins nd losses induced directly nd indirectly through hrvesting nd residue mngement opertions, quntifying net losses, gins nd pools of nutrients in the system through nutrient budget pproch THESIS STRUCTURE This thesis comprises literture review (Chpter 2) followed by generl site descriptions nd the experimentl methods nd lyout (Chpter 3). The min body of this thesis is compiltion of ppers covering the first five component studies prepred for peer-reviewed publictions 5

36 (Chpters 4 to 8). The finl chpter (Chpter 9) is generl discussion nd synthesis of the work. The significnce of the results nd their implictions for future reserch re highlighted. As much of the reserch presented in this thesis hs been published or is currently under review, Chpters 4 to 8 hve been presented s close to their published formt s possible nd include relevnt literture nd objectives specific to ech component study. Repetitive mterils nd methods content hs mostly been removed nd included in Chpter 3 to enhnce redbility. Tble nd figure numbering hs been ltered in ccordnce with this thesis structure. The ppers were written to stnd s seprte entities in ech publiction nd often necessitted the use of dt outside of the study described in this thesis. Some dt presented in the results of ech chpter repets or drws upon dt derived from other chpters. Chpters lso include dditionl comments nd dt s ppendices. 6

37 CHAPTER 2: GENERAL LITERATURE REVIEW 2.1. INTRODUCTION Globlly, plnttion forests re n importnt renewble resource tht in ddition to wood, provide wide rnge of products nd ecosystem services (Smpson et l. 2005; Nmbir 2008). Both worldwide, nd in South Afric, plnttion forest mngement ims t producing n economiclly profitble, sustinble wood supply through intensive mngement by improving genetics, silviculture nd hrvesting (Morris nd Smith 2002; Binkley et l. 2004; Gonclves 2004; Glvič nd Lukmn 2007). Despite incresing productivity of South Africn plnttion forests (consisting of Pinus, Euclyptus nd Acci species; (FSA 2009b)) concerns round the long-term sustinbility of yields re still being rised. This is due to the incresed productivity nd limited understnding of the ecologicl principles nd processes driving the productivity of plnttion ecosystems (Nmbir 1996; Lclu et l. 2003; Wtt et l. 2005; du Toit et l. 2010). Incresed productivity through intensive mngement prctices hs plced n incresed demnd on plnttion sites, ltering soil physicl properties, soil orgnic mtter fluxes nd nutrient dynmics (Smethurst nd Nmbir 1990; Blnco et l. 2005; du Toit et l. 2010; Rietz 2010). These intensive prctices, tht often include shorter rottion lengths, frequent hrvesting nd extreme silviculturl opertions (residue burning nd whole tree hrvesting) result in more frequent nd higher biomss removl while providing less time for the plnttion site (nd ecosystem) to recover (Tirks nd Rnger 2008; du Toit et l. 2010). The survivl of the forestry industry is ultimtely dependent on sustinble mngement prctices derived from sound reserch, nd is impertive not only to ensure continued profitble yields, but impcts on broder socil, environmentl nd economic sustinbility issues (Attiwill nd Leeper 1987; Costnz nd Ptten 1995; Nmbir 1996; Tomn nd Ashton 1996; Olbrich et l. 1997; Evns 1999; Nmbir 1999; Glvič nd Lukmn 2007; Nmbir nd Kllio 2008) SUSTAINABILITY The definitions of sustinbility re numerous nd lck common consensus, often formulted ccording to humn gends or within nrrow context. These definitions, in the most prt, 7

38 describe n outcome bsed prediction of sustinble resource supply nd utilise vriety of mesured indictors tht my include or exclude environmentl, economic nd socil components, covering vrious time scles nd endpoints (Attiwill nd Leeper 1987; Costnz nd Ptten 1995; Nmbir 1996; Tomn nd Ashton 1996; Evns 1999; Nmbir 1999; Glvič nd Lukmn 2007). Sustinbility in plnttion forestry hs in similr mnner focussed on predicting the supply of wood (fibre) nd other products from plnttion forests into the future, nd uses mesures of these forest products over time to determine sustinbility (Evns 1999; Morris nd Smith 2002). This pproch, termed nrrow sense sustinbility, gives little considertion to ecosystem processes nd is often msked by confounding fctors such s siliviculture, genetic improvements, the effects of wether ptterns, pest nd disese nd other fctors (Smith et l. 2005; du Toit 2006). Sustinbility in the broder sense encompsses n understnding of the ecologicl processes tht drive the productivity of the plnttion nd includes whole tree plus ecosystem productivity s opposed to the mrketble components considered by nrrow sense sustinbility (Smith et l. 2005). Although brod sense sustinbility offers improved understnding nd prediction, to be complete, it must cknowledge the nturl finite lifespn or renewl cycles of n ecosystem. Nturl systems when left undisturbed hve cyclicl lifespns of finite durtion (du Toit 2006; Voinov nd Frley 2007). A broder definition of sustinbility in the plnttion forestry context to encompss definitions nd constrints found in literture is suggested s follows: Mngement of plnttion forest ecosystem resources to ensure the ecologicl productivity of the system for the mximum expected lifespn while mintining continued economic, socil nd environmentl benefits derived from the plnttion ecosystem. As it is not possible to meet ll the requirements lid out by these objectives, mngement prctices re often formulted to llow for cceptble trdeoffs between the relised benefits derived from the plnttion nd the vrious sustinbility criteri (Attiwill nd Leeper 1987; Costnz nd Ptten 1995; Nmbir 1996; Tomn nd Ashton 1996; Evns 1999; Nmbir 1999; Morris nd Smith 2002; Glvič nd Lukmn 2007; Voinov nd Frley 2007). This review considers the interctions between mngement nd biogeochemicl nutrient cycling nd the role tht this plys in mintining the ecologicl sustinbility of plnttion forest 8

39 ecosystems for mintining economiclly vible yields into the future. Although this pproch tends towrds sustinbility in both the nrrow nd brod sense, it provides more in-depth understnding of ecosystem processes tht drive plnttion productivity. This literture review ttempts to introduce this ide, while not repeting detils nd literture derived dt given in ech of CHAPTERS 3 to PRODUCTIVITY AND PLANT GROWTH RESOURCES Plnttion forest gross primry productivity relies on the bility of trees to cpture light energy nd utilise this to convert crbon dioxide (CO 2 ) nd wter into biomss (Bttgli et l. 1998; Powers 1999). The bility of trees to cpture light is dependent on tree lef re, which in turn is relint on the vilbility of the resources of light, ir, wter, nd nutrients (not ignoring time nd lnd re/growing spce). Gross primry productivity (GPP) is the product of resource supply (Supply), the proportion or efficiency of resource cpture (RCE) nd the efficiency t which resources re utilised (RUE) (Monteith (1977), expressed s: Productivity (GPP) = Supply x RCE x RUE. Gross primry production is llocted to respirtion (including symbiotic orgnisms) nd net primry production (NPP), nd NPP is further llocted to plnt structurl components (including stemwood) nd growth functions (such s root nd cnopy turnover). Losses of NPP cn occur through pest nd disese interctions nd dverse wether. Gross primry production, NPP nd lloction ptterns re species dependnt, cn chnge over the durtion of rottion (physiologicl ge), nd cn be ffected by site fctors tht re ltered by nturl or mn induced shifts in resource vilbility or blnces (Cnnell nd Dewr 1994; Binkley et l. 2004; Stpe et l. 2004; Ryn et l. 2008). Productivity will be reduced if there is ny single resource limittion or imblnce through mngement prctices or nturl phenomenon tht lters Supply, RCE or RUE of tree growth resources (Binkley et l. 2004; Ryn et l. 2008). It is logicl tht permnent ltertions of ny of these will lter the inherent ecologicl productivity of site nd impct on the tree productivity in the long-term (sustinbility). The ecologicl productive potentil of plnttion forest is therefore determined through the physicl nd biologicl interctions between 9

40 individul trees, the trees nd other biot, the chemicl, biologicl nd physicl nture of soils (including depth) nd climte (Wring nd Ludlow 2008) PLANTATION MANAGEMENT AND SUSTAINABILITY Plnttion mngement uses vrious strtegies to enhnce or mintin productivity through mnipultion of site properties to lter resource Supply, RCE nd RUE. Mngement prctices hve little influence over nturl shifts in globl wether ptterns tht drmticlly lter the productive potentil of ny given rottion of trees (Pretzsch 2009). Such chnges cn only be dptively mnged through timing of silviculturl opertions to coincide with sesons, following rinfll events or plnting geneticlly tolernt tree species. The supply of wter nd nutrients from soil nd orgnic reserves re therefore the min biotic nd biotic fctors tht cn be mnipulted through mngement prctices, nd tht determine the productivity of plnttion grown Euclyptus (Lclu et l. 2003; Stpe et l. 2004; Nmbir 2008; Smethurst 2010). Current plnttion mngement therefore uses wide vriety of erly site preprtion nd silviculturl prctices tht cn enhnce plnttion growth or reduce production costs. These include hrvesting methods, the mngement of residues, site nd soil preprtion, fertilistion, site-species mtching, plnting prctices, tree spcing, thinning, pruning, pest/disese control nd weed control (Nmbir nd Kllio 2008; Tirks nd Rnger 2008). Although mngement prctices cn enhnce the growth of current tree rottion, such prctices if done without cre, cn hve negtive impct on plnttion productivity in the long term. This is through negtive impcts on site resources soil nd orgnic mtter tht cn be lost or degrded. Poor or incorrect mngement hs the potentil to severely degrde or depleted soil structure, chemistry, crbon (micro-orgnisms included) nd site nutrient supply nd storge (du Toit et l. 2010; Rietz 2010). The risk of these resources becoming degrded or depleted is dependent on site climtic nd edphic properties, tree species, mngement prctices nd the cpture, storge nd relese (fluxes) of crbon nd nutrients between tmospheric, soil nd biologicl pools (Lclu et l. 2003; Nmbir 2008; Smethurst 2010). The risk nd rte of such loss or degrdtion is however, site nd mngement dependnt. Binkley (1986b) for exmple, proposed tht nutrient losses re most relevnt on sites tht hve smll nutrient reserves in reltion to lrge removls nd hence most t risk of nutrient depletion. A number of interntionl studies lso confirmed this concept in tropicl Euclyptus plnttions, which suffered significnt productivity 10

41 declines fter excessive biomss removl (Crlyle nd Nmbir 2001; Nmbir 2008). This ws prticulrly evident in stnds grown on sndy soils (renosols) nd dystrophic loms (oxisols), with low nutrient buffer cpcities nd low orgnic mtter, where declines in growth were ttributed to reduction in nutrient vilbility or nutrient pool depletion (Mendhm 2003; Nmbir 2008; Sint-André et l. 2008; Tirks nd Rnger 2008; Lclu et l. 2010). The sustinble supply of nutrients for tree growth t specific site cn only be determined through understnding the size of the vrious nutrient reserves or pools, the fluxes of nutrients into (dditions), between (cycling) nd out of (losses) ech pool NUTRIENT POOLS AND FLUXES Nutrients in plnttion forest ecosystems re found in orgnic nd inorgnic forms in soil, in soil orgnic mteril, tree biomss (stem, roots, cnopy nd reproductive structures) nd forest floor litter lyers (Morris 1986; Lclu et l. 2003). Nutrients move (flux) between these pools nd re dded to nd lost from the ecosystem (Figure 1). Fluxes of nutrients re driven primrily by energy nd mtter flows tht re regulted by site environmentl fctors. These fctors principlly include wter, temperture, plnt vilble rdition, wind, phenology, time, physicl nd chemicl nture of ech component of the ecosystem (ir, plnt, orgnic mteril, soil nd wter). An understnding of these vrious pool sizes nd flux rtes nd mgnitudes during ech plnttion growth phse reltive to site types nd mngement prctice is necessry to determine the sustinbility of plnttion forests from nutritionl perspective (Wells nd Jorgenson 1978; Attiwill nd Leeper 1987; Attiwill nd Adms 1993; Morris 1997; Nzil et l. 2002; Lclu et l. 2005; Guo et l. 2006). A more comprehensive review of these pools nd fluxes is presented in CHAPTERS 5 to 9 below. Since wter is one of the min drivers of nutrient fluxes in plnttion forests (Smethurst nd Nmbir 1990; Crlyle nd Nmbir 2001), mngement prctices tht lter the hydrologicl properties of soils or even surfce properties cn hve drmtic effect on nutrient flux processes. Residue burning, for exmple, cn increse soil wter repellency which in turn cn reduce infiltrtion rtes nd increse runoff nd soil erosion on sloped lnds (Scott nd Vn Wyk 1990; Scott 1993; DeBno 2000; Scott 2000). The dependence of mny nutrient cycling processes on wter fluxes is obvious nd implied in the reminder of this review. Soil wter is centrl to this study nd more comprehensive literture review on soil wter in forest ecosystems is presented in CHAPTER 4. 11

42 Nutrient fluxes Nturl inputs of nutrients occur through minerl wethering, nitrogen (N) fixtion, tmospheric deposition nd number of other, often minor processes (niml nd colluvil movement) (Figure 2.1). A short review of the literture on tmospheric inputs of N, potssium (K), clcium (C) nd mgnesium (Mg) with rinfll, throughfll nd stemflow in forest ecosystems is presented in CHAPTER 5. Anthropogenic inputs re typiclly ssocited with fertilistion, while nutrient losses occur, directly nd indirectly, through hrvesting nd silviculturl mngement prctices. Fertiliser is used in commercil forests to increse productivity by improving nutrient vilbility (even on inherently fertile sites). Anthropogenic losses occur directly through hrvesting nd burning nd indirectly through soil erosion nd ccelertion of nturl loss processes. Nturl losses occur through leching, erosion, oxidtion during fires nd voltiliztion (denitrifiction of N) (Jorgenson et l. 1975; Binkley 1986; Attiwill nd Adms 1993; Kimmins 1994; Rnger nd Turpult 1999; Blnco et l. 2005). Nutrients re cycled within the system by retrnsloction, cnopy exchnge, biomss decomposition nd soil exchnge, which redistributes nutrients from nd between vrious ecosystem nutrient pools (soil, detritus, plnt, tmosphere) nd includes the immobilistion nd mobilistion of these nutrients (Rnger nd Turpult 1999; du Toit nd Scholes 2002). Nutrient lock-up in the forest floor or its relese through decomposition is dependent on fctors tht include temperture nd moisture regimes, lignin nd N contents nd rtios, soil funl chrcteristics nd litter ph (Hobbie nd Gough 2004; Prescott 2005). Wrmer nd wetter conditions, for exmple, induce more rpid decomposition nd nutrient relese (Jorgenson et l. 1975; Attiwill nd Leeper 1987). 12

43 Atmospheric pool Rinfll Living Biomss pool Cnopy leching Throughfll retrnsloction (stem, brk, leves, roots) Stemflow litterfll gseous losses fertilistion erosion & runoff uptke displcement leching Soil pools root turnover wethering, dsorption/desorption minerlistion, exchnge dissolution, precipittion Litter & residue pool decomposition minerlistion immobilistion Figure 2.1: Representtion of some mjor nutrient pools, dditions, losses nd cycling fluxes within mture plnttion ecosystem Nutrient pools in the soil The soil generlly contins disproportiontely lrge totl nutrient quntity reltive to plnttion biomss nutrient pools, lthough very smll portion of this is ccessible to the trees t ny one time (Fisher nd Binkley 2000). The size of the soil nutrient pool is generlly dependent on the volume of soil vilble for root explortion, the depth to which nutrient uptke cn occur, soil density nd the concentrtions of plnt vilble soil nutrients (Turner nd Lmbert 2011). Nutrients my be held in vrious orgnic or inorgnic forms in the soil tht my be redily vilble for root uptke or be trnsformed into plnt vilble forms within rottion or over mny rottions. The fluxes between these pools nd vilbility forms re complex nd specific to ech nutrient element in ech soil pool, but lso their interctions s this will control the 13

44 thermodynmic processes ffecting exchnge, sorption, precipittion, etc. (Fisher nd Binkley 2000). Nutrients held in orgnic forms re either microbilly minerlised or immobilised. Inorgnic nutrients trnsfer between the soil solution nd the soil solid phse through ion exchnge redox rections (ction or nion exchnge), re reduced or oxidised nd undergo fixtion or dissolution processes (Fölster nd Khnn 1997) Nitrogen Nitrogen, which is generlly held s orgnic forms in the soil crbon pool, cn be very lrge reltive to minerl forms, but minerlistion of the orgnic forms is dependent on microbil ctivity which my limit the rte nd mount of N relesed for uptke by trees (Binkley nd Hrt 1989). The soil crbon pool is mde up of plnt nd microbil mtter in ctive (redily oxidisble), intermedite (slowly oxidisble) nd pssive (reclcitrnt) frctions tht cn turnover t rtes of dys, yers, decdes, centuries or millenni (Attiwill nd Leeper 1987). Orgnic dditions to the soil N pool occur through root nd litter decomposition nd leching of orgnic compounds from the tree cnopy. Fertiliser, N fixtion (symbiotic nd non-symbiotic) nd tmospheric deposition dd inorgnic N to this pool (Rnger nd Turpult 1999; Son 2001). Nitrogen minerlistion by soil micro-orgnisms converts orgnic soil N from decomposed plnt mteril to mmonium, while further oxidtive nitrifiction cn convert portion of this mmoni to nitrite nd then nitrte (Rison et l. 1987). A loss of N from the soil pool cn occur through plnt uptke, leching of mmonium nd nitrte, voltilistion under lkline conditions, oxidtion during high intensity fires, or through denitrifiction under nerobic conditions (Groffmn 1995). Relese of N bound in orgnic forms thorough N minerlistion cn provide lrge proportion of tree uptke requirements (Fölster nd Khnn 1997; Scholes 2002). The relese of N from soil orgnic mteril through minerlistion depends on the physicl nd chemicl nture of the soil orgnic mtter, soil moisture nd soil temperture which includes ertion, C: N rtio, soil ph nd soil P vilbility (Tisdle et l. 1985; Gunderson et l. 1988; Hrris 1988; Crlyle et l. 1990; Vn Miegroet et l. 1990; Rice nd Hvling 1994; Gonclves nd Crlyle 1994 ; Brnes et l. 1997; Sierr 1997 ; O'Connell nd Rnce 1999). Chnges in these fctors with soil depth results in decresed N minerliztion with incresed soil depth (Cbrer 1993). Understnding these vribles hs llowed N relese rtes to be predicted t both temporl nd sptil scles using process-bsed models prmeterised with miniml set of site nd soil dt (O'Connell nd Rnce 1999; Pul et l. 2002). Net immobilistion of N cn 14

45 occur under conditions of low soil N vilbility, reltive to vilble orgnic crbon (C) (Smith nd du Toit 2005). Nitrogen nd N minerlistion re discussed further in CHAPTER Phosphorus Phosphorus (P) is often bundnt in the soil, existing in non-lbile orgnic (microbil, plnt detritus nd humus), primry ptite minerls, secondry compounds, s P tht is chemi-sorbed or occluded onto minerl surfces (iron nd luminium oxides, crbontes nd clys), lbile (exchngeble) P nd soil solution P (Wlker nd Syers 1976; Frossrd et l. 2000). Phosphorus fluxes occur between solid nd solution phses in the soil through dsorption, desorption of primrily forms of P ions on to functionl groups of soil sesquioxide surfces nd dissolution nd precipittion of P compounds (Hrley nd Gilkes 2000; Breemen nd Buurmn 2003b,2003). Orgnic to inorgnic trnsformtions occur through biologicl minerlistion nd immobilistion of orgnic nd lbile P (Klbitz et l. 2000). Although hving fr lrger pool in the soil thn the forest floor litter lyer, orgnic forms of P in the forest floor tend to be fr more vilble for plnt uptke thn soil P forms, resulting in the bulk of P uptke being derived from nd recycled bck to the orgnic P pools (Attiwill nd Adms 1993). The loss of the forest floor litter often results in notble deficiency of P in the trees (Nery et l. 1999). Nturl dditions of P occur through minerl wethering nd losses through leching (Breemen nd Buurmn 2003b). P is not prticulrly mobile soil element so leching is often only in soils tht lck sorption sites. Phosphorus leching rtes re often very low in undisturbed forest ecosystems. Greter losses occur through loss of prticultes (erosion) where P is sorbed or fixed on the fine mteril. South Africn plnttion forests re grown on highly wethered soils tht contribute insignificnt wethering P nd re often prone to P fixtion (Binbridge et l. 1995; du Toit nd Scholes 2002). Mycorrhizl ssocitions with tree roots cn enhnce P uptke from the soil, lbile P pools nd from (often inccessible) soil sources (Plssrd nd Dell, 2010). This my be importnt on P deficient soils where fungl excretion of orgnic nions such s oxlic cid cn relese insoluble P Bse ctions Bse ctions (K, C nd Mg) re predominintly held in the soil on the soil exchnge complex. The bility of soils to bind ctions depends on the ction exchnge cpcity (CEC) of the soil, which is dependent on soil cly type nd content, orgnic mtter (humus) content nd ph. 15

46 Potssium cn be held in the interlyer positions if illitic clys re present in the soil (Swhney 1972). Minerl wethering nd tmospheric inputs re the most importnt long-term sources of nturl bse ction inputs to plnttion forests. Minerl wethering is dependent on soil minerl composition nd exposed surfce re, soil depth, climte (which determines temperture nd moisture conditions), soil CO 2 nd soil cidity (Sverdrup nd Wrfvinge 1993). As plnt growth ffects the lst four fctors to vrible degree, it cn lter minerl wethering rtes nd exchnge processes (Kelly et l. 1998). Although minerl wethering cn be n importnt input of ctions in the long-term, the short-term supply is often insignificnt from plnt supply perspective (Jorgenson et l. 1975; Rnger nd Turpult 1999). The generl lck of long-term soil studies hs mde it difficult to quntify the rte of bse ction relese from minerl wethering (Richter nd Mrkewitz 2001). However, ction relese from minerl wethering plys minor role in the highly wethered Southern Africn soils (Owens nd Wtson 1979; du Toit nd Scholes 2002). Other fctors ffecting bse ction sttus re minerlistion of orgnic mteril, root exudtion nd turnover, nd cnopy leching. These contribute to the cycling of bse ctions through the soil, orgnic mteril nd plnt environment (Likens nd Bormnn 1995; Ouimet nd Duchesne 2005). Losses of bse ctions from the soil occur primrily through plnt uptke nd subsequent biomss removl or through leching, especilly in cidic soils nd soils tht hve low buffering bility (low crbon nd low cly soils) Nutrient pools in the biomss Considering rottion, the gretest potentil for site mnipultion nd nutrient loss occurs during the period between clerfelling nd cnopy closure (Nmbir 2008; du Toit et l. 2010; Smethurst 2010). A number of hrvest techniques nd residue mngement prctices re used in plnttion forestry resulting in prtil or full retention, removl or burning of residues. The choice is driven by wildfire risk mitigtion, economics, residue mrkets, environmentl considertions nd opertionl prcticlity. Residue cn be left where it flls fter hrvesting, distributed (brodcst) over the soil surfce, stcked in rows, mulched or incorported into the soil (du Toit et l. 2004; Smith nd du Toit 2005). The combintion of residues nd the previous crop litter lyer on the forest floor is often referred to s slsh. Hrvesting intensity, often referred to s residue removl, cn involve prtil or complete removl of vrious tree components in ddition to hrvested stem wood. Further removls might lso include the forest floor litter lyer. Mechnicl whole tree hrvesting, off-site debrking, residue burning nd fuel 16

47 wood collection cn incorporte both high intensity hrvesting nd residue removl (Nmbir 2008; Sint-André et l. 2008; Tirks nd Rnger 2008). Hrvesting nd residue mngement result in direct nutrient loss tht occurs s consequence of biomss removl nd burning, s well s indirect loss through ccelerted leching, runoff nd soil erosion following hrvesting (Fisher nd Binkley 2000; Pretzsch 2009). These losses re incresed by whole-tree hrvesting nd off-site debrking, s further biomss is removed in ddition to the stemwood (Nzil et l. 2002; Gonçlves et l. 2008; Sint-André et l. 2008). The quntity of nutrients lost is dependent on the mount of biomss removed nd the tree component tht is removed; the more metboliclly ctive components (leves nd brk) contining the highest nutrient concentrtions followed by brnches nd stem wood (Dmes et l. 2002; Nzil et l. 2002; du Toit et l. 2004; Blnco et l. 2005; Sfou-Mtondo et l. 2005; Sint-André et l. 2008; Dovey 2009). Often neglected in hrvesting loss ssessments is the timing of removl fter clerfelling s some nutrients (potssium nd phosphorus) my lech from biomss prior to collection if left to stnd in field. Burning of hrvest residues includes combustion of the litter lyer nd surfce soil orgnic mtter. This cn cuse substntil nutrient losses through oxidtion, voltiliztion, sh nd prticulte trnsport, followed by further leching nd erosion (Fisher nd Binkley 2000). These losses increse s lyer mss, nutrient content nd fire intensity increse; which in turn depend on wether conditions during burning, residue size distribution nd rrngement, nd moisture content (Attiwill nd Leeper 1987; Nzil et l. 2002; O'Connell et l. 2004; Ndel 2005; du Toit et l. 2010; Zvl et l. 2010). Indirect nutrient losses occur from the soil, forest floor nd hrvest residues prior to nd fter plnting. This is due to residue nd soil tempertures nd soil wter content becoming elevted through direct sunlight exposure nd miniml wter nd nutrient uptke. This results in more rpid residue decomposition nd nutrient displcement leding to ccelerted leching (ssuming dequte moisture), s well s voltilistion nd soil erosion in specific res (Morris nd Miller 1994; Fisher nd Binkley 2000; Fernández et l. 2004; Blnco et l. 2005; Gómez-Rey et l. 2008). Shding nd uptke of wter nd nutrients by the newly plnted crop grdully reduces the impct of these nutrient loss mechnisms. 17

48 The negtive impct of lrge nutrient losses on soil nutrient nd orgnic mtter content is most importnt on infertile sites. This cn reduce nutrient vilbility nd pool size nd mnifest s reduction in productivity in subsequent rottions nd in the long-term (Spngenberg et l. 1996; Corbeels et l. 2005; Gonclves et l. 2007; Deleporte et l. 2008; Sint-André et l. 2008). A network of interntionl studies funded through the Centre for Interntionl Forestry Reserch (CIFOR) investigted the effects of residue mngement fter clerfelling on subsequent productivity cross tropicl plnttion forests. Euclypts growth ws reduced fter residue removl on the low-fertility sites in Austrli (Sint-André et l. 2008), Brzil (Gonçlves et l. 2008b), Chin (Xu et l. 2008) nd the Republic of Congo (Deleporte et l. 2008). More fertile sites did not show the sme productivity declines in Austrli (Sint-André et l. 2008) or decresed only fter complete residue nd litter lyer removl in South Afric (du Toit et l. 2008). However, growth decline occurred with little evidence of soil chemicl chnges dignosed using conventionl soil chemicl nlyses. Despite evidence of nutrient relted growth decline in interntionl forests (Nmbir 2008; Sint-André et l. 2008; Tirks nd Rnger 2008), only few studies in southern Afric hve linked the loss of specific nutrients through certin hrvesting nd residue mngement prctices with subsequent long-term productivity decline. This hs been undertken for Pinus ptul grown on Gbbro derived soils (Morris 1986; Crous et l. 2008) nd Euclyptus grndis grown on shle/dolerite derived soils (Evns 1999; du Toit nd Scholes 2002). Successive rottion tree growth, fertiliser response, folir dignostics, nutrient pool ssessment nd nutrient budget blnces hve been used seprtely or in combintion s evidence of nutrient depletion nd growth decline in ech cse. Erly growth responses in both interntionl nd South Africn studies my hve been due to the removl of the nutrient pool (residue nd litter lyer) tht provided lrge proportion of redily vilble nutrients during erly growth t these study sites (Nmbir 2008; Sint-André et l. 2008; Tirks nd Rnger 2008). This incresed short-term nutrient vilbility fter clerfelling or burning tht msked the long-term effects of the lrge nutrient losses hs been termed the ssrt effect (Kimmins 1994). Dt describing the bility of the vrious South Africn plnttion sites nd soils to supply the nutrient demnd of trees is lso limited to only few studies (Morris 1986; du Toit nd Scholes 2002; Crous et l. 2008). Although some South Africn studies hve ssocited nutrient loss dt with subsequent growth nd uptke into the new crop, they re limited to specific soils, do not fully describe ll the nutrient loss nd ddition mechnisms nd my confound the effects of climtic vrition, genetics nd site conditions with the interprettion of results when forecsting to subsequent 18

49 rottions. As consequence it hs been difficult to link this erly growth reduction to long-term nutrient relted growth decline s mny study sites lck long-term evidence of response Nutrient budgets To be ble to fully describe nd understnd these processes in the plnttion forestry context, the interction between mngement prctices with plnttion nutrient uptke nd bio-chemicl cycling of nutrients needs to be understood (Rnger nd Turpult 1999; Lclu et l. 2005). The benefit of such understnding is the potentil to develop budget of nutrient inputs nd losses from site, using budget deficit to highlight nutrients tht re being rpidly depleted (Rnger nd Turpult 1999; Lclu et l. 2005). A nutrient budget however, is only useful if viewed in reltion to ecosystem pool size nd nutrient demnd nd supply (Binkley 1986b; du Toit nd Scholes 2002). This ws shown in du Toit nd Scholes (2002) where nutrient budget ws used with mesures of vilble nd potentilly vilble nutrient pool sizes s n pproch towrds developing nutrient sustinbility index pplicble to ny forest system. The index hs potentil to highlight sites sensitive to nutrient decline under intensive mngement prctices, provided dt is vilble cross multiple sites. The outcome of the budget therefore depends on the fertility of the soil reltive to the budget outcome nd tree rooting depth. Low or high soil fertility my lso be predetermined by pst griculturl prctices, lowered through repeted burning or rised through fertilistion. Deep rooted trees will hve ccess to lrger body of soil nutrients thn shllow rooted trees. Certin tree species cn lso be better dpted to low soil nutrients thn others nd better ble to tolerte nutrient stress. Some recognition hs been given to nutrient cycling, in South Africn plnttions in the pst (Morris 1992; Olbrich et l. 1997; Dmes et l. 2002; du Toit nd Scholes 2002; Louw nd Scholes 2002; Lowmn 2004; Ndel 2005; Cmpion et l. 2006; Dovey 2009), but comprehensive quntittive studies deling with nutrient cycling re scrce nd often limited to unpublished work. While this is primrily due to the finncil nd lbour resources required for such work, it is lso due to the difficulty in understnding the flux processes nd obtining the informtion required. Given these constrints, much of the sustinbility reserch performed interntionlly hs focused on the effects of nutrient removl on tree growth nd soil properties. Nutrient budgeting, discussed in Rnger nd Turpult (1999) highlights the need for nd benefits of crefully constructed nutrient budgets tht cn be used for erly wrning of negtive impcts of mngement prctices on tree productivity. 19

50 2.6. CONCLUSIONS FROM THE GENERAL LITERATURE REVIEW The productivity potentil of plnttion forest is determined through the physicl nd biologicl interctions between the trees, climte, the chemicl, physicl nd biologicl nture of soils, nd soil depth (Wring nd Ludlow 2008). These site qulity fctors qulittively nd quntittively determine the supply nd uptke of resources vilble for tree growth throughout the life of plnttion. Productivity nd the lloction of tree growth to stemwood cn chnge over the durtion of rottion nd re ltered by nturl or mn induced shifts in resource vilbility (Cnnell nd Dewr 1994; Stpe et l. 2004). Productivity will be reduced when tree growth resource becomes limited or imblnced (Binkley et l. 2004; Ryn et l. 2008). Mngement prctices tht lter the supply, uptke, use or blnce of tree growth resources will impct on productivity, while permnent resource ltertion will impct on long term sustinble productivity (Monteith 1977). It is therefore importnt to understnd the fctors tht ffect the quntity nd rte of supply of resources to trees, fctors tht limit or enhnce the uptke of the resources nd the optiml blnce tht ffords mximum growth in current nd future rottions. The risk of these resources becoming degrded or depleted is dependent on site climtic nd edphic properties, tree species, mngement prctices nd the cpture, storge nd relese (fluxes) of crbon nd nutrients between the vrious tmosphere, soil nd biologicl pools (Lclu et l. 2003; Nmbir nd Kllio 2008; Tirks nd Rnger 2008; Smethurst 2010). Sustinble productivity reserch nd mngement decisions re crucil to the survivl of the forestry industry, s not only do they ffect profits from wood yield, but impct on broder socil, environmentl nd economic sustinbility issues tht ultimtely constrin mngement decisions. Monitoring nutrient cycling processes through the hydrologicl, biologicl nd soil components of plnttion grown during the intensive inter-rottionl nd erly growth phse of plnttion is therefore necessry s it will enble closer ssessment of the impcts tht silviculturl mngement prctices hve on soil nd biologicl nutrient pools nd fluxes of site. 20

51 CHAPTER 3: SITE DESCRIPTION AND EXPERIMENTAL TREATMENTS 3.1. STUDY SITE The site selected ws locted on the Zululnd costl plins (northern KwZulu-Ntl, South Afric) where extensive res of sndy soil nd sub-tropicl climte result in res of high productivity potentil on soils with smll nutrient reserves. The experiment ws initited t the end of 2007 in 17.6 hectre comprtment of seven-yer-old clonl E. grndis x E. cmldulensis in the Siyqhubek owned Dukuduku plnttion (Figure 3.1). The site ws converted (circ 1955) from mosic of indigenous lowlnd costl forest nd grsslnd to commercil forestry. Using the climtic dt of (Schulze et l. 1997), the study re is chrcterised by reltively high nd vrible sesonl (summer) rinfll, (men nnul precipittion (MAP) of 920 mm), men nnul temperture (MAT) of 21.7 o C, APAN reference evportion of mm. This ws in greement with rinfll (Figure 3.2) nd temperture (Figure 3.3) dt observed over the pst 50 yers t nerby wether sttion. Wind ptterns re predominntly in north-esterly nd south-westerly directions long the cost (Swewcuk nd Prinsloo 2010). The site is 62 m bove se level, 15 km inlnd from the cost (Indin ocen) with its centre t 28 17' 51" S nd 32 18' 55" E. Although the nerest perennil river is found pproximtely 6 km to est, the site is 1.3 km from nerest delineted wetlnd re, the centre of which is t 45 m bove se level. The slope of the lnd is reltively flt with n incline of 0.17 o. Soils re deep (>30 m), free drining sndy soils (<5% cly) with low orgnic crbon content (<1%). Some bsic soil physicl nd chemicl properties re presented in Tble 3-1, showing the low soil orgnic C nd fertility sttus, with low ction exchnge cpcity nd high soil cidity of the sndy soils t the site. These dt were collected t16 points in strtified grid cross the experimentl site using 10 cm steel coring tubes. 21

52 Figure 3.1: Mp showing the loction of the plnttion re in southern Afric nd the study site loction within KwZulu-Ntl commercil plnttion forestry res. 22

53 Monthly men rinfll Month Figure 3.2: 50 yer men monthly rinfll observed from nerby wether sttion. Single stndrd devition is shown s I-brs. Monthly men tempertures ( C) Mx Min Month Figure 3.3: 50 yer men monthly mximum (Mx) nd minimum (Min) tempertures observed from nerby wether sttion. Stndrd errors shown s I-brs. 23

54 Tble 3-1: Bsic soil physicl nd chemicl properties of the top 100 cm (in 20 cm increments) t the strt of the study nd chemicl properties t 50 cm increments therefter Depth Bulk Density Cly Snd Silt ph OC TKN Bry 2 P K + C 2+ Mg 2+ N + Exch. cidity ECEC Acid St. (cm) (g cm -3 ) (%) (KCl) (g kg -1 ) (mg kg -1 ) (cmol c kg -1 ) % nd OC (WB): Orgnic crbon (Wlkley-Blck); TKN: Totl Kjedhl nitrogen; nd: not determined Stnding crop prior to clerfelling The clonl E. grndis x E. cmldulensis (GxC) trees were originlly plnted t spcing of 3 m x 3 m in rectngulr comprtment (92 x 213 trees) tht ws chosen s hving its long xis s close to north/south orienttion s possible (Figure 3.4). Tree volume t 7 yers ws derived from tree height nd dimeter t 1.3 m bove ground level (dbh) using tree volume nd tper functions of Morley nd Little (2011) s 143 m 3 h -1 for trees h -1 (99% survivl). The site index t bse ge 5 (SI 5, 80 th percentile of tree heights projected to 5 yers of ge) for this comprtment ws estimted s 19.9 using the dt of Coetzee (1992). Site index, tree growthbsed mesure of site qulity, ws within norml rnges typicl for this re (Smith et l. 2005b). The mture tree cnopies in this stnd (nd trees of the entire region) were lso infested with Thumstocoris peregrinus, peking during the summer months of Jnury to Mrch ech yer (Ndel et l. 2010). This insect pest reduces photosynthetic ctivity in the cnopy nd my cuse incresed litterfll (Ndel et l. 2010). Evidence of Gonipterus spp., lef-eting insect tht reduces lef re nd picl growth, ws lso found SETUP OF EXPERIMENTAL AREA At the end of 2007 the experimentl design ws overlin onto the entire stnding crop comprtment s zones for felling nd zones to be left undisturbed s stnding crop res (Figure 3.4). Instlltion of equipment commenced full yer prior to felling, llowing dequte time for soil nd equipment stbilistion. Photogrphic imges of vrious equipment instlltions, events nd the site in generl re given in Appendix 3.4 (Imge 3-1 to Imge 3-18). 24

55 Tretment implementtion Tretments were pplied to the study site fter clerfelling, s four replicte rndomised complete block design with two replictes of un-felled trees forming stnding crop tretment (Figure 3.4). The stnding crop res were delineted s 46 x 92 tree (138 x 276 m) zones t the northern nd southern end of the comprtment with surrounding buffer zone of 51 m width (17 trees) which ws clerfelled (Figure 3.4). Stnding crop plots were delineted in centrl positions of the northern nd southern res, with buffer zones of 17 trees. The north/south orienttion nd the lrge buffer res were designed to minimise the intrusion of dditionl shding nd/or solr penetrtion between felled nd stnding crop plots. The centrl portion of the comprtment ws felled, nd divided into 36 squre experimentl plots 42 x 42 m, ech with n internl 15 x 15 m plot used for tree growth mesurement. Felling of the buffer zone begn in September The buffer res were used s extrction routes for the min felling opertion t the end of November Timber ws mnully stcked in erly Jnury 2009 nd mechniclly extrcted. 25

56 Figure 3.4: Experimentl lyout of tretment plots showing the stnding crop zones nd internl felled Burn nd No-Burn tretment plots with dimensions given in metres. Additionl tretment codes pply to further components of this study not reported here (Whole-Tree s whole tree hrvest, Fert. No-Burn plus fertiliser replcement, Double - double residue lyer). Residues were mnully brodcst cross the entire felled re over the old crop litter lyer fter clerfelling, except for the whole tree hrvesting tretment (Whole-Tree) from which whole trees were removed, leving the litter lyer intct. The remining tretments included single residue lyer (No-Burn), double residue lyer (Double) (creted using the residues removed from the Whole-Tree plots), brodcst nd burn tretment (Burn) nd mcro-nutrient fertiliser replcement tretment (Fert). Fertiliser ws used to supply mjority of mcro nutrients lost 26

57 with stemwood removl during hrvesting clculted using seven yer old tree stem dt from the biomss smpling described lter in this chpter. Fertiliser ws pplied shortly fter plnting to ech tree in 15 cm rdius ring. Grnulted fertiliser ws pplied per tree ws 179 g limestone mmonium nitrte (28), 128 g superphosphte (10.5), 102 g KCl (50), 140 g CNO 3 (19, 15.5) nd 94 g of MgS0 4 (10). This dded N, P, K, C nd Mg to the soil in mounts of round 120, 22, 85,68, 16 kg h -1 respectively. The Burn tretment ws implemented in erly Mrch 2009 on hot cloudless dy under light, windy conditions to induce rpid complete burn. Weed nd coppice re-growth ws initilly mnully slshed, then eliminted using glyphosphte nd Triclopyr. The site ws mnully pitted (to 30 cm depth using mttock) in July 2009 nd plnted t 3 x 2 m espcement to G x C clone (GC 514) in ccordnce with compny policy chnge on spcing. Summry of tretments nd cronyms: Stnding crop (SC) Undisturbed crop no hrvest Whole-Tree (W) Removl of stem wood, brk, brnches nd folige t clerfelling Burn Residues brodcst nd burned No-Burn Residues brodcst nd retined Fertilised (Fert) Residues brodcst nd retined, Replcement of mcronutrients removed with stem wood Double (2S) Residues brodcst nd retined with ddition of n extr residue lod tht hd been removed from Whole-Tree plots. The tretments selected for intensive process monitoring in this study were the Burn nd No- Burn (residue retention) tretments, s these were considered to represent the most extreme prctices of the typicl residue mngement opertions, nd the stnding crop res were used to represent the site without hrvesting disturbnce. 27

58 3.3. DATA COLLECTION Wether Dt A fully-utomtic Cmpbell Scientific wether sttion ws instlled in n djcent open re to record rinfll, temperture, reltive humidity, solr rdition, wind speed nd wind direction. Dt were collected t hourly intervls nd summrised dily Wter fluxes Wter fluxes were monitored s precipittion, cnopy dringe (throughfll nd stemflow), soil moisture content nd dringe to 100 cm below the soil surfce throughout the felling, residue mngement, nd re-estblishment phses of the plnttion cycle until the newly plnted trees were 1.5 yers old Rinfll throughfll nd stemflow equipment Three white plstic funnels (14.1 cm dimeter) were instlled t 1.2 m bove the ground (4 replictes) in the felled plots to collect precipittion, nd similrly in the stnding crop plots to collect throughfll (Figure 3.5). The tree dimeter t brest height (dbh, i.e. mesured 1.3 m bove ground level) ws used to construct the size clss distribution, rnging between 13 nd 18 cm. The mjority of trees (75 %) were between 15 nd 17 cm in dimeter. The distribution of tree size clsses ws considered in selecting 4 x 4 tree plots into which the monitoring equipment ws instlled. This ws done to cpture the heterogeneity of the tree cnopy while representing the full tree dimeter distribution t ech smpling site. This design ws utilised considering the uniform cnopy of the clonl Euclyptus crop, the intended use of the smples in wter qulity ssessments nd cceptble error limits of the design (Kimmins 1973; Lwrence nd Fernndez 1993; Stpe et l. 2004; Mululo Sto et l. 2011). Wter drined from the funnels vi polyethylene tubing ws collected nd stored in 5 l white high-density polyethylene (HDPE) bottles. A smll piece of plstic mesh ws plced in ech funnel to prevent detritus from entering the tubing. The bottles were collectively housed in 25 l white HDPE bucket, buried to protect the bottles from het nd sunlight, with only the lid exposed. Four size clsses of tree dimeter were selected using the tree dimeter distribution, nd single tree ws selected from the midpoint of ech clss, onto which stemflow collectors were instlled. The stemflow collectors (Figure 3.6) were slotted 20 mm polyurethne tubes ttched to ech of the 4 trees 28

59 drining into seprte 5 l bottles contined in prtilly buried 30 l bucket (Levi nd Frost 2003). The tubes spirlled ech tree twice from 0.6 to 1.4 m bove ground level, held fst to the tree with plstic pckging belt threded through the tube. Contct between the tree nd tube length ws chieved using 100 % silicone selnt formultion. 3 m 1 3 m Tree position Collection funnel Buried storge bucket Figure 3.5: Lyout of funnel cluster nd storge continer reltive to tree positions. 29

60 Nrrow slit 1.3 m To buried collection bucket Figure 3.6: Side view of nrrowly slit stemflow collection tube ttched to tree stem Lysimeter instlltion A crescent-shped trench ws excvted to depth of 1.2 m to llow mesurement of soil moisture sttus nd flux on the site (Figure 3.7). The trench ws situted between the trees so tht the wll of the pit gve ccess to the inter-row of 6 trees, the front of the crescent fcing 2 trees, the rms turning digonlly cross the intr-row of 2 trees ech. This ws done to llow ccess to undisturbed soil t the pit fce while llowing the pit fce to cover s much soil nd tree size vribility s possible. Four replictes of zero tension plte lysimeters (30 cm x 30 cm) with 3 cm wlls on 3 sides were inserted horizontlly t depths of 15 cm, 50 cm nd 100 cm, into the wll of trench, by opening nrrow envelope into the pit fce t ech depth. Horizontl distnces between pltes were mintined t no less thn 60 cm. This instlltion ws repeted in ech tretment block. The lysimeter pltes were used s cost-effective method to collect smple of dringe wter for wter qulity dignosis, lthough they re known to hve collection efficiency of bout 10% (Weihermuller et l. 2007). This ws ssumed to give n integrted smple of grvitydrined soil solution, but necessitted the prediction of ctul dringe using the soil moisture flux model described lter. The depths chosen here were intended to reflect norml fine root length nd mss density distributions chrcteristic of the soil under Euclyptus. Highest 30

61 densities in clonl Euclyptus hve been shown to occur in the first 15 cm of soil, decresing mrkedly with depth to 50 cm nd nd grdully therefter until very few fine roots re found (Knight 1999; Lclu et l. 2001; Gonçlves nd Mello 2004; O'Grdy et l. 2005). Prior to insertion, soil from the envelope ws plced s slurry into the plte, the plte pressed to the top of the envelope to mke best contct, nd the gp under the plte ws filled with the remining soil. The pltes were instlled t slight ngle to llow for dringe to corner plug connected to polyurethne tube. A trench dug from the centre of the crescent led into pit excvted to 1.4 m, which llowed the 4 pltes t ech depth to grvity drin to plstic collection bottles. The crescent trenches nd inter-leding trenches were crefully bckfilled ensuring no disturbnce of the re bove the instlled pltes. The wlls of the ccess pits were reinforced with timber nd the pits were covered with corrugted lid to offer protection from sunlight nd rinfll. A second soil solute collection system ws instlled s suction cups between ech plte. These were held t constnt vcuum of -50 brs using vcuum pump regulted through vcuum switches nd Cmpbell CR10x dt loggers. This system filed to collect sufficient soil solution for nlysis nd hs been excluded from this study. 3 m 3 m Tree position Profile probe positions (reltive) Covered collection pit Lysimeter plte Bckfilled trench 31

62 Figure 3.7: Top view lyout of trench nd storge pit reltive to tree positions, showing lysimeter pltes tht re lterntively buried 15, 50 nd 100 cm. Profile probe positions shown reltive to trees Wter Smple collection nd nlysis Wter collection begn in December 2007, with weekly collection until erly Smple volume ws recorded nd sub-smples were decnted into 250 ml Teflon bottles nd trnsported in cool box. The sub-smples were ssessed for ph nd electricl conductivity (EC), refrigerted t round 5 C for no more thn four weeks nd bulked through weighted bulking prior to chemicl nlysis. Weighted bulking, representing four weeks smple collection, ws chieved using the frction of ech weekly volume over the totl four week volume. Bulking ws necessry to reduce lbortory nlyticl costs. Mesurements of ph nd EC were repeted fter bulking. Wter smples were nlysed for orgnic nd inorgnic N s Totl Kjeldhl-N [4500-N org method (Clesceri et l. 1998)], mmoni N (NH + 4 -N) nd nitrte (NO - 3 -N) [flow injection (Ptton nd Crouch 1977; APHA 1995)], the difference between Kjeldhl-N nd (NH N + NO 3 N) ws ssumed to be orgnic N (O-N). The TKN method included modifiction where slicylic cid ws dded to the ctlyst mixture. This converts NO 3 -N to nitric cid which nitrtes the slicylic cid. The nitro-compounds produced were then reduced to mmoni nd distilled by dding excess lkli. Included in the nlyses were the ctions K +, C 2+, Mg 2+ nd sodium (N + ) [Auto nlyser, (EPA 1984)]. Phosphorous ws initilly determined, but lter bndoned due to cost nd lbortory equipment filure. Cost lso excluded the nlysis of sulphur nd other micronutrient elements. Field blnks (pssing deionised wter through the field equipment) nd stndrd smples were submitted for nlysis to ensure nlyticl ccurcy nd test for field contmintion. The volumes of wter collected were used with funnel re to clculte precipittion nd throughfll in mm. Stemflow ws clculted (mm) using the volume dt for ech tree size to predict volume collection by ll trees nd scling this bck to the re represented by the trees on the stnd. Totl cnopy dringe ws clculted s the sum of stemflow nd throughfll. Solute concentrtions were multiplied with totl volume collected by ech collector for ech rinfll event to derive msses of elements contined in rinfll, throughfll nd stemflow. 32

63 Concentrtions tht were unresonbly high reltive to the other ion concentrtions (prticulrly K + nd orgnic-n for lrge rinfll events) were excluded s possibly contminted Soil moisture monitoring A soil cpcitnce profile probe ws used to mesure volumetric soil moisture content t weekly intervls t 10, 20, 30, 40, 60, 80, 100 cm below the soil surfce vi 5 ccess tubes instlled into ech plot. A micro-topogrphy ws pprent over the entire site, possibly cused by lifting of soil ner to trees, reltive to the soil level of the spces between trees (5 to 10 cm mplitude). The ccess tubes were therefore plced within nd between the tree rows to represent the vribility of soil moisture in ech plot (Figure 3.5C). Time-domin reflectometry (TDR) probes were instlled in one of ech of the tretment plots t 15, 50, 75 nd 100 cm, utomticlly recording volumetric soil moisture content t hourly intervls vi custom-built dt-logger. Soil smples for grvimetric moisture nlysis (oven drying t 105 o C until constnt mss) were collected periodiclly to vlidte soil moisture contents mesured with the electronic devices Soil chemicl properties Soil chemicl nlyses were undertken t the strt of the study, just prior to felling nd gin t the end of the study when the new trees were 1.5 yers old. Soils were collected from five points in ech plot using PVC core tht ws driven into the soil. Soils were seprted into incrementl lyers to represent lyers smpled by the lysimeter pltes (0-15 cm, cm nd cm) nd bulked cross ech depth for ech plot. Additionl smples collected monthly s 0 5 cm, 5-15 cm nd cm lyers using the sme methods bove were included in the nlysis. The smples were ir dried nd pssed through 2 mm sieve nd nlysed for ph, N, P, K C, Mg nd N using the procedures described in Hunter (1975), Frin (1981) nd Donkin et l. (1993). Nitrogen ws determined using the Kjeldhl method (Nelson nd Sommers 1980; Donkin et l. 1993; Mulvney 1996). Bry-2 extrctble P ws determined using the Bry nd Kurtz (1945) methodology, the filtrte utomticlly nlyzed colorimetriclly t 880 nm using segmented flow utonlyser (SAN plus SYSTEM; Sklr Anlyticl, Bred, The Netherlnds) with scorbic cid colour development (Murphy nd Riley 1962). Ction (C, Mg, K nd N) extrctnts (Donkin et l. 1993) nd Helmke nd D.L. (1996) diluted with n ionistion suppressnt (strontium or cesium) were determined by tomic bsorption spectrophotometry 33

64 (AAS; SpectrAA-10, Vrin Techtron Pty. Ltd., Mulgrve, Victori, Austrli). Soil ph ws determined in 1: 2.5 soil: 1M potssium chloride solution rtio (Thoms 1996) with using stndrd glss electrode (Metrohm Hersiu E396B; http: //products.metrohm.com) Wter repellency smples Soil smples collected to ssess N minerlistion described in CHAPTER 7 were used for further determintion of soil moisture nd wter repellency. A simple semi-quntittive ssessment of wter repellency ws conducted using wter dropper to pply 6 droplets of deionised wter to smoothed surfce of sub-smple of the ir-dried soils collected from ech of the tretments. The time tken until the wter drops were bsorbed (infiltrtion) into the soil ws recorded nd the verge time used s n indictor of wter repellency (Letey et l. 2000; Scott 2000) Growth nd biomss smpling Tree growth ws determined t six monthly intervls fter plnting (August 2009) up to two yers nd six months fter plnting. Tree height nd ground line dimeter (gld) were initilly recorded in the new crop followed by dimeter t 1.3 m bove ground level (dbh) once the trees were of sufficient size for these mesurements. Above ground biomss nd nutrient contents were initilly determined t six months, one nd two yers fter plnting. The two yer-old smpling only included biomss dt s nutrient dt ws not vilble t the time of publiction. Cnopy closure ws pproximted t the point where cnopies of djcent trees strted to overlp nd occurred t bout one yer fter plnting Stnd biomss nd nutrient mesurements Aboveground tree biomss ws estimted using 20 destructively hrvested trees cross the entire site. Destructive hrvesting utilised dbh size clss distribution, which ws divided into five size clsses from which trees were selected to represent the full rnge of tree sizes cross the site. Trees were selected from ech tretment in the new crop to cpture nutrient concentrtion differences, while reducing nlyticl costs. Hrvested trees were seprted into folige, brnches (live nd ded), brk nd stem wood. The initil hrvest in the new crop ws seprted 34

65 into folir nd non-folir (woody) mteril (stem, brk nd brnch). Field wet mss of ech component nd subsmples moisture contents fter drying to constnt mss t 60 C were used to derive the totl biomss of ech tree component. Additionl wood disk smples were used to determine wood density by mss displcement in wter. Specific lef re (SLA) ws determined by scnning leves from ech tree on flt-bed imge scnner with 5 mm clibrtion grid. Anlysis of individul lef re ws performed on the scnned imges using ImgeTool 3.0 softwre (UTHSCSA 2002). Oven dry mss nd re of btch of leves ws used to clculte SLA which ws multiplied by totl folir dry mss to derive cnopy lef re. Stnd level biomss nd lef re index (LAI, m² m - ²) ws estimted from single tree biomss components using correltions with gld nd dbh. A polynomil function produced the best predictive equtions with dbh in the mture crop, while nturl log trnsformed gld nd dbh gve the best fits for ll log trnsformed tree components of the new crop. All models hd R 2 vlues greter thn Sttisticl probbilities for predictive equtions were ll significnt (p <0.001), s were ll predictive prmeter estimtes (p < 0.001). Growth efficiency ws clculted in the old nd new crop with respect to incrementl bove ground woody biomss production per unit LAI nd expressed thus: Mg h -1 yr -1 LAI -1. LAI ws tken s the men vlue between biomss ssessment points. Tretment-specific llometric reltionship were lso tested for nutrient contents, but lcked significnce nd did not improve the predictions Litterfll nd residue Stnding crop litterfll ws collected weekly using 50% shde net held t 15 cm bove the ground in steel frmed (1.0 x 1.5 m) litter trps. Five litter trps were instlled in ech tretment positioned t vrious distnces from the trees. Litter ws seprted into folir, brnch nd brk components nd bulked four weekly. Litterfll ws not ssessed in the newly plnted crop s litter fll ws negligible up to cnopy closure nd tree cnopies were lso too low for effective litter collection. Forest litter lyer nd residue biomss ws collected t four weekly intervls using metl ring (34 cm dimeter) cross five rndom points in ech tretment plot. A loss on ignition (LOI; smples shed t 450 o C for 12 hours) ws used to estimte mss without of soil contmintion. The sme ring method nd nlyses were used before nd fter burning to ssess biomss nd nutrient loss fter burning. In ddition, 10 steel pltes (40 x 40 cm) were inserted under the residue (on the soil surfce) prior to burning to collect sh remining fter burning. 35

66 This dditionl method ws ttempted to void soil contmintion. Annul decy rte constnts (k) were clculted from percent residue biomss, using single negtive exponentil decy model X t /X 0 = e kt where X t is the residue biomss remining t time t, X 0 is the initil residue biomss nd k is the monthly decy constnt Olson (1963). Dried subsmples from ech tree component, litter, residue nd sh were individully ground nd homogenised then nlysed for nutrient concentrtion (N, P, K, C, Mg, N) using the methods described in Klr nd Mynrd (1991) t the Cedr lbortories in South Afric. Nutrient concentrtions were ssessed in four weekly bulked litterfll nd residue. Nutrient concentrtions for ech tree component in ech tretment were used with totl stnd component biomss to derive nutrient content per hectre. Residue, litter nd sh nutrient contents were clculted without LOI djustment. The quntity of nutrients relesed into the soil due to decomposition ws ssumed to be the difference between nutrient content t ech time of ssessment. Nutrient relese in the stnding crop ws clculted s the sum of chnge in forest floor nutrient content nd litterfll nutrient content. Vector nlyses fter Weetmn (1989) developed further in Slifu nd Timmer (2001) were performed for nutrient contents nd concentrtions in the folige using the No-Burn tretment s the reltive control. The weighted men concentrtion ws clculted from folir nutrient content nd folir mss. (Vlentine nd Allen 1989) described this method s more relible method to predict growth response to fertilistion thn folir dignostics lone s it considers both concentrtion nd growth in the ssessment. This method, for exmple, ccounts for the confounding effects of nutrient dilution tht my occur fter fertilistion induced growth response Sttisticl nlysis The effect of the tretments on boveground biomss, nutrient content, nd ccumultion nd growth rtes t ech ge were compred by generl ANOVA, with lest significnt difference (LSD 5% ) used to show tretment differences where significnce ws found. As only two reps could be produced for the stnding crop tretment, two reps were treted s missing dt. Tests for normlity using the Shpiro-Wilk test nd homogeneity of vrince using the Brtlett's test 36

67 were performed on the dt where pplicble. Differences in concentrtions were tested using Student s t-test. All sttisticl nlyses were performed using Genstt for Windows 12th Edition (Pyne et l. 2011). 37

68 3.4. APPENDIX OF PHOTOGRAPHIC IMAGES Imge 3-1: The sndy soil nd litter lyer Imge 3-2: Wood structure preventing soil collpsing into pit Imge 3-3: Pltes nd suction cups inserted into pit fce Imge 3-4: Slotted stemflow collector Imge 3-5: Automtic wether sttion in nerby clering Imge 3-6: Wood disk smples collected for mss, nutrient nd density determintion 38

69 Imge 3-7: Stnding crop litter pre-clerfelling Imge 3-8: Clerfelling of the buffer strips Imge 3-9: Clerfelled comprtment showing Stnding crop in bckground Imge 3-10: Opening soil pit cover Imge 3-11: Whole-Tree plot surfce fter clerfelling Imge 3-12: Residue burning 39

70 Imge 3-13: TDR probes inserted into the Burn tretment the dy fter burning Imge 3-14: Soil N-min cores in Burn tretment Imge 3-15: Ash lyer in soil t four weeks fter burning Imge 3-16: A Burn tretment plot t six months fter plnting Imge 3-17: A No-Burn tretment plot t six months fter plnting Imge 3-18: A Whole-Tree tretment plot t six months fter plnting 40

71 CHAPTER 4: A COMPARISON OF SOIL MOISTURE RELATIONS BETWEEN STANDING AND CLEARFELLED PLOTS WITH BURNT AND UNBURNT HARVEST RESIDUE TREATMENTS ABSTRACT The effects of clerfelling nd subsequent residue retention or burning on wter nd nutrient blnces needs to be understood nd quntified on forest sites tht re sensitive to loss, so tht the long-term sustinble productivity of such sites cn be mintined nd promoted. An experimentl site ws estblished in clonl Euclyptus comprtment on the Zululnd Costl Plin, to compre chnges in wter fluxes through the mture undisturbed Euclyptus stnd with those fter felling nd re-plnting, under 2 conditions: burning, nd retention of the hrvesting residues. The study ws locted in n re of high rinfll nd high stnd productivity, with sndy soils nd low soil crbon nd nutrient sttus; chosen so tht the effects of intensive demnds on wter nd nutrient fluxes on potentilly sensitive site could be investigted. This chpter presents only the hydrologicl component of the study. Dt collection included weekly determintion of rinfll, throughfll, stemflow nd soil moisture fluxes from the surfce to depth of 1 m. Dringe rtes through the profile were estblished using time domin reflectometry probes while wter dringe volumes were ssessed using shllow plte lysimeters. Despite slow growth in the stnding crop during the monitoring period (ttributed to pest infesttion), soil moisture depletion remined rpid nd dringe below 1 m remined low. Soil moisture ws rechrged within few months fter clerfelling, but becme rpidly depleted s the cnopy of new crop developed nd pproched cnopy closure. A decresed wetting-front velocity nd mrginlly higher field cpcity were proposed s evidence of pore clogging tht ppered to occur during the inter-rottion period. The soil profile under the unburnt residue mintined mrginlly higher soil moisture sttus nd lower dringe thn the soil profile under the burnt residue. Although soil moisture nd dringe in the burnt nd unburnt residue tretments becme similr to the stnding crop from cnopy closure onwrds, rinfll dditions to soil moisture were depleted fster under the new crop during the first few months fter cnopy closure. Smll differences in soil moisture sttus between the burnt nd unburnt residue tretments presented here my not be sufficient to influence residue mngement decisions. The length of the inter-rottion period nd prctice of residue burning my, however, need 1 Published (2011) in Wter SA 37(4):

72 considertion where soil crbon nd nutrient loss or displcement my negtively ffect the sustinbility of the site INTRODUCTION Wter use under commercil plnttion forestry hs been extensively studied in South Afric with the outcomes used to formulte guidelines for wter use chrges nd regultions (Dye nd Versfeld 2007). These regultions, n incresed demnd for wood products, nd numerous other politicl, economic nd environmentl fctors, hve pressurised the industry to seek innovtions to enble lrger quntities of timber nd biomss to be produced from decresing lnd bse (FSA 2009b). The loss of lnd re, 9% between 1999 nd 2009 (FSA 2009b) hs occurred through, inter li: wetlnd delinetion, successful lnd-clims tht hve subsequently resulted in lnd being converted out of forestry, nd the estblishment of wildlife corridors through plnttion forest res. Increses in productivity of 19%, for the sme 10-yer period, hve been relised through the selection of fster growing trees, improved mtching of tree species to sites for optiml timber growth, plnting of disese-resistnt clonl vrieties, shorter rottion lengths, more intensive silviculturl prctices, nd mechnistion. This hs lso substntilly incresed the rte nd quntity of biomss removl per unit lnd re of plnttion forests nd consequently incresed pressure on wter nd nutrient resources on the remining lnd re. It is therefore crucil tht the negtive impct of greter demnd on wter be understood nd quntified so tht long-term sustinble productivity cn be mintined nd promoted. Soil moisture is rpidly utilised under forestry with little evidence of long-term soil or ground wter rechrge under full-cnopied Euclyptus crops, s dynmic equilibrium is mintined between stomt nd lef re index controls of evpotrnspirtion nd soil moisture vilbily vilbily (Kienzle nd Schulze 1992; Dye 1996; Lclu et l. 2001). Trcking the processes within the hydrologicl component of commercil plnttion forest requires knowledge of the mgnitudes of the wter fluxes, tree wter use, nd the impct of site mngement on these fluxes. Site mngement t hrvesting, nd the conservtion, displcement or loss of hrvest residues through removl or burning during the inter-rottion phse, cn potentilly impct site. As the hydrologicl cycle is key driver of forest nutrient fluxes nd cycles, the effect of clerfelling nd subsequent residue mngement on soil moisture nd dringe my impct on wter vilbility to the subsequent crop nd nutrient loss or displcement from the ctive fine 42

73 root zone (top 50 cm of soil) (Smethurst nd Nmbir 1990; Crlyle nd Nmbir 2001). This is in ddition to nutrient nd crbon losses tht occur through hrvesting nd residue burning. Wter repellency of soils, which is common under Euclyptus plnttions, my be incresed with residue burning, reducing infiltrtion rtes, incresing surfce runoff nd soil erosion on sloped lnds (Scott nd Vn Wyk 1990; Scott 1993; DeBno 2000; Scott 2000). The deep sndy soils of the Zululnd costl ecosystem (Northern KwZulu-Ntl, South Afric) re low in cly, orgnic crbon content, nutrient nd wter storge cpcity, nd hve high dringe rtes (Hrtemink nd Hutting 2005). These soils re unble to buffer chemicl, physicl nd orgnic (biologicl) chnges, plcing them t risk of degrdtion, ultimtely limiting or reducing tree productivity under poor site-mngement prctices. The sustinbility of mngement prctices on sites which hve high productivity but low soil moisture nd nutrient storge cpcity is incresingly questioned, prticulrly under currently prescribed residue-burning opertions. A study ws initited to compre soil moisture content nd dringe volumes (using in situ mesurement techniques), in mture clonl Euclyptus stnd, with those fter felling, nd upon which residue burn nd no-burn mngement prctices were imposed, nd for the first 18 months of new crop growth. In ddition, soil moisture model (Hydrus 1D) ws clibrted with soil moisture content mesured t ech depth to verify the predictive bility of the model used. This study forms prt of lrger study tht imed to determine the impct of site mngement on long-term sustinble productivity with respect to nturl nd mngement-induced nutrient losses nd gins. The model will be discussed in future publictions s tool to predict dringe fluxes. Wter fluxes will be multiplied by mesured dissolved nutrient concentrtions to estimte nutrient fluxes due to grvity dringe, in similr mnner to tht used by Lclu et l. (2007) MATERIALS AND METHODS Mterils nd methods for this chpter tht re common to other chpters re given in CHAPTER 3. Methods used in soil moisture content re given here. 43

74 Model prediction of wter moisture nd dringe The 1-dimensionl Hydrus 1D soil moisture model (Šimůnek et l. 2008) ws prmeterised for the study site nd used to predict dily soil moisture content nd dringe t ech depth. Hydrulic conductivity prmeters were estimted using neurl network prediction fter Schp nd Bouten (1996) with mtrix potentil mesurements of similr soil tken from Rietz (2010) nd soil texturl frctions nd bulk density determined from smples tken t the site (Tble 4-1). Soil hydrulic prmeters re presented in Tble 4-1 s residul nd sturted soil moisture content (Qr, Qs, respectively), prmeters in the soil moisture retention function (Alph, n), nd sturted hydrulic conductivity (Ks). The vn Genuchten-Mulem soil hydrulic model ws used s the hydrulic conductivity component of the model (vn Genuchten 1980). Evpotrnspirtion ws estimted using the Penmn-Monteith eqution for the model nd meteorologicl dt collected t the site, nd lso evluted using TDR dt recorded t the site. Lef re index recorded t the site through destructive methods ws llowed to fluctute ccording to opticlly-determined LAI-2000 mesurements nd vlues given in Dye et l. (2004). A rdition extinction coefficient of 0.4 nd n lbedo of 0.2 were used fter Stpe et l. (2004) for the stnding crop trees. An lbedo of 0.3 ws used during the fllow period in the felled re (ten Berge 1986). Rinfll ws djusted for interception using reltionship developed between weekly rinfll nd weekly cnopy dringe from dt collected t the site. Interception of rinfll by the litter nd residue lyers ws estimted for the stnding crop tree litter lyer nd the felled crop residue lyer using reltionships between residue mss nd interception, fter Pul et l. (2003). Root distributions were tken from literture-derived vlues (Knight 1999; Lclu et l. 2001; Nouvellon et l. 2002; Gonçlves nd Mello 2004; O'Grdy et l. 2005) nd were vlidted from cores driven into the soil to depth of 350 cm t 5 points in ech of the 6 plots. Roots seprted from the soil were dried nd weighed to determine root mss per soil volume in lyers of 0-5, 5-15, , , nd 50 cm increments therefter, to 350 cm. A root wter uptke model Feddes et l. (1978) ws used to simulte root wter uptke ssuming trnspirtion to rnge between 1 nd 6 mm per dy (Dye 1996; Nouvellon et l. 2002). 44

75 Tble 4-1: Men vlues of soil orgnic crbon, texture, nd bulk density tken t incrementl depths nd soil moisture retention nd conductivity prmeters Smple Orgnic Bulk Cly Snd Silt Qr Qs Alph n Ks depth crbon Density (cm) % % % % g cm -3 cm 3 cm -3 cm 3 cm -3 cm -1 - cm dy RESULTS Rinfll nd cnopy dringe Annul rinfll for the first 2 yers of the study ws below the site long-term verge (920 mm). Rinfll mesured between October nd October of ech yer ws 696 mm for 2009, 721 mm for 2010, nd 918 mm for Liner regression nlysis between rinfll, throughfll, stemflow nd cnopy dringe (Tble 4-2) indicted tht round 90% of rinfll penetrtes the cnopy s throughfll fter the initil 1.4 mm of interception, while further 5% of rinfll penetrtes s stemflow. Interception s percentge of rinfll ws dependnt on rinfll intensity, with lrge rinfll events resulting in low interception percentge (0.25% with 218 mm) nd smll rinfll events resulting in high interception percentge (100% with 1 mm). Coefficient of vrition (CV) between throughfll collectors verged t 6.2% nd remined below 15% for the durtion of the study. Smller rinfll events (below 3.5 mm) produced the lrgest vribility between collectors. Stemflow volume incresed with tree size (dbh) nd ws correlted with rinfll ( sttisticl correltion (R) of 0.93 to 0.95 between collectors). On few occsions smll mount of precipittion ws collected under the tree cnopy without rinfll (possibly from dew or mist interception). 45

76 Tble 4-2: Reltionships between rinfll collected in n open re nd tht collected with throughfll nd stemflow derived through liner regression Sttistic Throughfll Stemflow Cnopy dringe Constnt *** 0 ns *** Slope *** *** *** R Regression men squre *** *** *** Residul men squre SE Constnt SE Slope A mss of 26 Mg h -1 (2.6 kg m -2 ) of hrvest residue remined on the site fter hrvesting. This ws dded to the pre-existing 24.6 Mg h -1 (2.5 kg m -2 ) of forest floor during hrvesting to yield 50.6 Mg h -1 (5.1 kg m -2 ) of residue. The 3-month dely between felling nd burning resulted in lrge portion (28%) of the residues decomposing prior to burning. Burning reduced the remining residue from 36.4 Mg h -1 (3.6 kg m -2 ) to 4.2 Mg h -1 (0.4 kg m -2 ), lyer of sh nd chr tht did not persist on the soil surfce fter the first week fter burning, during which 15 mm of rin fell. Only very smll quntity of corse chr remined on the soil surfce in smll mounds, the reminder leched into the soil with lrger prticles creting distinctive chr horizon between 5 nd 10 cm from the soil surfce. The soil surfce therefter ws predominntly sh-free with only white snd visible Chnge in soil moisture fter hrvesting Soil moisture content is presented in Figure 4.1 A - C s the men nd stndrd devition of dt sets from 10 probes dt in ech tretment recorded t 10 cm increments (Appendix 4-3A - F). Soil moisture rechrge in the felled plots begn within 1 month fter felling (nd 72 mm of rinfll) t shllow depths nd fter 3 months t 100 cm with further 230 mm of rinfll. Rechrge in the stnding crop plots, lthough often less thn in the felled res, ws rpidly reduced fter rinfll during the first few summer months fter felling (Figure 4.1 A- C). During ech winter of the study in the stnding crop tretment wter content ner 100 cm depth remined t level slightly bove wilting point despite being reduced to wilting point t the shllower depths. This my indicte reduction in root ctivity t this depth during winter. Soil moisture contents in the No-Burn nd Burn tretments were consistently higher thn the stnding crop tretments nd seemed to remin bove field cpcity for period of pproximtely 2 46

77 months during the inter-rottionl period, even during prolonged periods without rinfll. Differences in soil moisture content between residue mngement tretments, lthough reltively smll, remined consistent throughout the inter-rottionl period. The No-Burn tretment remined moister thn the Burn tretment with up to 3% higher wter content fter burning during the period from felling to bout 6 months of ge of the new crop. Differences in soil moisture content between the stnding crop nd both the Burn nd No-Burn tretments decresed s the new crop pproched cnopy closure. Soil moisture mesurements fter cnopy closure were slightly higher in the No-Burn tretment thn the Burn tretment, while soil moisture in both Burn nd No-Burn plots tended to remin below tht of the stnding crop tretment. 47

78 15 cm Vol. Wter Content (A) 17% 16% 15% 14% () (b) (c) (d) 13% 12% SC 11% Burn 10% No-Burn 9% 8% 7% 6% 5% 4% 3% 03/08 06/08 09/08 12/08 03/09 06/09 09/09 12/09 03/10 06/10 09/10 12/10 Dte (mm/yy) Precipittion (mm dy -1 ) 50 cm Vol. Wter Content 17% 16% 15% 14% 13% 12% 11% 10% (B) 100 cm Vol. Wter Content (C) 9% 8% 7% 6% 5% 4% SC Burn No-Burn () (b) (c) 3% 03/08 06/08 09/08 12/08 03/09 06/09 09/09 12/09 03/10 06/10 09/10 12/10 17% 16% 15% 14% 13% 12% 11% 10% 9% 8% 7% 6% 5% 4% SC Burn No-Burn () (b) Dte (mm/yy) (c) 3% 03/08 06/08 09/08 12/08 03/09 06/09 09/09 12/09 03/10 06/10 09/10 12/10 Dte (mm/yy) Figure 4.1(A-C): Volumetric soil moisture content s mens of the stnding crop (SC), residue burned (Burn) nd residue retined (No-Burn) tretments t 15, 50 nd 100cm from 200 dys before hrvesting to 18 months fter plnting. Dshed lines represent () felling; (b) residue burning; (c) plnting nd (d) cnopy closure. Rinfll events re shown s verticl lines scling to n inverted secondry Y xis in 5. Single stndrd devitions re given s verticl I brs. (d) (d) 48

79 Vribility (stndrd devition) between probes nd CV within ech tretment plot ws linked primrily to the probe position reltive to the trees. Probes t the centre of 4 trees reflected greter wetting fter rinfll nd more rpid subsequent drying thn probes positioned ner to trees. Stndrd devitions nd CV cross ll probes t ech depth within ech residue mngement tretment were lso greter fter rinfll s differences in soil moisture content between probe positions reltive to the trees becme more exggerted. This ws most pprent in the stnding crop tretment nd lest pprent in the felled tretment plots during the interrottion period, vribility being greter in the Burn tretment t the soil surfce. Vribility between probe positions begn to decrese to similr levels s the stnding crop tretments s new crop growth progressed. The CV between probe positions remined below 15% t the soil surfce, decresing with depth to below 5% s differences between probe positions becme less. Mesured wter contents obtined vi probes nd grvimetric determintion greed well, indicting dequte probe clibrtion for the site. Differences in wetting nd drying ptterns between probe positions my hve been due to limited runoff nd run-on cross the microtopogrphy of the site. Rpid drying my hve been through higher root densities in the wetter depressed zones (Lclu et l. 2001). Although wter use ws pprently high in the stnding crop, very little growth occurred during the study period, with the biomss incresing by 4.6 % (4.2 Mg h -1 ) during the first yer fter plnting the new crop. While stem wood ccounted for 3.2 Mg h -1 of this growth litterfll dded further 7.7 Mg h -1 to bove-ground biomss production in the stnding crop during this period. Lef re index ws clculted s 1.68 m 2 m -2 cross the site t felling, fluctuting between 1.75 in the wet seson nd 1.42 in the dry seson of the stnding crop plots. Wter-use efficiency for stem wood production ws therefore poor, given the high rte of wter loss nd limited growth. New crop growth ws initilly greter in the Burn tretment thn in the No-Burn tretment, but slowed in the Burn tretment fter cnopy closure. Differences in biomss in the Burn tretment compred with the No-Burn tretment mounted to 29% more t 6 months, 17% more t 1 yer nd 5% more t 18 months. Although biomss in the Burn nd No-Burn tretments ws not sttisticlly different from 18 months onwrds, growth in the Burn tretment continued to slow reltive to the No-Burn tretment. Lef re index ws mesured s 1.9 nd 1.5 in the Burn nd No-Burn tretments, respectively, t 6 months of ge nd 2.7 nd 2.4, respectively, t 1 yer (cnopy closure). An bove-ground biomss of 4.1 nd 3.1 Mg h -1 ws estimted in the Burn nd No-Burn tretments, respectively, t 6 months of ge, nd 11.5 nd 9.9 Mg h -1 t 1 yer fter plnting. 49

80 During the inter-rottion to cnopy closure period t 3 to 14 months fter clerfelling (Februry 2009 to April 2010), hourly TDR nd weekly profile probe soil moisture dt gve some evidence indicting n ltertion in soil hydrulic conductivity (infiltrtion rte) nd field cpcity in the felled No-Burn nd Burn tretments. These prmeters were not mesured directly during this period s this chnge ws not expected. Hourly TDR dt produced consistent wetting-front dvnce rtes on ech test plot for number of rinfll events prior to felling cross ll tretments, nd throughout the study period for the stnding crop tretment. Figure 4.2 presents evidence of wetting front (using TDR probe mesurements) fter prolonged dry period nd the onset of evening rinfll. An evening smple of the TDR dt ws selected to reduce the potentil effects of dytime evpotrnspirtion. A wetting front ws detected t 100 cm depth within 2 to 3 h of detection of incresed moisture t 15 cm, in greement with the expected time for wter to move this distnce through the profile for the given soil hydrulic conductivity of the soil. After felling nd during the inter-rottionl period (Figure 4.2), wetting front rtes slowed in the felled Burn nd No-Burn tretment plots while remining unchnged in the stnding crop tretment plots. The decrese in rte of the wetting front velocity ws most pprent from round 3 months fter felling in the Burn nd No-Burn tretments, becoming less pprent over the months tht followed. Penetrtion time in the wter drop repellency test ws found to vry by few seconds, though no significnt difference (P<0.05) ws found between tretments. This suggested tht the wter repellency of the soil did not lter wter infiltrtion rtes between tretments, though the infiltrtion rte my hve been ffected by soil surfce condition (presence or bsence of residue nd site micro-topogrphy). Chnge in volumetric soil wter (%) 16% 14% 12% 10% 8% 6% 4% 2% SC 15cm Burn 15cm No-Burn 15cm rinfll SC 100cm Burn 100cm No-Burn 100cm Rinfll (mm) 0% Hour of dy 0 50

81 Figure 4.2: Demonstrtion of the time dely between wetting front detection t 15 cm nd 100 cm s percentge chnge in soil moisture content fter onset of rinfll in the stnding crop (SC), residue burned (Burn) nd residue retined (No-Burn) tretments. Clculted s difference from pre-rinfll soil moisture content, (zero vlues not shown) Wter collected by plte lysimeters The volumes of wter collected were dependent on soil moisture content prior to rinfll events nd the intensity of ech rinfll event (Figure 4.3A-C). Low-intensity rinfll events resulted in more wter collection thn similr-quntity high-intensity rinfll events, s the lower intensity of rinfll my hve reduced the frction of wter lost by pltes overflowing. Cumultive rinflls during ech time period were 418, 211, 641, 286 nd 727 mm, respectively (Figure 4.4). Volumes of wter collected decresed with depth nd were generlly the lowest in the stnding crop tretment (Figure 4.3A C, Figure 4.4). Lrger totl volumes of wter were collected under the Burn tretment t ech depth thn under the No-Burn tretment. Despite the trends of differences between tretments, no significnt (P<0.05) differences between tretments were found. 51

82 16 14 () (b) (c) (d) cm lysimeter collection (mm) SC Burn No-Burn rinfll Precipittion (mm dy -1 ) (A) 0 03/08 06/08 09/08 12/08 03/09 06/09 09/09 12/09 03/10 06/10 09/10 12/10 Dte(mm/yy) cm lysimeter collection (mm) SC Burn No-Burn () (b) (c) (d) (B) 100 cm lysimeter collection (mm) 0 03/08 06/08 09/08 12/08 03/09 06/09 09/09 12/09 03/10 06/10 09/10 12/ SC Burn No-Burn () (b) Dte(mm/yy) 0 03/08 06/08 09/08 12/08 03/09 06/09 09/09 12/09 03/10 06/10 09/10 12/10 (c) (C) Dte(mm/yy) Figure 4.3(A-C): Men volumes (± SD) of wter collected t 15, 50 nd 100 cm depth with plte lysimeters in the stnding crop (SC), residue burned (Burn) nd residue retined (No-Burn) tretments. Dshed lines represent () felling; (b) residue burning; (c) plnting nd (d) cnopy closure. I brs represent single stndrd devitions. Rinfll events re shown s verticl lines scling to n inverted secondry Y xis in 6(A). (d) 52

83 90 Lysimeter collection (mm) cnopy closure to 18 months six months to cnopy closure Burning to six months Felling to Burning cm 50 cm 100 cm SC Burn No-Burn SC Burn No-Burn SC Burn No-Burn Depth (cm) & Tretment Figure 4.4: Totl volume of wter collected t 15, 50 nd 100 cm depth with plte lysimeters in the stnding crop (SC), residue burned (Burn) nd residue retined (No-Burn) tretments from felling to 18 months fter plnting. Stcked brs represent time periods from felling to burning, nd six monthly time periods from plnting to cnopy closure onwrds. I brs represent 5% lest significnt differences for tretment nd collection period Predicted wter content using HYDRUS model Tble 4-3 presents the liner regression vribles nd reltionships between wter content predicted with the Hydrus model nd tht mesured within the profile probe t ech depth for the stnding crop plots. The model dequtely estimted wter content t ll depths with ner 1: 1 reltionship between predicted nd observed vlues. Model estimtes were poorer t the soil surfce, but improved with soil depth; this is reflected in the increse in the R 2 vlues with incresing depth. Soil surfce estimtions my hve been less relible s the surfce soils hd more vrible orgnic crbon content nd lower bulk density, especilly in the 0 to 10 cm soil lyer. A further estimtion of soil moisture content to 10 m depth ws performed for the stnding crop tretment, ssuming constnt soil properties to depth nd the bsence of wter tble bove 10 m depth. The model estimted no dringe beyond 2 to 3 m depth, ssuming ll wter to be tken up by the stnding crop. However, this ws not vlidted with ctul mesurements. 53

84 Tble 4-3: Reltionships between wter content predicted with the Hydrus 1D model nd mesured with the profile probe t ech depth for the in the stnding crop (SC) tretments. Sttistic 10 cm 20 cm 30 cm 40 cm 60 cm 100 cm Constnt ns ns ns ns ns ns Slope *** *** *** *** *** *** R Regression men squre Residul *** *** *** *** *** *** men squre SE Constnt SE Slope *** Superscript implies F- or T- test significnce t p < 0.001, ns superscript implies no significnce Model predictions of wter content were poorer thn the stnding crop for the No-Burn nd Burn tretments during the inter-rottion to cnopy closure period, 3 to 14 months fter clerfelling. Predicted wter content during this period ws significntly below vlues mesured in the field. Reducing soil hydrulic conductivity nd incresing field cpcity in the Hydrus 1D model, for the period in question, to levels determined with the probe mesurements improved the model s predictive cpbilities. An inverse model prediction of soil hydrulic properties ws lso used to produce improved soil moisture estimtion. Although djusting soil hydrulic conductivity in the Hydrus 1D model llowed for much closer fit between predicted nd observed soil moisture content during this time period, chnges in other hydrulic prmeters could only be estimted. Figure 4.5 nd Figure 4.6 present the Hydrus 1D predicted totl volumes of wter pssing the 15, 50 nd 100 cm soil depths for the durtion of the study for the stnding crop nd felled tretments res. Vlues in Figure 4.6 re totl volumes from felling to 18 months fter plnting, subdivided into time periods similr to Figure 4.4. Dringe volumes s predicted by Hydrus (Figure 4.6) were higher thn those mesured in the field (Figure 4.4). Actul dringe ws 4 to 6% of predicted dringe in the felled plots, nd 6 to 27% of predicted vlues in the stnding crop plots. Plte dringe rte my hve been too slow nd wter my hve been diverted round the pltes or by pltes overflowing, n occurrence common in such lysimetery studies (Weihermuller et l. 2007). 54

85 300 () (b) (c) (d) SC 50 SC 100 SC Predicted Dringe (mm) Felled 50 Felled 100 Felled /08 06/08 09/08 12/08 03/09 06/09 09/09 12/09 03/10 06/10 09/10 12/10 Dte (mm/yy) Figure 4.5: Predicted dringe volumes t 15, 50 nd 100 cm for felled nd stnding crop (SC) res of the study site. Verticl dshed lines represent () felling; (b) residue burning; (c) plnting nd (d) cnopy closure cnopy closure to 18 months six months to cnopy closure Burning to six months Predicted Dringe (mm) Felling to Burning rinfll sc Felled sc Felled sc Felled Depth (cm) & Tretment Figure 4.6: Rinfll nd Predicted dringe volumes t 15, 50 nd 100 cm for felled nd stnding crop (SC) res of the study site. Ech portion of the brs represents dringe predicted for ech period in Figure DISCUSSION This study shows the chnges in soil moisture regimes fter clerfelling clonl Euclyptus crop on the well-drined sndy soils of the Zululnd costl plin. While soil moisture rechrge did occur to field cpcity under the living tree cnopy, soil moisture ws rpidly reduced over the entire mesured soil profile through evpotrnspirtion. Chnges in soil moisture fter clerfelling indicted rpid soil moisture rechrge with rinfll fter clerfelling; with surfce evportion nd weed nd coppice re-growth reducing soil moisture content. The incresed soil 55

86 moisture sttus fter hrvesting ws temporry s the new trees hd developed reltively lrge lef re by the time of cnopy closure tht fforded rpid reduction in soil moisture in the upper soil lyers. The residue lyer on the No-Burn tretment nd the bsence of this lyer on the Burn tretment (with exposed snd) my hve been responsible for the smll differences in soil moisture content. The retention of residue results in both higher interception of rinfll prior to entering the minerl soil nd lower soil surfce evportion rtes. Burning eliminted interception of rinfll prior to entering the minerl soil, but incresed soil surfce evportion rtes through surfce exposure. These my hve effectively reduced overll mesured differences between Burn nd No-Burn tretments. Although soil moisture levels in the new crop were similr to those in the stnding crop by cnopy closure, reltively fster soil moisture depletion occurred under the lrger cnopy of the new crop, prticulrly in the Burn tretment. Despite the incresed cnopy lef re fter burning hving reltively lrger wter demnd, the dditionl growth in this tretment my imply incresed wter use efficiency with respect to biomss production. This erly biomss increse is often used by plnttion mngers s n incentive towrds residue burning. The fster-developing cnopy reduces weed growth, thereby reducing opertionl weeding costs. This increse in biomss ws short-lived in this study, s growth in the burned plots begn to decline reltive to the unburnt plots fter cnopy closure. An incresed lef re fter burning my hve reduced the new crop's bility to cope with wter deficits or drier soil conditions fter cnopy closure, hence reducing growth rtes. This my imply tht trees tht hve developed lrger cnopy in response to residue burning my hve reduced bility to cope with drought conditions, should these occur. Not ll processes tht ffect soil hydrology re clerly understood or ccounted for in plnttion forest hydrologicl models. Stemflow hs been shown to chnnel wter nd nutrients cptured during rinfll or mist events directly to tree root systems, providing wter to trees tht is often not detected in the bulk soil or ccounted for in hydrologicl models (Johnson nd Lehmnn 2009). Preferentil flow long old tree root chnnels s mechnism of sub-surfce wetting or ground wter rechrge my lso be poorly ccounted for (Le Mitre et l. 1999). The interctions between pest infesttion nd wter use efficiency my lso need investigtion, prticulrly where pests cuse no reduction in lef re, but increse lef wter loss through dmge to epiderml lef tissue or photosynthetic cpcity. The continued wter use during pest 56

87 infesttion nd the drstic reduction of growth in this study clerly demonstrtes the negtive impct nd cost of pest infesttion to productivity nd wter use efficiency. Orgnic substnces relesed from the rpidly-decomposing residues my hve plyed role in ltering the soil during the inter-rottion. Orgnic exudtes, root decomposition, prolifertion of fungl hyphe nd microbil biomss in the soil my hve temporrily ltered soil physicl properties nd disrupted hydrulic flow pths through biologicl soil-pore clogging. Reserch hs shown tht orgnic mtter hs strong influence on wter retention nd sturted hydrulic conductivity (Rwls et l. 2003). Blnco-Cnqui nd Ll (2007) presented evidence of mulches dded to the soil surfce cusing n increse in soil porosity nd soil moisture content t wilting point nd decrese in soil bulk density. Chnges in hydrulic conductivity induced through microbil ctivity hve lso been shown in column studies tht simulted sturted soils (Seki et l. 1998), lthough these effects ssocited with forest residue brekdown need to be confirmed with further study. A wter repellent lyer in the soil produced by fine chr prticles my lso ct to increse the wter retention chrcteristics of soil (Moore 1996; Woods nd Blfour 2010; Onoder nd Stn 2011). Lrge quntities of nutrients locked up in the residues re rpidly relesed through residue decomposition into the soil solution during the inter-rottionl period. Solutes my be displced beyond the shllow fine-rooting zone or lost where lrge quntities re further leched. This my be concern where nutrient replcement through nturl processes occurs t insignificnt rtes nd where fertilistion is not possible. Although high evpotrnspirtion rtes nd wter use of Euclyptus, which cn led to decresed strem flow, re disdvntge on wter-limited sites where there re other competing humn, griculturl, industril or environmentl requirements for wter, high evpotrnspirtion rtes nd wter use my limit nutrient loss from plnttion forests. This my be of importnce to infertile sites where loss or displcement of nutrients with dringe cn negtively ffect growth. This principle ws demonstrted in studies conducted on the sndy soils of Congo nd Brzil where wter did not drin beyond depth of 5 m, despite fr higher rinfll thn tht recorded for the present study site (Nouvellon et l. 2002). The loss of nutrients in tht study hd fr more negtive effect thn the loss of wter. Sites t risk of nutrient loss my therefore require specil mngement. Allowing temporry coppice re-growth nd plnting to chieve erlier cnopy closure my limit dringe nd nutrient leching loss tht my occur if plnttion res re left fllow for prolonged periods. 57

88 4.6. CONCLUSION Despite the disdvntges of high wter use, n implied reduction in nutrient leching or displcement under full-cnopied crop my llow more nutrients to be retined in the ctive rooting zones under the undisturbed forest stnd. The impct of pest dmge on wter use efficiency in the full-cnopied crop my lso need further investigtion, s the loss to productivity, coupled with the high cost of wter, compounds economic losses relted to wter use. Although soil moisture nd dringe incresed fter felling, differences between residue retention nd residue burning presented here my not be enough to ffect the decision between burn nd no-burn mngement policies. Bsed on dt presented here, it is lso unlikely tht the residue mngement prctices presented here will hve ny further direct impcts on soil moisture content nd dringe beyond the period presented in this study. Reducing wter use through increses in productivity s consequence of superior genetics nd clonl forestry my still be one of the best strtegies for improving wter use efficiency, essentilly producing more timber per unit wter use (Stpe et l. 2004). The possibility of incresed wter use resulting from potentilly lrger tree cnopy, induced in response to plnting fter residue burning, my however increse the drought susceptibility of the new crop round the time of cnopy closure. Deeper dringe nd leching or displcement of nutrients during the period directly fter clerfelling my be of concern to forest mngers for sites tht re sensitive to soil nutrient losses. This my give reson for plnttion forest mngers to shorten the inter-rottion length, with erlier plnting to enble more rpid depletion of soil moisture through erlier cnopy closure, thereby retining more nutrients in the soil nd in the growing trees. This, however, my only be necessry for sndy soils with low nutrient retention cpcity. Rpid replnting is obviously lso positive from finncil point of view. In ddition to soil crbon nd nutrient loss with biomss removl nd burning, reducing dringe loss or the displcement of nutrients on sites with low wter nd nutrient storge cpcity my be n importnt fctor driving the efficient, nd ultimtely the sustinble, use of wter nd nutrient resources of such sensitive sites. 58

89 4.7. APPENDICES Appendix 4-1: Further Comments bout tree root Systems Although tree root systems hve been extensively studied, the vst mjority of these studies hve been with seedling plnted trees (Knight 1999). A pot study of cutting propgted E. globulus trees were found by Ssse nd Snds (1996) to hve lower wter use under wter stress thn seedling propgted trees. Although decresed wter use in clonl forestry hs not been shown, increses in productivity s consequence of superior genetics nd clonl forestry hs been shown to increse wter use efficiency, essentilly producing more timber per unit wter use (Stpe et l. 2004). The shllower root system nd the bsence of well defined tproot in cutting propgted trees (Mri João et l. 2005) my result in decresed in wter uptke t depth, prticulrly from the cpillry fringe bove the wter tble. This my chnge the view of wter use by clonl forests in Zululnd s previous model estimtions of ground wter use (Kienzle nd Schulze 1992) ssume the presence of tproot. Appendix 4-2: Further Comments bout wter use efficiency South Africn reserch over severl decdes hs shown tht plnttions commonly use more wter per unit of time nd spce thn indigenous vegettion tht it replces (Scott nd Smith 1997; Gush et l. 2002). However, Plnttions usully hve higher wter use efficiency thn indigenous forests (Gush nd Dye 2009; Gush et l. 2011), minly becuse they produce lrge quntities of biomss per unit of wter trnspired. Binkley et l. (2004) lso demonstrted tht resource use efficiency t the lndscpe scle is greter when there is n increse in totl resource use. Rther thn fforesting more lnd, incentives should perhps revolve round mintining nd idelly incresing productivity on currently fforested sites llowing for more timber to be produced on limited lnd re. Commercil plnttion owners my therefore need to ccept the high wter use nd focus on chieving high productivity on this limited lnd bse while sustining soil crbon nd nutrient stocks, prticulrly on nutrient poor sites. This my be relised through investing in number of key res in plnttion forestry reserch nd mngement tht include: () improved genetics, not only to produce higher vlue products, but to improve tolernce to unfvourble climtic conditions, pest nd disese, (b) Improved silviculture nd hrvesting prctices tht mintin or build crbon nd nutrient stocks, nd (c) improvements in pest, disese nd wildfire prevention. Essentilly mintining nutrient nd 59

90 crbon resources nd improving wter use efficiency re the only options tht hve ny prcticl fesibility in commercil plnttion forestry, llowing the gretest economic vlue to be obtined from n uncontrollbly high wter use while conserving ll other biologicl reserves. 60

91 10 cm Vol. Wter Content A 20 cm Vol. Wter Content B 30 cm Vol. Wter Content C 18% 17% 16% 15% 14% 13% 12% 11% 10% SC () (b) (c) (d) Burn No-Burn 9% 8% 7% 6% 5% 4% 3% 03/08 09/08 03/09 09/09 03/10 09/10 03/11 18% 17% 16% 15% 14% 13% 12% 11% 10% () (b) (c) (d) SC Burn No-Burn 9% 8% 7% 6% 5% 4% 3% 03/08 09/08 03/09 09/09 03/10 09/10 03/11 18% 17% 16% 15% 14% 13% 12% 11% 10% () (b) (c) (d) SC Burn No-Burn 9% 8% 7% 6% 5% 4% 3% 03/08 09/08 03/09 09/09 03/10 09/10 03/11 Dte (mm/yy) 61

92 40 cm Vol. Wter Content D 60 cm Vol. Wter Content E 100 cm Vol. Wter Content 18% 17% 16% 15% 14% 13% 12% 11% 10% 9% 8% 7% 6% 5% 4% SC Burn No-Burn () (b) 3% 03/08 09/08 03/09 09/09 03/10 09/10 03/11 18% 17% 16% 15% 14% 13% 12% 11% 10% 9% 8% 7% 6% 5% 4% SC Burn No-Burn () (b) F Dte (mm/yy) Appendix 4-3(A-F): Volumetric soil moisture content s mens of ech depth in the stnding crop (SC), residue burned (Burn) nd residue retined (No-Burn) tretments for the durtion of the study. Dshed lines represent () felling; (b) residue burning; (c) plnting nd (d) cnopy closure. Single stndrd devitions re given s verticl I brs. (c) (c) 3% 03/08 09/08 03/09 09/09 03/10 09/10 03/11 18% 17% 16% 15% 14% 13% 12% 11% 10% 9% 8% 7% 6% 5% 4% SC Burn No-Burn () (b) (c) 3% 03/08 09/08 03/09 09/09 03/10 09/10 03/11 (d) (d) (d) 62

93 CHAPTER 5: NUTRIENT FLUXES IN RAINFALL, THROUGHFALL AND STEMFLOW ABSTRACT Atmospheric deposition ws ssessed t two sites over four yer period in post cnopy closed (mture) Euclyptus stnds in the northern Dukuduku nd southern KwMbonmbi commercil plnttion forestry res of Zululnd, South Afric. The im of this study ws to determine the mgnitude nd relevnce of nutrient ddition with rinfll, throughfll nd stemflow to commercil forestry in this region. Cnopy ction exchnge ws used with rinfll nd cnopy dringe to derive wet, dry nd totl tmospheric deposition. Nutrient concentrtions mesured in the rinfll, throughfll nd stemflow vried widely throughout the study period, nd between sources nd sites. Rinfll ws slightly cidic t both sides, but becme less cidic upon pssing through the tree cnopies. Cnopy exchnge nd collection of dry deposition resulted in incresed ction concentrtions under the tree cnopy, while the cnopy generlly bsorbed nitrogen (N), from the rinfll, reducing the below cnopy concentrtions. Atmospheric deposition ws shown to be responsible for lrge quntities of nutrients dded to the Euclyptus stnds t ech site. Annulised deposition verged cross ll yers t ech site mounted to N, clcium (C) mgnesium (Mg) nd potssium (K) fluxes of 11.0, 6.0, 2.7 nd 10.2 kg h -1 yr -1 t Dukuduku nd , 7.5 nd 18.8 kg h -1 yr -1 t KwMbonmbi. Orgnic N fluxes contributed further 8.1 kg h -1 yr -1 t Dukuduku nd 7.1 kg h -1 yr -1 t KwMbonmbi to the totl N deposition. Although K deposition vlues were high, dditions of ll other nutrients, lthough lso high, were within the rnges reported in locl nd interntionl reserch. Over the course of full rottion, the tmospheric deposition levels recorded t these sites my hve potentil to supply lrge proportion of the nutrients tht re lost during stem-wood hrvesting. This study dds vlue to understnding of nutritionl sustinbility of fst growing plnttion forests, demonstrting the importnce of tmospheric deposition s nutrient ddition source to plnttion grown Euclyptus long the Zululnd costl plin INTRODUCTION The South Africn forestry industry contributes significntly to South Afric s gross domestic product nd employment from reltively smll nd decresing portion of South Afric s totl 2 Published (2011) in Southern Forests 73(3-4):

94 lnd re (FSA 2009). The loss of lnd re, 9% between 1999 nd 2009 (FSA 2009) hs occurred through, inter li: wetlnd delinetion, successful lnd-clims tht hve subsequently resulted in lnd being converted out of forestry, nd the estblishment of wildlife corridors through plnttion forest res. Increses in production of wood from South Afric s intensively mnged plnttions (19% for the sme 10-yer period) hs occurred through improvements in genetics, silviculture nd hrvesting (Schutz 1982; Burger 1996; Buhus et l. 2002). Although gins in productivity hve been relised through these improvements, questions re still being rised round the long term sustinbility of yields. The principles nd processes driving nutritionl sustinbility (mintennce of site fertility) in prticulr re not fully understood or quntified in commercil plnttion forestry (Blnco et l. 2005; Lclu et l. 2005b; Wtt et l. 2005). The impct tht nutritionl sustinbility hs on plnttion productivity is dependent on the biogeochemicl cycling of nutrients within the forest ecosystem nd the bility of trees to cquire, use nd cycle these nutrients (Rennenberg nd Schmidt 2010). Nutrient cycling is importnt in plnttion forestry where ddition of nutrients through processes such s minerl wethering, nitrogen (N) fixtion nd tmospheric deposition my prtilly offset losses incurred through hrvesting nd indirectly through other mngement prctices. The role nutrient cycling processes plys in site fertility hs been described in nturl forests nd few plnttion forests in interntionl literture (Jorgenson et l. 1975; Wells 1976; Binkley 1986; Attiwill nd Adms 1993b; Rnger nd Turpult 1999; du Toit nd Scholes 2002; Blnco et l. 2005; Lclu et l. 2005b; Smethurst 2010). Here, the blnce of dditions, losses nd fluxes within the ecosystems hs been used to develop nutrient budgets tht scrutinise present nd future sustinble nutrient supply (Lclu et l. 2003b). Nturl nutrient ddition my become more importnt in offsetting nutrient losses where high costs nd environmentl concerns hve reduced the use of fertiliser. Tht is, provided tht mngement prctices do not promote or ccelerte other loss mechnisms, such s soil erosion nd nutrient leching. Frequent removl of lrge quntities of timber on low fertility sites my led to nutrient depletion under conditions where nutrients re not replced by nturl processes or fertilistion. In ddition, further biomss removl hs the potentil to excerbte nutrient loss. Little work hs been done in South Afric to directly quntify nutrient cycling processes for commercil plnttion forestry. This is lrming since mny highly productive sites re locted on soils tht re both highly wethered nd limited in nutrient storge cpcity (du Toit nd Scholes 2002). 64

95 Although the contributions mde by most nturl processes re often too smll to mke ny significnt contribution (Jorgenson et l. 1975; Binkley 1986; du Toit nd Scholes 2002) some reserch dt indictes the potentil of tmospheric deposition to significntly contribute certin elements in South Afric (vn Wyk 1990; Olbrich 1993; Zunckel et l. 2000; Lowmn 2004). Atmospheric deposition is the ddition of nutrients s wet deposition (precipittion, including mist interception), dry deposition (dry prticulte fllout) nd s folir gseous uptke (EPA 2001). Compounds found in tmospheric deposition re derived primrily from industril pollution (minly fossil fuel burning), biomss burning, lightning nd costl windblown sespry or mist (Hill et l. 2005). Wet nd dry tmospheric deposition cn dd N s orgnic-n compounds nd minerl forms (mmonium (NH + 4 ), nitrte (NO - 3 ) nd nitrite (NO - 2 ) to the ecosystem (Zimmermnn et l. 2003; Hill et l. 2005). In ddition, clcium (C 2+ ), mgnesium (Mg 2+ ) nd potssium (K + ) re typiclly dded in lrge mounts, while only smll mounts of phosphorus (P) my be dded through tmospheric deposition (Binkley 1986; Rnger nd Turpult 1999; de Vries et l. 2003). As rinfll penetrtes the tree cnopy, ions re exchnged between the cnopy nd the rinfll, resulting in either uptke of ions or leching of ions from the tree cnopy (Drijers nd Erismn 1995). Interception nd interction of rinfll with tree cnopies, which South Africn reserch hs neglected in the pst, my produce significntly different dry deposition fluxes (Lindberg et l. 1986) to those derived though pst methods. In ddition, studies crried out in South Afric re generlly not crried out in commercil plnttions nd seldom include cnopy exchnge processes. This pper describes nutrient dditions with rinfll, throughfll nd stemflow of two clonl Euclyptus stnds on the est cost of South Afric over three successive growing sesons (2007 to 2011). The objective of this study ws to determine the order of mgnitude of nutrient ddition through tmospheric deposition to the northern nd southern commercil forestry regions of the Zululnd Costl plins of South Afric. These costl plins were selected due to the inherently low fertility of their Aeolin soils (Fey 2010) nd the high productivity potentil nd hence lrge hrvesting nutrient demnd nd loss potentil of the sites. These fctors increse the risk of rpid site fertility decline nd consequentil drop in future productivity. The continued growth of plnttions in these res with respect to nutrient sustinbility hs been mjor concern in recent yers s there hs been no conclusive evidence of nutrient relted growth decline. This is despite these plnttions undergoing mny rottions of intensive nutrient removl mngement prctices, such s burning nd dditionl biomss removl t hrvesting. A study ws initited in erly 2007 t two sites in this region to guge the mgnitude of 65

96 tmospheric deposition s nutrient source nd the degree to which this flux compenstes for nutrient losses incurred through hrvesting MATERIALS AND METHODS Second study sites The Zululnd Costl plins nd Dukuduku study site re described in CHAPTER 3. A second study site ws chosen in the southern forestry region of Zululnd to generte n erlier comprtive dt set nd to enble n understnding of smple hnding nd lbortory processes. Wter collection begn in April 2007 t the southern site. Both smpling sites were selected centrlly nd representtively of the commercil forestry res in the northern nd southern regions of the Zululnd costl plins (Figure 3.1). For the two sites, growth nd site informtion dt existed nd both crried mture (post cnopy closure) clonl Euclyptus spp. (Tble 5-1). Tble 5-1: Generl chrcteristics of the smpling sites selected for tmospheric deposition monitoring Northern site Southern site Nme Dukuduku KwMbonmbi Stnd ge t strt 6 yers 5 yers Clonl hybrid E. grndis x E. cmldulensis E. grndis x E. urophyll Spcing 3 m x 3 m 2.6 m x 2.6 m Altitude 62 m 60 m Distnce from Ocen 15 km 15 km Ltitude 28 17' 51" S 28 37' 54" S Longitude 32 18' 55" E 32 07' 43" E MAP 920 mm 1196 mm MAT 22 C 22 C Site Index (5 yers) 19.9 m 27.1 m Clcultion of deposition components Dry deposition nd cnopy exchnge nutrient fluxes were derived lgebriclly through cnopy ion exchnge models described in Drijers nd Erismn, (1995) nd Erismn et l. (2002) first reported in (Bredemeier 1988). Rinfll nutrient content ws ssumed to closely pproximte wet deposition while the sum of throughfll nd stemflow (cnopy dringe) contents ws 66

97 ssumed to pproximte totl deposition plus cnopy exchnge. The exchnge of N + ions through the tree cnopy ws ssumed to be negligible nd so N + in cnopy dringe ws tken s the sum of rinfll (wet deposition) nd dry deposition. This ssumption ws confirmed through evidence of N substitution for K not being found in the folige. The rtio between N + dry deposition nd N + wet deposition clculted s (throughfll N rinfll N )/rinfll N ws ssumed to be the sme for C 2+, Mg 2+ nd K +. Multiplying this rtio by the wet deposition of ech of these elements yielded derived vlue of dry deposition for ech element fter Drijers nd Erismn, (1995) nd Erismn et l. (2002). The difference between cnopy dringe nd derived dry deposition ws ssumed to be cnopy exchnge. Cnopy exchnge of NH + 4 ws clculted ssuming the sum of C 2+, Mg 2+ nd K + exchnge to be equl to the sum of NH + 4 nd six times hydrogen ions (H + ) exchnge minus the cnopy leching of C 2+, Mg 2+ nd K + ssocited with the excretion of wek cids from the tree cnopy (Drijers nd Erismn 1995). Cnopy leching of NO - 3 ws ssumed to be negligible. Results were summed over six month periods to reflect the wet summer period October to Mrch (S) nd the dry winter period April to September (W). This ws done to represent sesonlity s experienced in the re with the wetter summer period hving higher rinfll, lower vpour pressure deficit nd modertely higher tempertures thn the drier winter period. Cnopy exchnge of O-N ws ssumed to be negligible nd clculted s the difference between rinfll O-N nd cnopy dringe O-N. Reserch describing the occurrence of uptke or exchnge of O-N by tree cnopies is scrce (Hill et l. 2005) RESULTS Rinfll nd cnopy dringe (stemflow nd throughfll) re shown in Figure 5.1 nd b for ech sesonl period. Rinfll t Dukuduku (Figure 5.1) does not include the first two months owing to the mesurements hving commenced in December Although rinfll t both sites ws below the long term men for the mjority of the study it ws significntly higher t the southern KwMbonmbi site (Figure 5.1b, 1072 mm) thn t the northern Dukuduku site (Figure 5.1, 809 mm). Around 85% of rinfll penetrted the cnopy s throughfll t ech site, while further 5% of rinfll penetrted s stemflow t the Dukuduku site nd 2% t the KwMbonmbi site. Interception s percentge of rinfll ws dependnt on rinfll intensity, with lrge rinfll events resulting in low interception percentge (0.25% with 218 mm) nd smll rinfll events resulting in high interception percentge (100% with 1 mm). Coefficient 67

98 of vrition (CV) between throughfll collectors verged t 6.2% nd remined below 15% for the durtion of the study. Smller rinfll events (below 3.5 mm) produced the lrgest vribility between collectors. Stemflow volume incresed with tree size (dbh) nd ws correlted with rinfll ( sttisticl correltion (R) of 0.93 to 0.95 between collectors). On few occsions smll mount of precipittion ws collected under the tree cnopy without rinfll (possibly from dew or mist interception). A regression reltionship between rinfll nd cnopy dringe t ech site gve slightly less cnopy dringe reltive to rinfll t the KwMbonmbi site (cnopy dringe = x rinfll ; R 2 = 0.991) thn t the Dukuduku site (cnopy dringe = x rinfll ; R 2 = 0.992). Regression reltionships between cnopy dringe nd rinfll were significnt t both sites, hving F nd t probbilities <0.001 for ech prmeter estimte. Overll interception ws round 12% of rinfll t KwMbonmbi nd 10% of rinfll t Dukuduku. The differences in rinfll between summer nd winter period divisions re clerer t the Dukuduku site, with single high rinfll event during winter 2009 t KwMbonmbi (Figure 5.1, b). Very high rinfll occurred during the summer 2010 period t both sites. 68

99 RAINFALL (mm) W-07 S-07 W-08 S-08 W-09 S-09 W-10 S-10 Dukuduku Rinfll Cnopy dringe 50 () 0 MONTH RAINFALL (mm) W-07 S-07 W-08 S-08 W-09 S-09 W-10 S-10 Kwmbonmbi Rinfll Cnopy dringe 50 (b) 0 MONTH Figure 5.1: Monthly rinfll nd cnopy for ech month t the northern Dukuduku site nd southern KwMbonmbi site. Verticl dshed lines represent the strt nd end of ech six month wetter summer (S) nd dryer winter (W) periods for ech yer ph Volume weighted ph (weighted s H + concentrtion) is shown in Figure 5.2 for rinfll nd throughfll t ech site using the sme six month periods s Figure 5.1. Rinfll nd throughfll were slightly cidic, with ph tending to remin below 6 for most mesurements. The rinfll ph (CV 10%) ws more vrible thn the throughfll ph (CV 7%) t ech site. Rinfll ws buffered upon pssing through the tree cnopy resulting in the nrrower throughfll ph rnge. Dryer winter period ph tended to be higher thn the wetter summer period ph vlues for ech source. Rinfll collected t KwMbonmbi hd lower ph thn tht collected t Dukuduku (p <0.001 using the unpired Student s T-test on individul event dt), while throughfll ph did not followed ny site trends. 69

100 7 W-07 S-07 W-08 S-08 W-09 S-09 W-10 S ph KwMbo Rinfll Duku Rinfll Throughfll Throughfll MONTH Figure 5.2: ph of rinfll nd throughfll solutions volume weighted for ech month t the northern Dukuduku (Duku) site nd southern KwMbonmbi (KwMbo) site. Verticl dshed lines represent the strt nd end of ech six month wetter summer (S) nd dryer winter (W) periods for ech yer. (Volume weighting ws clculted using ph derived H + concentrtion) Ction concentrtions in throughfll nd rinfll Volume weighted concentrtions re shown in Figure 5.3 to d for the elements ssessed in rinfll nd throughfll t ech site for ech six month period. Volume weighted men concentrtions of K +, C 2+ nd Mg 2+ (Figure 5.3, b, c) were often doubled s rinfll pssed through the tree cnopy fter undergoing cnopy exchnge processes nd collecting dry deposition from the tree cnopies. K + ws on verge more concentrted thn C 2+ nd C 2+ more thn Mg 2+ in the rinfll nd throughfll t both sites. Although N + (Figure 5.3d) ws the most concentrted of ll the elements in the rinfll, its concentrtion did not increse by the sme extent in pssing through the cnopy. Concentrtions of the ctions tended to be higher in smller rinfll events nd incresed s the length of time between rinfll events incresed. Concentrtions of ctions were lso higher in ll sources during the six-month winter periods t both sites. The lrgest differences in ction concentrtions between rinfll nd throughfll occurred during the winter periods s spikes in throughfll ction concentrtions. Ction concentrtions tended to be lower t KwMbonmbi thn t Dukuduku in both the rinfll nd throughfll, except for Mg 2+ in the rinfll nd K + in the throughfll (students T-test p <0.001). Although ctions were highly correlted with EC nd ech other, no significnt reltionships could be found between the ion concentrtions in the rinfll nd ny other site fctor. 70

101 W-07 S-07 W-08 S-08 W-09 S-09 W-10 S-10 KwMbo Rinfll Throughfll Duku Rinfll Throughfll K+ (mmolc l -1 ) () 0.0 MONTH 0.6 W-07 S-07 W-08 S-08 W-09 S-09 W-10 S-10 C2+ (mmolc l -1 ) KwMbo Rinfll Duku Rinfll Throughfll Throughfll 0.1 (b) 0 MONTH Mg2+ (mmolc l -1 ) W-07 S-07 W-08 S-08 W-09 S-09 W-10 S-10 KwMbo Rinfll Throughfll Duku Rinfll Throughfll (c) 0 MONTH 71

102 N+ (mmolc l -1 ) W-07 S-07 W-08 S-08 W-09 S-09 W-10 S-10 KwMbo Rinfll Throughfll Duku Rinfll Throughfll (d) 0 Figure 5.3: Concentrtions of () K, (b) C, (c) Mg nd (d) N (mmol c l -1 ) in rinfll nd throughfll solutions volume weighted for ech month t the northern Dukuduku (Duku) site nd southern KwMbonmbi (KwMbo) site. Verticl dshed lines represent the strt nd end of ech six month wetter summer (S) nd dryer winter (W) periods for ech yer Nitrogen concentrtions in throughfll nd rinfll The three forms of N ssessed in the rinfll nd throughfll re shown s weighted men concentrtions for ech monthly period (Figure 5.4 to c). The units mg l -1 re used here rther thn ionic concentrtions to compre the three forms. Men concentrtions of O-N, nd NO 3 - -N were slightly higher in throughfll thn in the rinfll t both sites, while NH 4 + N concentrtions were often lower in the throughfll thn the rinfll, being reduced through cnopy exchnge processes. O-N concentrtions in the throughfll were on verge the highest followed by NH 4 + N nd NO 3 - -N t both sites. Rinfll NO 3 - -N concentrtions were lowest t both sites with rinfll NH 4 + N nd O-N concentrtions being reltively similr. Concentrtions of the three N forms (Figure 5.4-c) were generlly higher during the winter periods thn during the summer periods t both sites, with throughfll N generlly higher thn rinfll throughout ll periods. O- N, NH 4 + N nd NO 3 - -N were less concentrted t KwMbonmbi (students T-test p <0.001), with NO 3 - -N often nd NH 4 + N occsionlly below nlyticl detection limits. Although stemflow volumes were comprtively very low, concentrtions of ll elements were n order of mgnitude higher in the stemflow solutions thn in the rinfll nd throughfll (Appendix 5-1: nd Appendix 5-2: ). MONTH 72

103 5 W-07 S-07 W-08 S-08 W-09 S-09 W-10 S-10 Orgnic-N (mg l -1 ) KwMbo Rinfll Duku Rinfll Throughfll Throughfll 0 () MONTH NH4-N (mg l -1 ) (b) W-07 S-07 W-08 S-08 W-09 S-09 W-10 S-10 KwMbo Rinfll Throughfll Duku Rinfll Throughfll MONTH NO3-N (mg l -1 ) (c) W-07 S-07 W-08 S-08 W-09 S-09 W-10 S-10 KwMbo Rinfll Duku Rinfll MONTH) Throughfll Throughfll Figure 5.4: Concentrtions of () Orgnic-N, (b) NH4 N nd (c) NO3-N (mg l -1 ) in rinfll nd throughfll solutions volume weighted for ech month t the northern Dukuduku (Duku) site nd southern KwMbonmbi (KwMbo) site. Verticl dshed lines represent the strt nd end of ech six month wetter summer (S) nd dryer winter (W) periods for ech yer. Note the different scle for the Y xes. 73

104 Ction mss fluxes The six monthly mss dditions of ctions with rinfll (bulk wet deposition) nd cnopy dringe re shown for ech site in Tble 5-2 with clculted cnopy exchnge, dry deposition nd nnulised totls. Totl deposition s well s the sum of wet nd dry deposition, is shown s nnulised vlues only. Stemflow (Appendix 5-1 nd Appendix 5-2) contributed smll frction to the totl nutrients dded with cnopy dringe, round 10% of the ctions nd 5% of N t ech site. The cnopy exchnge models yielded results suggesting substntil exchnge between nutrient deposition nd cnopies which differed for N species nd the bse ctions (Tble 5-2). Lrge quntities of N + dominted ction fluxes t both sites (Appendix 5-1 nd Appendix 5-2) followed in decresing order by K +, C 2+ nd Mg 2+ t both sites. Ctions ddition with cnopy dringe (Tble 5-2) ws lrger thn ddition through rinfll, nd rinfll lrger thn derived dry deposition t both sites. Rinfll, cnopy dringe nd dry ction ddition ws higher t the KwMbonmbi site thn t the Dukuduku site throughout the study period. Cnopy exchnge processes nd collection of dry deposition on the tree cnopy gretly incresed the quntities of ctions in the rinfll fter cnopy dringe t both sites. This increse ws lrgest for Mg 2+ nd lest for K + t both sites. Lrge quntities of bse ctions were lost from the tree cnopies through exchnge processes (positive numbers for cnopy exchnge in Tble 5-2). Although the trends were not consistent, lrger quntities of C 2+ nd K + nd lower quntities of Mg 2+ were exchnged t KwMbonmbi thn t Dukuduku. These vlues, in most cses, tending to be higher during the six month summer periods thn the winter periods t both sites. Summer Mg 2+ exchnge ws on verge higher t Dukuduku thn t KwMbonmbi, while C 2+ nd K + were on verge higher t KwMbonmbi thn t Dukuduku during both sesons. The cnopy exchnge models on verge clculted lrger ction dry deposition t KwMbonmbi thn t Dukuduku Nitrogen mss fluxes Orgnic-N ws mjor contributor to N fluxes nd more N ws dded through wet deposition in ech form thn through dry deposition t both sites. Rinfll nd cnopy dringe ddition of 74

105 NO - 3 -N ws lower thn O-N nd NH + 4 -N rinfll nd cnopy dringe ddition t both sites. Although the rinfll t KwMbonmbi ws higher thn Dukuduku (Figure 5.1), less N ws dded through wet deposition nd cnopy dringe t KwMbonmbi thn Dukuduku (Tble 5-2); owing to differences in concentrtions between the two sites (Figure 5.4). The predominnt cnopy exchnge process with respect to N ws tht of uptke (negtive numbers for cnopy exchnge in Tble 5-2). Uptke of NO - 3 -N within the tree cnopies ws most pprent t Dukuduku, with miniml exchnge t KwMbonmbi due to undetectble concentrtions t tht site for most of the study. The verge uptke of NH + 4 -N ws slightly higher t KwMbonmbi nd tended to be higher during the summer periods sesonl periods (most notble t KwMbonmbi). Derived dry deposition of NH + 4 -N ws however higher t KwMbonmbi thn t Dukuduku. Similr NO - 3 -N dry deposition ws derived for both sites. The ddition to rinfll O-N fter cnopy dringe ws lower during the six-month winter periods t Dukuduku (Tble 5-2) thn during the summer periods. Although the six-month verges show KwMbonmbi O-N to increse in the rinfll fter pssing through the tree cnopy, sesonl ptterns were wek, with slight increses during the summer periods. Higher quntities of O-N were observed more frequently in cnopy dringe t Dukuduku thn t KwMbonmbi. 75

106 Wet deposition Tble 5-2: Addition of N species, C 2+, Mg 2+ nd K + by wet deposition, cnopy dringe, nd dry deposition inferred from cnopy exchnge clcultions for six month wetter summer (S) nd dryer winter (W) periods for ech yer t the northern Dukuduku site nd southern KwMbonmbi sites. DukuDuku Kwmbonmbi Seson- NH + 4 -N NO - 3 -N O-N C 2+ Mg 2+ K + NH + 4 -N NO - 3 -N O-N C 2+ Mg 2+ K + yer kg h -1 W-07 n/ n/ n/ n/ n/ n/ S W S W S W S men nnul Cnopy exchnge Cnopy dringe Dry deposition W-07 n/ n/ n/ n/ n/ n/ S W S W S W S W-07 n/ n/ n/ n/ n/ n/ S W S W S W S W-07 n/ n/ n/ n/ n/ n/ S W S W S W S wet dry cnopy exchnge Totl Negtive vlues imply loss from rinfll nd cnopy uptke, positive implies gin to rinfll nd loss from the cnopy 76

107 5.5. DISCUSSION Mny interntionl nd locl studies nd monitoring progrmmes hve recorded tmospheric deposition for vrious purposes nd hve yielded brod rnges in nutrient quntities dded. Rnges in totl dditions cross Europe, North nd South Americ, Asi, Oceni, Afric nd South Afric re presented in Tble 5-3. These studies cover n rry of wet, dry nd totl deposition cross numerous regions, include forestry of multiple species (both nturl nd plnted), nd represented mny replictes of smples collected for periods of between five nd 20 yers. Dt from South Afric only includes wet nd dry deposition outside of forestry cnopies. The rnges in nutrient ddition through tmospheric deposition were strongly influenced by tmospheric pollution levels, distnce from industril res, wind direction, distnce from the ocen, wind fetch nd rinfll ptterns. The ddition of mcro-nutrients by tmospheric deposition in our study flls within the rnges described in Tble 5-3 for C 2+, Mg 2+, NO - 3 N, NH + 4 -N t both sites. Addition of K + t both sites ws t the higher end of the rnges presented in Tble 5-3. Orgnic-N ws lso bove these rnges, lthough very few sites ssessed O-N. Sites upon which O-N ws ssessed in Tble 5-3 only recorded dissolved O-N nd not prticulte O-N. 77

108 Tble 5-3: Mens nd rnges in nutrient dditions with bulk rinfll derived from numerous globl studies (kg h -1 yr -1 ). Rnges re shown in round brckets nd numbers of studies re shown in squre brckets Region NO 3 - -N NH 4 + -N DON TN C 2+ Mg 2+ K + N + P References Afric Europe North Americ South Americ Oceni Asi South Afric nd (Adeniyi 2006; Gly-Lcux et l. 2009; Lclu ( ) ( ) (nd) ( ) ( ) ( ) ( ) ( ) ( ) et l. 2010b; Josipovic et l. 2011) [17] [17] [0] [18] [7] [7] [10] [7] [2] (Jordn et l. 1980; Gundersen et l. 1998; ( ) ( ) ( ) (3.4-33) ( ) ( ) (2-9) ( ) ( ) Andersen nd Hovmnd 1999; Jenssen et l. 2002; Zimmermnn et l. 2002; de Vries et l. [11] [16] [1] [20] [17] [12] [13] [8] [2] 2003; Tlkner et l. 2010) ( ) (1-3.7) (1-2.7) ( ) ( ) ( ) ( ) (1-10.5) ( ) [5] [4] [2] [10] [21] [11] [11] [9] [10] ( ) ( ) ( ) ( ) ( ) ( ) ( ) (2-16.6) ( ) [5] [8] [1] [9] [13] [9] [8] [5] [5] (Scheider et l. 1979; Jordn et l. 1980; Johnson et l. 1995; Willims nd Melck 1997) (Jordn et l. 1980; Myer et l. 2000) ( ) (0.5-1) ( ) ( ) (0-12.5) ( ) (2-5.1) ( ) ( ) (Jordn et l. 1980; Adms nd Attiwill 1991) [3] [3] [3] [3] [5] [4] [4] [4] [3] nd ( ) ( ) (nd) (8-21.6) ( ) ( ) ( ) ( ) ( ) (Zhng et l. 2006) [1] [1] [0] [2] [3] [1] [2] [2] [1] nd nd (vn Wyk 1990; Olbrich 1995; Zunckel et l. 2000; Lowmn 2004; Gly-Lcux et l. 2009; Josipovic et l. 2011) (0-24.9) (1-7.2) (nd) ( ) ( ) ( ) ( ) (nd) (0.5-1) [14] [12] [0] [12] [18] [18] [18] [6] [3] nd = no dt 78

109 The Zululnd plnttions re likely to receive deposition from number of sources including the ocen, industril pollution, biomss burning, vehicle emissions, rurl fires nd possibly inlnd col fired power sttions (Annegrn 2005). The southern plnttion re is in close proximity to the Richrds By industril re while the northern re is surrounded by plnttion forests nd sugrcne plnttions tht re burned on regulr bsis. Isotopic nlyses of ir borne dust for n inlnd site showed tht ctions were derived primrily from biomss burning (46%) nd col burning (47%) (Piketh nd Annegrn 1994), while the contribution of ocen sources ws smll, being distnt from the ocen. Annegrn (2005) presented some evidence tht pollution is crried from s fr s centrl southern Afric, pssing over the northern Zululnd region, recircultion from the ocen bck onto the lnd. This is substntil pollution source from inlnd biomss burning tht hs not been ccounted for in the pst. Lr et l. (2005) shows sugrcne burning to contribute mjor proportion of fine prticulte mteril in Southestern Brzil, followed by dust. Soil prticles, oil combustion nd sugrcne burning mde up the mjor proportions of corse prticulte mteril in tht study. The vlues for below cnopy ction deposition were 2 to 4 times tht dded by rinfll. Potssium hd the lrgest mss ddition of the ctions in our study. Myer et l. (2000) lso presented dt with lrge increses in ctions under the tree cnopy (2 to 13 times) in tropicl rin forest, with K + enrichment being the lrgest. Cnopy exchnge in Tlkner et l. (2010) under Germny beech stnd ws lso high nd the lrgest for K +. Cnopy exchnge ws however fr lower in two clonl Euclyptus studies (Congo nd Brzil) with zero K + exchnge t the Congo site (Lclu et l. 2010b). The infesttion by the sp-sucking Thumstocoris insects in our study sites my hve induced lrger levels of K + leching from the tree cnopy through the epiderml dmge to the leves, possibly explining the high ction loss from the tree cnopies. Quntities of O-N mesured in this study re substntil nd provided it is in minerlisble form, my provide significnt source of N to the trees. The contribution of O-N to deposition hs been questioned in recent yers by number of uthors who hve expressed its neglect nd importnce (Cornell 2011). These uthors cite O-N to mke up round 30% of totl N deposition (Fless nd Hunt 2001; Neff et l. 2002). Long rnge dust nd smoke trnsport nd mrine (se spry) hve been noted s possible sources of tmospheric O-N. Studies hve shown se spry to be highly enriched with orgnic mtter, prticulrly in res of high mrine 79

110 biologicl ctivity (Rinldi et l. 2010). This work is firly recent nd numerous uthors stte the problem of lck of dt nd highly vrible results (Cornell 2011). Growth on infertile sites chrcterised by reltively low soil orgnic crbon nd totl N levels (in Brzil, Congo, Chin, Austrli, similr to our sites), ws reduced where lrge quntities of residues were removed Nmbir et l., (2004) summrised in Tirks nd Rnger, (2008). Residue removl did not result in tree growth responses in the newly plnted crop up to two yers of ge (Chpter 6). The high O-N concentrtions found in our study nd the high concentrtions tht were excluded lso seemed to coincide with surfce soil N minerlistion spikes recorded t the Dukuduku site (unpublished dt). This my give further evidence supporting the high levels of N recorded t our study sites, which will be discussed in future publictions. Results presented in our study my however be n rtefct of the cnopy exchnge model used, model produced for ion exchnge processes. The lesser increse in N + concentrtions in pssing through the cnopy gives some greement towrd the ssumption tht N + present in the rinfll is not tken up by the tree cnopy to ny significnt degree. In development of this method, there my hve been n exchnge of N + in the tree cnopy tht ws kept in blnce through N + uptke by the root system resulting in the throughfll N + levels seeming to remin unffected. Cnopy uptke of N + cn occur during lef development s reported for deciduous trees in Stelens et l. (2007) which is prticulrly problem in high N + mrine environments (Stelens et l. 2008). An uptke of N + ions by the tree cnopy will therefore result in this model underestimting dry deposition nd overestimting cnopy exchnge of ctions Significnce of the dt A scenrio of nutrient loss with hrvesting is presented in Tble 5-4 considering n verge oven dry stem-wood hrvest of 150 Mg h -1 over seven yer rottion nd using simplistic ssessment of nutrient loss for plnttion grown Euclyptus (Dovey 2009). Assuming similr totl deposition vlues to those in Tble 5-2 over the seven yer rottion (excluding the temporry unplnted period between felling nd plnting) dditions with tmospheric deposition hve been clculted in Tble 5-4. The scenrio given in Tble 5-4 depicts n verge to high 80

111 site productivity rnge for the Zululnd region with mny sites hving productivities flling bove or below this vlue within the Zululnd region. Productivity rnges reported by Smith et l. (2006) showed men nnul increments to rnge between 18 nd 58 m 3 h -1 yr -1, with site index t five yers rnging between 18.0 to 27.3 m with rottion lengths between five nd eight yers. Tble 5-4: A hypotheticl clcultion of nutrient loss by removl of vrious tree components ssuming 150 Mg h -1 of stemwood nd potentil ddition of nutrients over seven yer rottion (Minerl nd orgnic N shown seprtely) Component biomss Totl N C Mg K Mg h -1 kg h -1 stem-wood Brk Brnches Totl Site Totl Deposition Minerl-N Orgnic-N C Mg K Dukuduku KwMbonmbi These dt suggest tmospheric deposition to ply mjor role in the ddition of mcronutrients to commercil forest plnttions in this region. Of the ctions K + nd Mg 2+ my be replced in quntities exceeding removls with stem-wood hrvesting t the KwMbonmbi site, while only smll frction of C 2+ is replced from tmospheric sources. The KwMbonmbi site, with n pprently lrger ction flux, my therefore regin ction losses more rpidly thn Dukuduku, prticulrly for Mg 2+ nd K +. The ddition of N s NO - 3 -N nd NH + 4 -N lone hs potentil to replce significnt quntities of N tht my be lost with hrvesting. Although the fte of O-N is unknown within the forest nutrient cycle, it is deposited in quntities sufficient to outweigh N losses ssocited with stem-wood hrvesting when dded to inorgnic N. Higher productivity sites will incur greter nutrient loss with hrvesting, reltive to the gins for given rte of tmospheric deposition. Mny other mngement scenrios tht increse nutrient losses must lso be included in the ssessment, such s removl of tree folige, burning of residues nd the burning nd removl of the forest litter lyer. Significnt improvements in stnd growth hve been observed fter fertiliztion with N fter re-estblishment on the Zululnd costl plin (du Toit nd Oscroft 2003). On degrded sites, even lrger responses hve been recorded to fertiliztion contining minly N (du Toit et l. 2001). This supports the findings of 81

112 the current study, nmely tht the system remins relint on inputs of N (nd possibly C) s supplement to tmospheric deposition inputs, if high levels of productivity is to be mintined. Other nturl loss mechnisms (such s leching), not included here, should lso be dded to the nutrient loss clcultion nd should be included in ny nutrient budget ssessment. It is lso cler from Tble 5-4 tht the removl of brk nd/or brnches for off-site debrking, or s n energy fuel source hs potentil to significntly increse nutrient losses, especilly C 2+. Under this scenrio it is unlikely tht C 2+ will be replced over single rottion under the levels of tmospheric deposition recorded in this study Further comments This study needs to be continued nd replicted cross wider rnge of sites nd include vrious tree species nd plnting densities. Crbon, sulphur nd micronutrient deposition my lso need to be included in future studies in order to drw more complete cnopy exchnge models, nutrient blnces nd ssess deposition rtes of other nutrient elements. Such work lso needs to continue into the long term s the results presented here represent the period of collection nd my not represent future deposition ptterns. Furthering this work will crete n extremely vluble dtset of tmospheric nutrient flux ptterns, contributing significntly to nutrient budget clcultions used in ssessing sustinble nutrient mngement prctices, while potentilly explining or predicting responses to fertiliser ddition. Atmospheric deposition should first be ssessed on highly productive, short-rottion, nutrient-poor forestry sites given the potentil for these sites to export lrge quntities of nutrients with hrvesting. This is importnt when ttempting to understnd the blnce between nutrient loss nd gin under vrious mngement regimes. Such informtion, in conjunction with soil nd biomss nutrient pool dt, my be used to indicte the key processes ffecting nutritionl sustinbility nd consequently the site specific long-term productivity of forestry plnttions in South Afric CONCLUSION Although this study considers just three yers of dt t two sites, it gives evidence towrds tmospheric deposition being n importnt nutrient source within the commercil forestry regions of the Zululnd costl plins of South Afric. Given the rtes recorded here tmospheric deposition is likely to ply mjor role in nutrient supply to the plnttions within this re, provided tht mngement prctices do not ccelerte nutrient losses through incresed 82

113 biomss removl, shorter rottions or more nutrient demnding mngement prctices. Under the conditions of these fst-growing plnttion forests it my however become necessry to pply nutrients through fertilistion (Rnger et l. 2002) (orgnic, minerl, dded N 2 fixing species) to compenste for the nutrients lost by stemwood nd dditionl biomss removl fter stnd hrvesting. 83

114 5.7. APPENDICES Appendix 5-1: Dukuduku rinfll, throughfll nd stemflow solution concentrtion mens (Men), stndrd devition (SD) nd 5% confidence intervls (CI). Totl N is the sum of mmonium, nitrte nd orgnic N. Source medin ph Ec N C Mg K Totl N NH 4 -N NO 3 -N Stts μs cm -1 mg L -1 ug L -1 Rinfll Throughfll Stemflow SD Men CI SD Men CI SD Men CI Appendix 5-2: Kwmbonmbi rinfll, throughfll nd stemflow solution concentrtion mens (Men), stndrd devition (SD) nd 5% confidence intervls (CI). Totl N is the sum of mmonium, nitrte nd orgnic N. ph Ec N C Mg K Totl N NH 4 -N NO 3 -N Source Stts μs cm -1 mg L -1 ug L -1 Rinfll Throughfll Stemflow SD Men CI SD Men CI SD Men CI

115 Appendix 5-3: Rinfll, ph nd nutrient ddition with bulk rinfll t Dukuduku Dte Volume ph N K C Mg O-N mm kg h -1 NH 4 -N NO 3 -N Dec Feb Mr Apr My Jun Jul Aug Sep Oct Nov Dec Jn Feb Mr Apr My Jun Jul Aug Sep Oct Nov Dec Jn Feb Mr Apr My Jun Jul Aug Sep Oct Nov Dec Jn Feb

116 Appendix 5-4: Throughfll, ph nd nutrient ddition with throughfll t Dukuduku Dte Volume ph N K C Mg O-N mm kg h -1 NH 4 -N NO 3 -N Dec Feb Mr Apr My Jun Jul Aug Sep Oct Nov Dec Jn Feb Mr Apr My Jun Jul Aug Sep Oct Nov Dec Jn Feb Mr Apr My Jun Jul Aug Sep Oct Nov Dec Jn Feb

117 Appendix 5-5: Stem flow, ph nd nutrient ddition with stem flow t Dukuduku Dte Volume ph N K C Mg O-N NH4-N NO3-N mm kg h -1 Dec Feb Mr Apr My Jun Jul Aug Sep Oct Nov Dec Jn Feb Mr Apr My Jun Jul Aug Sep Oct Nov Dec Jn Feb Mr Apr My Jun Jul Aug Sep Oct Nov Dec Jn Feb

118 CHAPTER 6: EFFECTS OF CLEARFELLING AND RESIDUE MANAGEMENT ON NUTRIENT POOLS, EARLY GROWTH AND ABOVE GROUND NUTRIENT ACCUMULATION ABSTRACT Reserch on sndy tropicl soils with low soil nutrient retention nd storge cpcity hs shown growth reductions through nutrient loss fter vrious levels of residue removl. Growth nd bove ground nutrient pools nd fluxes were therefore compred on sndy low nutrient clonl Euclyptus site on the Zululnd Costl Plin in South Afric fter clerfelling to yer fter cnopy closure. Clerfelled res were subjected to residue mngement tretments tht included retention with nd without fertilistion, residue burning, whole tree removl nd residue doubling. An dditionl tretment ws included s point of reference by leving undisturbed replictes of trees on-site. Stem only removl retined lrge quntities of nutrients on-site, while whole tree removl induced the gretest nutrient loss, exceeded only by N lost during residue burning. Growth nd nutrient ccumultion ws initilly most rpid in the burned nd fertilised tretments. However, growth slowed in the burned tretment resulting in no significnt difference from the remining tretments from yer fter plnting onwrds. Fert. tretment growth remined high while growth in ll other tretments remined sttisticlly similr. Nutrient supply in the Burn tretment remined dequte. Folir vector nlysis suggested fertilistion to hve overcome N nd P t six months nd Mg nd C deficiencies t one yer fter plnting compred to the No-Burn tretment. Results in our study conflicted with reserch on similr sndy tropicl sites tht experienced growth reduction fter residue removl nd growth increse fter residue doubling. Nutrient pools in the residues, litter lyer nd soil t our site supplied nutrients in excess of erly growth demnds. Mture crop dt indicted nutrient recycling to limit demnd lter, llowing high tmospheric deposition rtes to prtilly replenish nutrients lost during clerfelling. These fctors my slow the rte of nutrient decline, prticulrly nitrogen under low nutrient removl mngement prctices. The risk of these sites becoming depleted in subsequent rottions is incresed s lrger nutrient removl plces hevier relince on belowground pools. 3 Submitted for publiction to Southern Forests. 88

119 6.2. INTRODUCTION Plnttion forest mngement hs imed t producing sustinble wood supply from intensive mngement through improvements in genetics, silviculture nd hrvesting (Gonclves 2004). Despite productivity gins, sustinbility of yield in the long-term remins mjor concern to mngement nd scientists, nd reserch into principles nd processes driving site productivity is ongoing (Nmbir 1996; du Toit nd Scholes 2002; Lclu et l. 2003; Wtt et l. 2005; du Toit et l. 2010). Poor mngement hs potentil to cuse degrdtion of soil structure, chemistry, crbon (micro-orgnisms included) nd site nutrient supply nd storge (du Toit et l. 2010). The erly stge of short-rottion pulpwood plnttion growth cycle, from felling to post cnopy closure of the subsequent crop, represents the period of gretest opportunity for site mnipultion, s the most silviculturl opertions tke plce during this period (Nmbir nd Kllio 2008; Tirks nd Rnger 2008; du Toit et l. 2010; Smethurst 2010). This period lso presents opportunities to undertke mngement prctices tht promote sustinbility, s the silviculturl opertions undertken here hve potentil to impct on future site resource supply nd productivity (Nmbir nd Kllio 2008; Tirks nd Rnger 2008). A few studies in southern Afric hve demonstrted lrge quntities of nutrients potentilly lost through certin hrvesting nd residue mngement prctices, prticulrly residue burning (Morris 1986; du Toit nd Scholes 2002; Ndel 2005). There re some dt describing the effects of post hrvesting residue mngement on subsequent growth of Euclyptus in South Afric (Ndel 2005; Smith nd du Toit 2005; du Toit et l. 2010; Rietz 2010), These re mong limited number of studies for which nutrient loss dt hs been ssocited with subsequent growth nd uptke into the new crop. In ddition, few of these studies hve been crried out on sites considered sensitive or susceptible to degrdtion. The sndy structureless soils of the Zululnd costl ecosystem (in KwZulu-Ntl) my be susceptible to nutritionl degrdtion nd productivity decline. These soils hve smll nutrient reserves nd buffering cpcity s they re low in cly nd orgnic crbon content nd hve high dringe rtes (Hrtemink nd Hutting 2005; Fey 2010). The sub-tropicl climte nd reltively high rinfll in this region results in res of high productivity potentil (Smith nd du Toit 2005). This implies lrge nutrient removl on soils with low nutrient reserves. A sustined production therefore, my not be possible in this region; prticulrly if nutrient removl over 89

120 ech rottion is incresed. Fertilistion responses on old griculturl (sugrcne) sites tht hve been converted to plnttions my be evidence of nutritionl degrdtion in such cses (du Toit et l. 2001; du Toit nd Oscroft 2003). Growth decline observed cross the brod sugrcne growing regions hs been ttributed to loss of soil orgnic mtter nd nutrients, decresed soil ph nd incresed soil bulk density (Qongqo nd vn Antwerpen 2000; Dominy et l. 2001; Dominy et l. 2002). However, there is little conclusive evidence towrds nutrient relted productivity decline on plnttion sites despite numerous rottions of intensive silviculturl mngement prctices nd frequent wildfires. A study ws implemented to ssess the chnges in bove ground biomss growth, nutrient pools nd fluxes in Euclyptus stnd during the period from felling to post cnopy closure, through the imposition of rnge of site mngement prctices. An undisturbed crop is included to compre mture crop nutrient pools nd fluxes with the new crop. The objective ws to determine the risk of nutrient depletion nd productivity decline in subsequent rottions. The hypothesis is tht the low orgnic mtter Aeolin snd derived soils in the subtropicl environment of the Zululnd costl ecosystem re t risk of nutrient depletion nd yield decline of successive rottions MATERIALS AND METHODS Mterils nd methods for this chpter tht re common to other chpters re given in CHAPTER Nutrient Audit A nutrient udit of dt in this chpter ws clculted s prt of the publiction of this chpter prior to vilbility of leching dt presented in CHAPTER 8.. An udit of bove ground nutrients fluxes between felling to cnopy closure ws clculted s nutrients relesed between felling nd plnting. Nutrients ddition to the soil included relese with residue decomposition between plnting nd cnopy closure with sh-bed contents in the Burn tretment nd fertiliser ddition in the Fert. tretment. Fertiliser ws considered to dd nutrients to the soil in ddition to residue decomposition during the plnting to cnopy closure 90

121 phse. Nutrient relese in the Burn tretment ws clculted between felling nd burning, dding the nutrient content of the remining sh to the nutrients relesed through decomposition prior to burning. Stnding crop vlues were clculted to include litterfll ccumulted to the end of ech period, which ws dded to the litter lyer mss t the strt of ech period. Nutrient uptke into the boveground tree biomss between felling nd cnopy closure ws used with relese to clculte the difference between bove ground uptke nd potentil nutrient supply. These clcultions ssume ll nutrients relesed from the residue, dded by fertiliser or remining s sh fter burning to hve entered the soil. This clcultion ws done to indicte the potentil supply of nutrients under ech residue mngement prctice reltive to boveground nutrient demnd. It gives n indiction of the extent to which trees rely on belowground nutrient pools nd tmospheric inputs RESULTS Tempertures nd rinfll were within the norml men long-term rnge of the site. Monthly totl rinfll ws however often below the long-term men for the first two yers of the study, but rose shrply t the end of the study (Appendix 6-1). Rinfll nd temperture divided the yer into two distinct sesons, wet seson nd dry seson. The wet summer seson ws the six month period from October to Mrch nd the dry winter period from April to September Stnding crop biomss nd nutrients Nutrients held in the stnding crop were within rnges given by Dovey (2009) for Euclyptus grown in South Afric, except for the brk C concentrtion, which ws three fold the reported vlues. Brnch C nd stem wood N were lso t the higher end of the reported vlue rnges. Growth rtes in the stnding crop incresing over the study period nd lef re index remined in nrrow rnge. A 3.5 Mg h -1 increse in bove ground biomss occurred between the time of felling nd plnting of the new crop, followed by 4.2 Mg h -1 during the first yer of new crop growth nd 9.6 Mg h -1 in the second yer. Stnding crop litterfll summed over the whole study period (Tble 6-2) ws smller thn decomposition (Tble 6-1) resulting in net loss of forest floor biomss nd nutrients. Folige dominted the litterfll for the durtion of the study, with brnch-fll only greter thn folir litterfll on few occsions (Tble 6-2, Appendix 6-2). Nitrogen nd C, which hd similr content in the litterfll, ws most dominnt, followed by Mg, K nd P which were n order or mgnitude lower thn N nd C. Retrnsloction nd cnopy 91

122 exchnge (during rinfll) ccount for the lrge differences between fresh equivlent nd litterfll folige contents. High C in the litterfll ws through mss loss increses in C concentrtions prior to litterfll, C not being retrnslocted (Fife et l. 2008). Tble 6-1: Biomss nd nutrients contined in the litter lyer nd residues for tretments t times of felling, burning, plnting nd cnopy closure. Residue nd litterfll re clculted from felling to pre-burning, nd from felling to plnting nd cnopy closure, respectively. Mss N P K C Mg Mg h -1 kg h -1 Point in time Tretments nd effects Felling Residue (October 2008) Stnding crop No-Burn, Burn, Fert Whole-Tree Double Burning Pre burn (Mrch 2009) Post burn ring Ash Post burn plte Ash Plnting Stnding crop litterfll (August 2009) Stnding crop No-Burn, Fertilised 28.6 b b 11.3 b b 52.4 b Whole-Tree b 35.8 Double 40.5 c b 14.8 c 31.7 b 978 c 80.5 c Cnopy Stnding crop litterfll closure Stnding crop b (August 2010) No-Burn, Fertilised 21.9 b b b 35.0 Whole-Tree b 7.8 b 9.1 b b 26.9 Double 28.2 c c c 44.1 b 1 Estimte for the three tretments from summing residue nd litter lyer. 2,3 Stnding crop litterfll for the period (1) October 2008 to August 2009 nd (2) August 2009 to August Letters (,b,c ) tht re different indicte significnt tretment differences t ech stge (LSD5%, p <0.001). Tble 6-2: Annulised mss of folir nd cnopy mcro-nutrients deposited s litterfll in the stnding crop during the period between felling nd cnopy closure of the new crop. Fresh folige mss is the litterfll equivlent mss of nutrients using folir concentrtions nd indictes potentil retrnsloction. Mss N P K C Mg Mg h -1 yer -1 kg h -1 yer -1 Leves Brk Brnch Totl Fresh folige

123 Litterfll results (not included in publiction) Although smll brnch-fll mde up the bulk of brnch-fll, less frequent nd more sptilly spred lrge brnch-fll resulted in higher vribility between the litter-trps (Appendix 6.2). Some extreme wether conditions (wind, hil nd hevy rinfll) lso resulted in incresed litterfll vribility. Brk litterfll generlly occurred t the beginning of the wet sesons, nd s it ws limited to the regions round ech tree, ws lest effectively cught in the litter trps. Nutrient concentrtions in the litterfll chnged with rinfll nd the nture of the litterfll (folige, brk or brnch) (Appendix 6.2A-F). Totl litterfll concentrtions of N, P C nd Mg remined in reltively nrrow rnge, though were generlly higher during the wet seson nd lower during the dry (Appendix 6-4 nd Appendix 6-5). Potssium hd the lrgest fluctutions in concentrtion following both sesonl ptterns nd rinfll ptterns. The dily quntities of mcro-nutrients flling with litterfll were influenced by litterfll mss nd nutrient concentrtion, which were dependnt on the rtios of tree components mking up the litterfll Post felling residue nutrient fluxes Hrvesting nd residue retention (stem wood removl) left lrge quntities of mcro-nutrients cross the site; prticulrly C nd N (Tble 6-1). Residue biomss ws lmost double forest floor biomss. Uneven mss trnsfer nd distribution resulted in the Whole-Tree nd Double tretments hving slightly lower residue lods thn expected. Decomposition ws initilly rpid in residue retined tretments; fster during wet thn during dry periods (Figure 6.1). Residue decy hlf life in the No-Burn tretment ws clculted s 16 months with n nnul k fctor of 0.42 (R 2 =0.965). Hlf life ws longer in the Double tretment (12 months) with n nnul k vlue of 0.55 (R 2 =0.948). Decomposition of the old litter lyer in the Whole-Tree tretment ws slow fter felling, hving n 18.5 month decomposition hlf life. The dely between felling nd plnting (39 weeks) resulted in significnt loss of residue biomss (Tble 6-1) nd nutrients. Prior to burning t 19 weeks fter the strt of felling, 28% of the residue biomss hd decomposed (Tble 6-1) nd further losses of N, P, nd Mg hd occurred from the residues. The following 48 weeks between plnting nd cnopy closure resulted in lrger N nd C residue decomposition losses thn prior to plnting. 93

124 BIOMASS (Mg h -1 ) () (b) (c) (d) Stnding crop No-Burn Burn Whole tree Double residue MONTH/YEAR Figure 6.1: Forest floor litter lyer nd residue biomss from prior to felling, through felling nd up to cnopy closure for ech tretment. Error brs represent single stndrd devition between smpling points. Dshed lines represent () felling; (b) residue burning; (c) plnting nd (d) cnopy closure. Burning resulted in lrge loss of nutrients from the residues through oxidistion, in smoke nd wind erosion of fine sh. The smllest loss being 8% of the C nd the highest being 52% of the N compred with pre-burn pools (Tble 6-1). Lrger biomss nd N loss occurred through residue burning thn through whole tree removl. More P, K, C nd Mg were lost through whole tree removl thn through burning. The two sh ssessment methods gve similr biomss losses when clculting LOI djusted ring mss nd undjusted plte msses, but produced different nutrient losses (Tble 6-1). A hotter burn ws visully evident on the pltes s less blck crbon nd more white sh on the pltes thn on bre soil. Ring sh smples were therefore used in ssessing burning losses Residue nd litter results (not included in publiction) Although the reduction in residue biomss of different residue tretments ws less thn 10% in ll tretments during the first four weeks fter felling, concentrtions of nutrients were significntly reduced. Nitrogen, P, K, C, nd Mg concentrtions were reduced to 88%, 46%, 20%, 72% nd 75% nd 55% of their originl vlues, respectively. In the period fter the first four weeks, concentrtions remined reltively rnge bound cross ll tretments, following 94

125 different relese ptterns with decomposition (Appendix 6-3 B-F). N concentrtion, lthough remining stble from felling onwrds, ws highest in the stnding crop fluctuting slightly in ll other tretments over time (Appendix 6-3 A). K nd Mg concentrtions decresed while C concentrtions incresed in ll tretments between felling nd cnopy closure, except the stnding crop tretment in which the concentrtions of these elements remined within nrrow stble rnge. P remined rnge bound in ll tretments with smll fluctutions in concentrtion New crop growth During the first prt of the wet seson (October to Mrch) bsl re growth rtes followed the tretment order Burn > Fert. > No-Burn > Whole-Tree > Double (Figure 6.2), with sttisticl significnces overlpping (p < 0.001). At 1.0 yers of ge the Fert. tretment hd the highest bsl re followed by Burn nd the remining tretments (p < 0.001). Burn nd Fert. tretment differences were not significnt t this point. At 1.5 yers, nd into the next wet seson, trees in the Fert. tretment becme significntly lrger thn the other tretments (p < 0.001). Differences between ll other tretments were not significnt. This pttern of significnt difference continued to 2.5 yers fter plnting (p < 0.001). Burn tretment growth rtes tended to decrese over the 2.5 yer growth period reltive to No-Burn nd ll other tretments (Appendix 6-7). Height growth differences were not significnt prior to two yers fter plnting (Figure 6.3). Height becme significntly lrger in the Fert. tretment t two (p = 0.049) nd two yers six months yers fter plnting (p = 0.030). 95

126 BASAL AREA (m 2 h -1 ) Fertilised Burn No-Burn Whole tree Double residue b bc c c b b b b YEARS AFTER PLANTING Figure 6.2: Bsl re of ech tretment t six monthly intervls fter plnting. 0.5 yer bsl re dt clculted using gld, others clculted using dbh. Different, b, c superscripts denote significnt differences between tretments t ech ge in the tretment order of the legend (LSD 5% ). b b b b b b b b b b b b Fertilised Burn TREE HEIGHT (m) No-Burn Whole tree Double residue b b b b b b b b YEARS AFTER PLANTING Figure 6.3: Men heights of trees in ech tretment t six monthly intervls fter plnting. Different, b superscripts denote significnt differences between tretments t ech ge in the tretment order of the legend (LSD 5% ). 96

127 Above ground biomss nd nutrient ccumultion Woody biomss, LAI nd folir biomss were ll significntly lrger (p < 0.01) in the Fert. nd Burn tretments t six months thn in ll other tretments Figure 6.4. The Fert. nd Burn tretment were not significntly different from ech other. The remining tretments were not significntly different from ech other either. At one yer fter plnting (cnopy closure) woody biomss, folir biomss nd LAI followed the sme order of significnce. These mesures remined lrger in the Fert. nd Burn, nd lrger thn ll other tretments (p < 0.05). Above ground biomss nd LAI in the Fert. tretment ws 44.7 Mg h -1 nd 3.2 m 2 m -2 t two yers fter plnting, significntly more thn ll other tretments, including the Burn tretment (p < 0.001). No significnt differences occurred between ll other tretments in two yer biomss nd LAI between ll other tretments which hd men boveground biomss of 37.3 Mg h -1 nd LAI of 2.7 m 2 m -2. Lef re growth efficiency t six months ws 3.6 Mg h -1 yr -1 LAI -1 in the Burn nd Fert. tretments which were significntly different from n verge 3.5 Mg h -1 yr -1 LAI -1 for ll other tretments (p < 0.01). Although growth efficiency incresed to 9.3, 9.0, 8.6, 8.7 nd 8.1 Mg h -1 yr -1 LAI -1 in the Fert, Double, No-Burn, Whole-Tree nd Burn respectively, differences were not significnt. BIOMASS (Mg h -1 ) Stnding crop b B AB No Burn Burn Fertilised Whole tree Double residue TREATMENT, YEARS AFTER PLANTING Figure 6.4: Biomss ccumulted in the bove ground tree components between the plnting to 0.5 yers, nd plnting to cnopy closure (1.0 yers). Vlues shown on brs with LAI (m² m - ²) on the top. (stnding crop is the sum of growth nd litterfll) Different, b, c superscripts denote A b B LAI Folir Woody 1.5 b B

128 significnt differences between tretments; lowercse for 0.5 yers nd uppercse for 1.0 yers (LSD 5% ). Totl bove ground nutrient ccumultion followed the sme tretment rnkings s differences in biomss (Tble 6-3). However, significnce t six months of ge ws found only for N. The Fert. tretment hd the highest N contents followed by Burn, No-Burn, Double, nd Whole-Tree with significnt differences overlpping (Tble 6-3). At one yer fter plnting, the Fert. tretment hd ccumulted significntly more N thn ll other tretments (p < 0.01). Nitrogen in the woody biomss ws the highest in the Fert. tretment, significnce overlpping cross ll other tretments. There were no further significnt tretment differences in woody biomss contents. The Fert. tretment ccumulted significntly more P (p < 0.01), K (p < 0.01) nd Mg (p < 0.05) into the folige thn ll other tretments (Tble 6-3). Folir ccumultion of C ws highest in the Burn. Folir ccumultion of these elements ws less in the Double nd No-Burn, nd lest in the Whole-Tree tretment with overlpping orders of significnt differences. In ddition to C, the Burn tretment ccumulted significntly more P nd K thn the No-Burn, Whole-Tree nd Double tretments. There were no significnt differences between W, No-Burn nd Double tretments. 98

129 Tble 6-3: Mcro-nutrients ccumulted in the bove ground tree biomss t 0.5 nd 1.0 yers fter plnting (cnopy closure). Superscript nd subscript vlues re folir nd woody component msses respectively. Letters tht re different indicte significnt difference between tretment men bove ground mss t ech ge (LSD 5% ). Tretment 0.5 yers fter plnting 1 yer fter plnting N P K C Mg N P K C Mg Stnding crop * No-Burn 32.7 bc b bc c c b Burn 39.6 b b b b b Fertilised b Whole tree 31.2 c b c c c b Double 31.7 bc b bc bc bc b *stnding crop is the sum of growth nd litterfll occurring during ech period fter plnting the new crop. 99

130 Folir nutrient dignostics No folir nutrient deficiencies were found t ny stge from plnting to cnopy closure using stndrds for euclypts (Bordmn et l. 1997) nd rtio method (Linder 1995). However, vector nlysis showed fertilistion to llevite both N nd P deficiencies t six months (Appendix 6-8). All other mcro-nutrients were diluted in the Fertilised tretment. The burn tretment ws not nutrient limited t six months. Wek depletion responses occurred in the Whole tree tretment for N, P, K, nd C, vector lengths less thn 10 t six months for ll mcronutrients. Clcium nd Mgnesium deficiencies were strongly llevited by fertilistion in the Fert. tretment t one yer fter plnting. All other elements were between sufficiency nd dilution rnges in the remining tretments. Deficiencies of N nd C were llevited in the Burn tretment t one yer fter plnting; ll other mcro-nutrients were diluted. Depletion of P nd K ws lso found through vector nlysis in the Whole tree tretment for the sme period. Twoyer dt showed N deficiency llevition in the fertilised tretment nd no nutrient limittions in ny other tretment Nutrient Audit Lrge quntities of nutrients were potentilly dded to the soil during the 39 week felling to plnting fllow period (Tble 6-4 i). Residue nutrient relese ws lower during the plnting to cnopy closure period therefter (48 weeks) for ll elements nd tretments except N in the No- Burn tretment (Tble 6-4 ii). Fertiliztion resulted in gin of ll mcro-nutrients to the soil. Nutrient supply from decomposition in the Whole-Tree tretment ws less thn bove ground uptke of P, K, nd C throughout the entire felling to cnopy closure period. Negtive C vlues (gin) in the Whole-Tree tretment were through n increse in residue C concentrtions. However, net loss of C occurred between plnting nd cnopy closure. Residue decomposition nutrient relese ws lrger thn bove ground nutrient ccumultion in ll other felled tretments up to yer fter plnting. Atmospheric inputs plyed mjor role in nutrient supply to the study site. Combined throughfll nd stemflow in the stnding crop contributed 16.3 kg h -1 of N (6.8 kg h -1 minerl nd 9.6 kg h -1 orgnic) nd 17.2, 15.1 nd 6.2 kg h -1 of K, C nd Mg respectively between plnting nd one yer fter plnting (CHAPTER 5). Rinfll contributed 10.6 kg h -1 of N (

131 kg h -1 minerl nd 3.8 kg h -1 orgnic) 5.5, 7.7, nd 1.4 kg h -1 of K, C nd Mg. Phosphorus ddition ws not recorded (below detection). Chnges in the supply rtes of redily vilble mcronutrient species from the soil pool s consequence of the different tretments could not be included s these were difficult to mesure. However, sequentil coring estimtes of N minerlistion for the period from felling to one yer fter plnting yielded 46.7 nd 24.5 of N minerlised in the No-burn nd Burn respectively (unpublished dt). Immobiliztion occurred in the stnding crop tretment (-5.7 kg h -1 yr -1 ). Nutrient relese during the plnting to cnopy closure period ws fr less thn boveground nutrient ccumultion for most tretments (Tble 6-4 ii nd iii). This implies tht the trees were relint on nutrients relesed into the soil prior to plnting, minerlised or exchnged from the soil pools, nd dded through tmospheric inputs. Fertilistion in ddition to residue decomposition supplied n excess of nutrients reltive to boveground biomss ccumultion during the yer fter plnting. Decomposition supplied only smll proportion of boveground nutrient requirements in the Whole-Tree tretment (Tble 6-4 iv). More P, C nd Mg were relesed from the residues in the Double tretment thn were ccumulted in boveground growth, while less N nd K ws relesed thn ccumulted in boveground growth. A smll shortfll of P, K nd C ws pprent in the stnding crop (Tble 6-4 iv), but this ws smll reltive to the tmospheric inputs given bove. 101

132 Tble 6-4: An udit of boveground nutrient loss, relese (s relese of nutrients with decomposition nd fertiliser ddition) nd bove ground uptke for ech tretment between felling nd cnopy closure (kg h -1 ). It Includes clcultion of the difference between relese nd uptke. Potentil nutrient relese to soil Felling to plnting (i) Plnting to cnopy closure (ii) N P K C Mg N P K C Mg Stnding crop No-Burn (65) 17 Burn Fertilised Whole-Tree (84) 18 9 (1) Double Aboveground crop uptke (iii) Relese nd uptke difference (iv) Felling to cnopy closure N P K C Mg N P K C Mg Stnding crop No-Burn Burn Fertilised Whole-Tree Double Vlues in prenthesis indicte n increse in residue nutrient content; ll remining nutrients re ssumed vilble fter burning DISCUSSION Stnding crop nutrient fluxes Stnding crop dt demonstrtes reltive blnce between nutrient dditions nd losses with bove ground nutrient cycling, (retrnsloction, litterfll nd forest floor decomposition) responsible for significnt proportion of growth relted nutrient demnds. Atmospheric deposition supplied excess nutrients to the stnding crop over the study period, while soil leching losses beyond the root zone were unlikely due to high wter use (CHAPTER 4). These dt provide good bseline ginst which to compre nutrient fluxes in disturbed nd undisturbed plnttion systems. It demonstrtes tht growth prior to site disturbnce ws not limited by the nutrients ssessed in our study. However, stnding crop growth ws possibly limited by the pest infesttion. This ws evident in the high cnopy turnover reltive to biomss ccumultion nd growth increse s infesttion levels visully declined. 102

133 Tretment effects on nutrient cpitl nd erly growth Lrge quntities of nutrients lost through hrvesting, residue removl nd residue burning reduced the quntities of nutrients vilble to the next crop nd site nutrient pools. However, residue mngement hd very little effect on erly growth nd biomss development t this site up to 2.5 yers fter plnting. Despite significnt nutrient losses from residue nd forest floor, the initil growth nd nutrient ccumultion response in the Burn tretment my hve been through incresed minerl nutrient vilbility fter burning. Clcium remined in quntities tht exceeded erly growth demnd. Erly effects diminished over time, the initilly higher growth fter burning slowing sufficiently for bove ground biomss to mtch the other residue mngement tretments. Whole-Tree removl did not reduce growth to 2.5 yers, despite lrge nutrient losses. A combintion of tmospheric inputs, soil nutrient supply tht included decomposition of the old root system my hve provided erly growth nutrient requirements to cnopy closure. Loss of nutrients from the whole tree hrvest tretment my limit growth t lter stge. Although the lck of erly growth response to residue doubling ws unexpected, nutrient lockup in the residues my hve reduced soil nutrient vilbility. The long dely between felling nd burning my lso hve permitted nutrient return to the soil through decomposition nd leching prior to burning (Figure 5 i). However, mngement usully limits the durtion of the fllow period fter clerfelling. This will llow smller quntities of redily vilble nutrients to be present t estblishment. It lso underscores the reson for significnt responses obtined with smll fertilizer pplictions in close proximity to young trees during the period immeditely fter plnting. The initil fertiliser response my hve been through incresed nutrient vilbility in close proximity to the roots of the young trees. Fertilistion t plnting exceeded the N, P nd K recommended rtes of up to 70, 17, 17 kg h -1, respectively du Toit nd McLennn (2000). Pst fertiliser experiments used to derive these rtes in the sme region hve responded to nutrient ddition t plnting, even fter residue burning. Depending on site type, N nd/or P nd/or S ppliction t plnting improved volume growth by m 3 h -1 over 6 yer rottion cross five site types (du Toit nd Oscroft 2003). Although fertilistion significntly incresed ccumultion of folir nd woody N nd P t our study t one yer, there ws no evidence tht these elements were responsible for the growth response. The vector nlysis pointed to C nd Mg hving improved growth pst cnopy closure. This my indicte shift in nutrient requirements, N nd P being dequte in our study fter the estblishment phse. Although 103

134 growth response ws through n increse in tree cnopy size (LAI), growth efficiency my hve plyed smll role during erly growth. Growth efficiencies were similr to vlues in clonl Euclyptus t two yers in Congo Lclu et l. (2009) nd t three yers in Brzil (Stpe et l. 2008) Comprison with similr interntionl studies A network of interntionl studies co-ordinted through the Centre for Interntionl Forestry Reserch (CIFOR) used similr tretments to investigte the effects of residue mngement fter clerfelling on subsequent productivity cross tropicl plnttion forests (Tirks nd Rnger 2008). Growth ws reduced in Euclyptus plnttions fter residue removl on sites chrcterised by reltively low soil orgnic crbon nd low totl soil nutrient levels in Austrli (Sint-André et l. 2008), Brzil (Gonçlves et l. 2008b), Chin (Xu et l. 2008) nd the Republic of Congo (Deleporte et l. 2008). More fertile sites either did not experience productivity decline (Austrli (Sint-André et l. 2008)) or experienced decresed growth only fter complete residue nd litter lyer removl (South Afric (du Toit et l. 2008)). The low orgnic crbon nd low nutrient sites in Brzil, Congo, Chin nd Austrli were similr to our site, but demonstrted growth improvement with residue doubling nd retention nd growth reduction fter residue removl. Our study site experienced similr zero response to the more fertile CIFOR network sites. Soil crbon nd nutrient levels reported t these fertile sites were n order of mgnitude higher thn t our study site. Incresed nutrient vilbility cn lso increse bove ground growth through reduction in below ground crbon lloction nd my explin pst nd present responses. The proportion of NPP llocted to below-ground fine root production nd respirtion is decresed t higher nutrient vilbilities (Cnnell et l. 1988; Snds nd Mullign 1990; Dickmnn et l. 1996; Misr et l. 1998; Girdin et l. 2004; du Toit 2008). Coyle nd Colemn (2005) disgree with this pproch nd rgue tht belowground chnges re rther through n ccelertion of physiologicl growth mturity Tretments used nd mngement Although most of the residue mngement prctices described in this study re representtive of mngement prctices commonly used the SA forestry industry, there is nowdys strong trend 104

135 towrds prctices involving greter biomss removl. These less conservtive prctices my be used to reduce fire risk, to fcilitte mechnised hrvesting or to llow the utilistion of tree residues for energy production. Mechnised whole tree hrvesting my become more common prctice in South Afric. Where whole-tree residue hrvesting or burning re required, it my be beneficil to leve residues on site for s long s possible to llow nutrients (prticulrly K nd P) to move into the soil. This is provided the more mobile K ions cn be stored on the soil exchnge nd not leched beyond the rooting zone CONCLUSION This site occurs on dystrophic sndy soil with low nutrient retention nd storge cpcity nd smll belowground nutrient reserves. Despite this, there ws little evidence of n erly growth reduction fter lrge nutrient removl. This grees with the lck of evidence towrds nutrient relted decline cross the Zululnd region. The results of this study my be deceptive if viewed in terms of tree responses growth lone. It therefore ccounts for the consequences of mngement prctices on nutrient loss nd demonstrtes the effect of plnttion mngement on nutrient reserves nd future nutrient supply. Nutrient pools in the residues nd litter lyers t felling ply mjor role in mintining nutrient reserves on this site. Further to this, nutrient recycling processes in the mture crop limits nutrient demnd while the lrge tmospheric inputs prtilly replenish nutrients lost t clerfelling. Incresed nutrient loss (whole tree removl nd burning) removes bove ground pools nd plces more hevy relince on belowground (soil nd soil orgnic) nutrient reserves. Given the smll soil nutrient pools t our site, it is likely tht soil nutrients will become depleted under mngement prctices tht induce lrger losses thn nturl nd nthropogenic replcements. Retention of residues my therefore be necessry to conserve nutrients on such sites in Zululnd nd fertilistion my be required to replenish nutrients lost in quntities tht exceed nturl replcements. Although this study needs to continue to rottion end, fertilistion responses nd smller tmospheric inputs my point to depletion of Mg lredy in effect. However, this needs further testing through fertilistion studies. Further work using more intensive nutrient removl must be undertken to determine the threshold of nutrient removl t which soil nutrient depletion occurs cross sites in Zululnd. This must lso be extended cross brod site types nd species rnges to extrpolte effects sptilly nd into the long term. Strtegies to reduce nutrient losses nd meliorte sites where nutrient depletion hs lredy occurred my then be required. 105

136 6.7. APPENDICES RAINFALL (mm) () (b) (c) (d) TEMPERATURE ( C) 0 01/08 04/08 07/08 09/08 12/08 03/09 06/09 09/09 12/09 03/10 06/10 09/10 12/10 MONTH/YEAR Appendix 6-1: Totl monthly rinfll (solid brs) nd long term mens (dotted line on br); men monthly temperture (line) nd long term mens (fine dotted line) recorded from felling to post cnopy closure. Verticl dshed lines represent the times of () felling; (b) residue burning; (c) plnting nd (d) cnopy closure () (b) (c) (d) LITTERFALL (Mg h-1 yr-1) Folige Brk Brnches 0 01/08 04/08 07/08 09/08 12/08 03/09 06/09 09/09 12/09 03/10 06/10 09/10 12/10 MONTH/YEAR Appendix 6-2: Annul folir, brnch nd brk litterfll rtes per litter trp collection, verged cross the stnding crop plots. Error-brs represent the single stndrd devition between ll litter trps. Verticl dshed lines represent the times of () felling; (b) residue burning; (c) plnting nd (d) cnopy closure. 106

137 1 () (b) (c) (d) Nitrogen (mg kg -1 ) Stnding crop No-Burn Whole tree 0.1 (A) 0 01/08 04/08 07/08 09/08 12/08 03/09 06/09 09/09 12/09 03/10 06/10 MONTH/YEAR 0.05 () (b) (c) (d) Phosphorus (mg kg -1 ) Stnding crop No-Burn Whole tree (B) 0 01/08 04/08 07/08 09/08 12/08 03/09 06/09 09/09 12/09 03/10 06/10 MONTH/YEAR 107

138 0.25 () (b) (c) (d) 0.2 Potssium (mg kg -1 ) Stnding crop No-Burn Whole tree /08 04/08 07/08 09/08 12/08 03/09 06/09 09/09 12/09 03/10 06/10 (C) MONTH/YEAR 2 () (b) (c) (d) Clcium (mg kg -1 ) Stnding crop No-Burn Whole tree 0.5 (D) 0 01/08 04/08 07/08 09/08 12/08 03/09 06/09 09/09 12/09 03/10 06/10 MONTH/YEAR 108

139 0.25 () (b) (c) (d) 0.2 Mgnesium (mg kg -1 ) Stnding crop No-Burn Whole tree 0 01/08 04/08 07/08 09/08 12/08 03/09 06/09 09/09 12/09 03/10 06/10 (E) MONTH/YEAR () (b) (c) (d) Sodium (mg kg -1 ) Stnding crop No-Burn Whole tree (F) /08 04/08 07/08 09/08 12/08 03/09 06/09 09/09 12/09 03/10 06/10 MONTH/YEAR Appendix 6-3(A-F): Concentrtions of N, P, K, C, Mg nd N in the litter nd forest floor of the stnding crop, Whole-Tree nd No-Burn tretments. Verticl dshed lines represent the times of () felling; (b) residue burning; (c) plnting nd (d) cnopy closure. 109

140 0.4 K Mg () (b) (c) (d) K, Mg nd N (mg Kg -1 ) N 0 01/08 04/08 07/08 09/08 12/08 03/09 06/09 09/09 12/09 03/10 06/10 09/10 12/10 MONTH/YEAR Appendix 6-4: Concentrtions of K, Mg nd N in the litterfll collected under the stnding crop. Verticl dshed lines represent the times of () felling; (b) residue burning; (c) plnting nd (d) cnopy closure. 1.2 () (b) (c) (d) N nd C (mg kg -1 ) N C P P (mg kg -1 ) /08 04/08 07/08 09/08 12/08 03/09 06/09 09/09 12/09 03/10 06/10 09/10 12/ MONTH/YEAR Appendix 6-5: Concentrtions of N, C nd P in the litterfll collected under the stnding crop. Verticl dshed lines represent the times of () felling; (b) residue burning; (c) plnting nd (d) cnopy closure. 110

141 BASAL AREA (m 3 h -1 yr -1 ) Fertilised Burn Whole tree Double residue YEARS AFTER PLANTING Appendix 6-6: Reltive growth rtes s the difference between bsl re increment of ech tretment nd bsl re increment of No-Burn tretment t six monthly intervls fter plnting. 111

142 Folir A 80 Woody B NITROGEN (kg h-1) bc B b c B bc B (A) SC 0.5 No Burn Burn FS TREATMENT, YEARS AFTER PLANTING W S BC B A Woody Folir BC 4 C PHOSPHORUS (kg h-1) (B) SC 0.5 No Burn Burn FS TREATMENT, YEARS AFTER PLANTING W S AB A 60 Woody Folir C C BC 50 POTASSIUM (kg h-1) (C) SC 0.5 No Burn Burn FS TREATMENT, YEARS AFTER PLANTING W S

143 90 80 Woody Folir 70 CALCIUM (kg h-1) C A AB C BC 10 (D) SC 0.5 No Burn Burn FS TREATMENT, YEARS AFTER PLANTING W S Woody Folir B AB A B B 14 MAGNESIUM (kg h-1) (E) SC 0.5 No Burn Burn FS TREATMENT, YEARS AFTER PLANTING W S 1.0 Appendix 6-7 (A-E): Nutrients ccumulted in the bove ground tree components between the plnting to 0.5 yers, nd plnting to cnopy closure (1.0 yers). Vlues shown on brs with LAI (m² m - ²) on the top. Stnding crop (SC) is the sum of growth nd litterfll) Different, b, c superscripts denote significnt differences between tretments; lowercse for 0.5 yers nd uppercse for 1.0 yers (LSD5%). 113

144 NUTRIENT CONCENTRATION (A) Nitrogen (0.5 yers) No-Burn Burn 2S W FS NUTRIENT CONTENT Tretment Mss Conc Content Vector length Burn Fertilised No-Burn WholeTree Double NUTRIENT CONCENTRATION (B) Nitrogen (1.0 yers) W Burn 2S No-Burn FS NUTRIENT CONTENT Mss Conc Content Vector length NUTRIENT CONCENTRATION No-Burn 2S W (C) Phosphorus (0.5 yers) Burn FS NUTRIENT CONTENT Tretment Mss Conc Content Vector length Burn Fertilised No-Burn WholeTree Double NUTRIENT CONCENTRATION No-Burn Burn W 2S (D) Phosphorus (1.0 yers) FS NUTRIENT CONTENT Mss Conc Content Vector length

145 NUTRIENT CONCENTRATION No-Burn 2S W (E) Potssium (0.5 yers) Tretment Mss Conc Content Vector length Burn Fertilised No-Burn WholeTree Double NUTRIENT CONCENTRATION (G) Clcium (0.5 yers) FS Burn NUTRIENT CONTENT No-Burn Burn 2S FS W NUTRIENT CONTENT Tretment Mss Conc Content Vector length Burn Fertilised No-Burn WholeTree Double NUTRIENT CONCENTRATION W No-Burn (F) Potssium (0.5 yers) NUTRIENT CONCENTRATION 2S Mss Conc Content Vector length (H) Clcium (1.0 yers) Burn FS NUTRIENT CONTENT W 2S No-Burn Burn FS NUTRIENT CONTENT Mss Conc Content Vector length

146 NUTRIENT CONCENTRATION (I) W 2S No-Burn Burn FS NUTRIENT CONTENT Mgnesium (0.5 yers) Tretment Mss Conc Content Vector length Burn Fertilised No-Burn WholeTree Double NUTRIENT CONCENTRATION S No-Burn (J) Mgnesium (1.0 yers) W Burn FS NUTRIENT CONTENT Mss Conc Content Vector length Appendix 6-8(A-J): Vector nlysis figures for ech mcro-nutrient element t 0.5 nd 1.0 yers fter plnting using No-Burn s the (100%) control nd interprettions fter Slifu nd Timmer (2001) (units re s percentge of the control). (FS =fertilised; W = whole tree; 2S=double). Vector tble is given below ech figure showing folir mss, nutrient concentrtion (conc) nd nutrient content reltive to No-Burn (set t 100) nd vector length. Appendix 6-9: Equtions used to predict tree component msses nd lef re (LA) t 6 using gld (cm) nd 12 months fter plnting using dbh (cm). 6 months 12 Months Stnding crop (t felling) x dbh 2 + b x dbh + c Component Eqution R 2 Eqution R 2 b c R 2 LA (m 2 ) x gld x dbh Folige (kg) x gld x dbh Woody (kg) x gld x dbh Brnch (kg) x dbh Stem (kg) x dbh Brk (kg)

147 Appendix 6-10: Equtions using dbh to predict tree component msses nd lef re (LA) of the stnding crop Tree Component Eqution R 2 LA (m 2 ) x dbh Folige (kg) x dbh Brnch (kg) x dbh x dbh Brk (kg) x dbh x dbh Stem (kg) x dbh x dbh Woody (kg) x dbh x dbh Appendix 6-11:: Biomss nd mcro nutrients in the tree biomss nd litter lyer t felling Pre-hrvest component vlues Biomss N P K C Mg (Mg h -1 ) (kg h -1 ) Litter lyer Stem Brk Brnches leves Residues Residues re the sum of the litter lyer nd tree remins fter stem wood removl Appendix 6-12: Bsl re nd men heights of ech tretment t six monthly intervls fter plnting. Age (yers) No-Burn Burn Fertilised WholeTree Double LSD 5% p Bsl re (m 2 h -1 ) 0.5* 1.36 bc b 1.31 c 1.26 c 0.23 < b 3.54 b b 3.22 b 0.5 < b 6.80 b b 6.52 b 0.59 < b 8.35 b b 8.22 b b 9.52 b b 9.51 b 0.66 <0.001 Men tree height (m) b b b b b b b b *0.5 yer bsl re dt clculted using gld, others clculted using dbh Different, b, c superscripts denote significnt differences between tretments t ech ge (LSD5%). 117

148 CHAPTER 7: NITROGEN MINERALISATION IN A CLONAL EUCALYPTUS PLANTATION ON SAND AS AFFECTED BY CLEARFELLING AND RESIDUE MANAGEMENT ABSTRACT A study ws set out to compre in situ soil N minerlistion, surfce leching displcement nd root uptke fluxes in n undisturbed Euclyptus crop, clerfelled nd re-estblished fter residue burning (Burn) nd residue retention (No-Burn). This ws crried out using sequentil open nd closed 0 to 30 cm soil lyer coring method on fst growth, sndy low soil N site. Minerlistion nd immobilistion in the undisturbed stnding crop remined ner zero with net immobilistion of 3.4 kg h -1 yer -1. Surfce N uptke ws estimted t 71.3 kg of N h -1 yer -1 while N displcement ws limited by high throughfll (27.4 kg of N h -1 yer -1 ) nd high wter use. Clerfelling incresed N minerlistion rtes, mobile NO 3 -N concentrtions in the soil nd surfce displcement. 121 kg h -1 of N ws lost during residue burning leving 111 kg h -1 of N in sh/chr. Net N minerlistion ws reduced fter burning by nerly 53% over the 20.1 month period. Net N minerlistion in the No-Burn plots incresed over time. Net N minerlistion in the No-Burn plots (45.7 kg h -1 ) ws greter thn in the Burn plots (24.5 kg h -1 ) over the 20.1 month period. Burning hd no significnt impct on 0 to 30 cm soil N displcement or root uptke. As growth ws improved over the first six months fter burning it ws suggested tht fctors other thn N supply were limiting erly growth. Loss of N nd orgnic substrtes through residue burning cn excerbte N loss fter clerfelling. A reduced minerlistion cn limit N supply to trees lter in the rottion. Atmospheric N inputs were substntil in this study site nd my offset losses to some degree. Residue retention my be necessry to conserve soil N nd mintin N supply to trees on these highly productive but low- N soils INTRODUCTION Minerlistion of nitrogen (N) is crucil in forests soils where limited N vilbility hs the potentil to reduce tree growth rtes during periods of high N demnd (Binkley nd Hrt 1989; Mithni et l. 1998; Smethurst et l. 2004). The supply of N through minerlistion, reltive to 4 Submitted for publiction in New Forests 118

149 tree N demnd, is n importnt spect in forest nutrient mngement s it is lso good indictor of ecosystem helth, hving strong influence on plnttion tree growth (Reich et l. 1997; Morris nd Boerner 1998). Nitrogen minerlistion involves the mmonifiction of orgnic-n compounds to NH 4 -N nd the oxidtive nitrifiction (consumption) of portion of this mmoni to nitrite nd then nitrte (NO 3 -N) by soil mico-orgnisms lso occurs. Net N minerlistion or immobilistion is the blnce of ll these processes (Rison et l. 1987). Plnt uptke, nitrte leching displcement or losses nd dsorption onto soil exchnge surfces cn result in further reductions in soil N, while N fixtion, tmospheric deposition nd culturl dditions (e.g. fertilizers) cn increse soil N (Binkley 1986). The retention of dded N strongly depends on soil orgnic mtter content nd the form of N deposited to the site or relesed through microbil processes fter being deposited (Zhu nd Wng 2011). Mngement of hrvesting nd post-hrvest residues plys n importnt role in N losses during the inter-rottion period s soil temperture, soil moisture content, orgnic mtter content nd soil chemistry re ffected by residue mngement (Li nd Herbert 2004; Gonclves et l. 2007). The potentil for N losses from plnttion forest nutrient pools during periods when net N minerlistion exceeds plnt uptke is of prticulr concern during the inter-rottionl (fllow) period, from clerfelling to re-estblishment. Cesstion of wter nd nutrient uptke combined with higher soil tempertures nd soil moisture contents cn result in incresed losses through enhnced minerlistion leching nd denitrifiction (Fisher nd Binkley 2000). Further losses of orgnic crbon nd N ssocited with burning (vrying ccording to fire intensity) cn lso led to n increse in soil NH 4 -N relese through heting nd sh deposition (Weston nd Attiwill 1996). Although biomss removl nd burning hs been shown to contribute mjor portion of N loss during the inter-rottion period (Morris nd Miller 1994; Spngenberg et l. 1996; Gonclves 2004; Corbeels et l. 2005; Snkrn et l. 2005; Gonclves et l. 2007), further N leching or displcement losses resulting from ccelerted N minerlistion nd incresed leching cn compromise the sustinbility of future N supply (Crlyle et l. 1998b; Pitek nd Allen 1999). Given the incresing demnd for forest biomss (Crickmy 2005), the fte of N following clerfelling disturbnce nd residue mngement is key to the conservtion of N nd to site recovery on N limited sites (Weston nd Attiwill 1996; Gomez-Rey et l. 2007). Conservtion of N following clerfelling cn be chieved by mintining high C : N rtio fter clerfelling to 119

150 induce N immobilistion nd rpidly re-estblishing vegettion cover (Weston nd Attiwill 1996). Impcts of mngement prctices on N supply nd subsequent future growth need to be considered more crefully s prt of N mngement strtegies (Bris et l. 2002). Studies investigting N minerlistion processes in South Africn plnttion forestry re limited predicting fertiliser requirements (Louw nd Scholes 2002) nd do not chronologiclly describe chnges fter clerfelling nd residue mngement. Our study imed to ssess soil N fluxes during the inter-rottion period of Euclyptus stnd on site chrcterised s hving smll N pools nd rpid N-flux rtes. The objective of this study ws to determine the effect of clerfelling nd two extremes of stndrd mngement prctices (residue retention nd residue burning) on in situ minerlistion nd nitrifiction; giving n indiction of leching nd root uptke N fluxes. A lrger impct ws expected fter hrvesting nd residue burning thn fter residue retention MATERIALS AND METHODS Mterils nd methods for this chpter tht re common to other chpters re given in CHAPTER 3. Methods used in sequentil coring nd soil temperture determintion re described here seprtely Sequentil coring nd nlysis A sequentil coring method ws used to ssess in situ 0-30 cm N-minerlistion rtes, including the net effects of leching, denitrifiction nd tmospheric deposition (Adms nd Attiwill 1986; Rison et l. 1987; Adms et l. 1989). Sequentil core smpling commenced yer before clerfelling (September 2007). Fifteen PVC cores (5 cm internl dimeter nd 40 cm in length) were inserted into the soil, to depth of 30 cm, in ech smple plot. Five cores were collected immeditely to represent time-zero undisturbed smple, while five closed (cpped) nd five open (uncpped) cores were left in situ for 28 dy incubtion. This durtion ws chosen s it ws ssumed to be n dequte period to llow smll differences in physicl conditions while enbling detection of N concentrtion chnges (Jussy et l. 2004). At the end of ech incubtion period the closed nd open cores were collected nd trnsported in cool storge for further nlysis. Fifteen new cores were inserted to replce the extrcted cores t the end of ech 28 dy 120

151 incubtion, nd included the collection of new time-zero core smples. Cores were inserted t vrious positions between the trees to represent the vribility of soil micro-topogrphy, while voiding timber extrction routes nd res disturbed by prior smpling. This generted one smple per lyer per tretment in ech replicted block. The collected cores were divided into three lyers of 0-5 cm, 5-15 cm, nd cm, bulked ccording to core type nd soil lyer, homogenised sub-smple ws plced into seled plstic vils tht were refrigerted t 5 C until chemicl nlysis the following dy. A sub-smple of ech soil smple ws used to determine grvimetric wter content by oven drying t 105 o C for 24 hrs. The smples were nlysed for extrctble NH 4 -N nd NO 3 -N fter shking 25 g soil with 50 ml 2 M KCl for 1 hour nd filtering the extrcts through 42 um (Whtmn No. 42) filter pper. The concentrtions of NH 4 -N nd NO 3 -N were ssessed colorimetriclly using segmented flow nlysis with Perstorp Flow Solution III uto-nlyser. The sodium slicylte-sodium nitroprusside-hypochlorite method ws used for NH 4 -N (Alves et l. 1993) nd the sulphnilmide-nphthyl-ethylenedimine method for NO 3 -N plus NO 2 -N fter reducing nitrte to nitrite with copperized cdmium wire (Willis nd Gentry 1987). Nitrogen minerlistion, nitrifiction (nd immobilistion) were clculted for ech incubtion period s the difference between the closed core (fter in situ incubtion, dy 28) nd the time-zero core (pre in situ incubtion, dy 0) NH 4 -N nd NO 3 -N concentrtions. Nitrogen minerlistion ws clculted in ech core set s follows: ΔClosed = Closed (t+1) NI t ΔOpen = Open (t+1) NI t ΔNI = NI (t+1) NI t Where ΔClosed is the N minerlistion in closed cores over ech incubtion period, NI t is the non-incubted bulk soil core t t (time of core insertion). Closed (t+1) is the closed core smple tken fter incubtion t t+1. ΔOpen is N fluxes in the open cores ssumed to be minerlistion minus leching plus deposition. ΔNI is N fluxes in the bulk soil ssumed to be minerlistion minus leching plus deposition minus root uptke. The difference between closed nd open core N fluxes ws used to estimte leching nd the difference between open cores nd bulk soil N fluxes ws used to estimte root uptke (Rison et l. 1987; Smethurst nd Nmbir 1989). A 121

152 clcultion of differences between core fluxes ws therefore used to estimte leching plus deposition nd root uptke: Net losses (Leching minus deposition) = ΔOpen ΔClosed This should be root uptke = ΔOpen - ΔNI. Alterntively, As Rison formultes it: Closed (t+1) - NI (t+1) - (net losses) Which is the sme s Closed (t+1) - NI (t+1) - [ΔOpen - ΔClosed] The contribution of residue nd litter minerlistion (nd leching) to soil N ws excluded from this study due to the size of the mteril nd finncil constrints, but is included in Chpter 8 to depth of 1 m. The effect of residue mngement on soil N ws therefore prioritised. Net N fluxes were clculted s the sum of NH 4 -N nd NO 3 -N. Men soil bulk density t ech depth (using five smples per depth in ech plot) ws used with mesured N concentrtions to scle nutrient content to per hectre bsis Atmospheric deposition Atmospheric inputs described in CHAPTER 3 nd CHAPTER 5 were cumulted over ech incubtion period nd correlted with the N flux dt tken from the core smples In field mesurements Soil tempertures were mesured using copper constntn thermocouple probes t depths of 2.5 cm, 10 cm nd 22.5 cm which represented the midpoint of ech core smple depth. The ccess tubes nd thermocouples were plced within nd between the tree rows to represent the vribility of soil moisture in ech plot. To increse the monitoring re, soil temperture probes were instlled s three clusters per depth in ech plot, ech cluster comprising four to five interconnected probes, giving n verged reding from the interconnected probes. Cmpbell Scientific Cr10x dt loggers mesured tempertures cross ech probe cluster t one minute intervls. 122

153 Sttisticl nlyses Differences between tretments, soil smple depths nd smpling dtes were compred using repeted mesures ANOVA cross time mesurements nd n ANOVA ws used for cumultive N mesures, growth nd biomss with lest significnt difference (LSD 5% ) used to determine significnce of tretment differences. Person correltion ws used to test for correltion between N fluxes nd soil mesures. All sttisticl nlysis ws performed using GenStt for Windows 12th Edition (Pyne et l. 2011) RESULTS Chnges in system N pools during the study period Effects of clerfelling nd burning on mcro-nutrient pools nd subsequent tree growth re given in CHAPTER 3. A brief summry is given here for N nd in Tble 7-1. Clerfelling removed 90.5 Mg h -1 of stem wood contining kg h -1 of N leving 50.6 Mg h -1 of residue nd forest floor contining kg h -1 of N (Tble 7-1). During the three month dely between clerfelling nd burning the residue biomss nd N content decresed by 14.2 Mg h -1 nd 71.2 kg h -1 of N. Burning reduced the remining residue to 4.2 Mg h -1 lyer of sh nd chr with loss of kg h -1 of N. Only very smll quntity of corse chr remined on the soil surfce t week fter burning. The reminder leched into the soil with lrger prticles creting distinctive chr horizon between 5 nd 10 cm from the soil surfce. Growth in the Burn tretment ws initilly more rpid thn in the No-Burn tretment, but slowed reltive to the No-Burn tretment fter cnopy closure (CHAPTER 6). Differences between the Burn nd No-Burn tretments were no longer significnt from one yer to 2 yers six months fter plnting (time of publiction). Height growth ws not significntly different between the Burn nd No-Burn tretments t ny ge. The stnding crop ccrued 1.3 Mg h -1 of biomss nd 2.5 kg h -1 of N into the bove-ground tree components during the period between clerfelling to plnting of the new crop (Tble 7-1). Litterfll during this period ws fr greter thn biomss ccretion, totlling 6.9 Mg h -1 contining 57.2 kg h -1 of N. The No-Burn tretment ccrued significntly less biomss nd N thn the Burn tretment t six months fter plnting. At cnopy 123

154 closure lrge quntity of N nd biomss remined in the No-Burn tretment residues. The stnding crop tretments hd reduced forest floor biomss nd N compred to tht t clerfelling. Tble 7-1: Nitrogen pool sizes nd ccretion t nd between the times of clerfelling, burning, nd cnopy closure for stnding crop, Burn nd No-Burn tretments. Chnge in forest floor/ residues pools given in prenthesis. Nitrogen (kg h -1 ) Component Stnding crop No-Burn Burn Post-Clerfelling Pools (October 2008) Forest floor/residues Clerfelled tretments implemented but prior to Burning (Mrch 2009) Forest floor/residues * Accretion Burning tretment implemented but prior to plnting (August 2009) Forest floor/residues Accretion New crop plnted nd grown up to cnopy closure (June 2010) Forest floor/residues Accretion Above-ground stnding crop ccretion includes litterfll; * is sh remining fter burning Soil nd ir temperture Men dily ir temperture (Figure 7.1) remined below soil temperture for most of the study period nd ws chrcterised by lrger vribility thn soil temperture. Soil temperture ws higher t the surfce, decresing with depth (for the depths recorded) during the summer monthsappendix 7.7. Surfce tempertures were modertely lower during winter with wrmer tempertures few centimetres below the soil surfce. Soil temperture vrition ws slightly greter t the surfce, with the lrgest vrition occurring in the Burn tretment. Temperture differences between Burn nd No-Burn were significnt t ll mesured depths from burning to four months fter plnting (p < 0.001) (Figure 7.1; Appendix 7.7). Differences between the stnding crop nd No-Burn tretments were between -2.0 nd 2.0 ºC fter clerfelling, decresing to differences of between -1.0 ºC nd 1.0 ºC t plnting. Men dily soil tempertures incresed by mximum of 10.0 ºC higher in the Burn thn the No-Burn tretment immeditely fter burning, but rpidly decresed to mximum difference of round 3.0 ºC from two weeks fter burning (Figure 7.1, period (b) to (c)). This initil difference ws due to the lbedo effect of the blck sh, the subsequent reduction in differences occurred with the sh 124

155 being lost into the soil with rinfll (nd possibly wind). Temperture differences between tretments continued to decrese over time becoming incresingly similr (non-significntly different) s the new crop pproched cnopy closure () (b) (c) SC Burn No-Burn Air (d) Temperture ( C) /08 12/08 03/09 06/09 09/09 12/09 03/10 06/10 Dte (mm/yy) Figure 7.1: Men dily ir nd soil (10 cm depth) tempertures for the stnding crop (SC), single residue (No-Burn) nd burned residue (Burn) tretments. Dshed lines represent () clerfelling; (b) residue burning; (c) plnting nd (d) cnopy closure Soil moisture Soil moisture content (CHAPTER 4) incresed in the Burn nd No-Burn tretments reltive to the stnding crop tretment fter clerfelling (Figure 7.2), the moisture contents becoming similr ner cnopy closure of the new crop. Soil moisture content in the No-Burn tretment ws slightly higher thn in the Burn tretment from My 2009 to December 2009, with the lrgest differences occurring shortly fter hevy rinfll events. Wter contents determined in the core smples fter extrction were modertely higher in the open cores thn in the closed nd timezero smples. Differences in soil moisture contents between cores types were lrgest where hevy rinfll occurred during core incubtion. Soil moisture content ws lower t the surfce (0-5 cm), incresing with depth in the Burn nd No-Burn tretments. Wter contents t 0-5 cm were similr between the Burn nd No-Burn tretments from clerfelling to cnopy closure. 125

156 15% 14% () (b) (c) (d) % SC 20 12% Burn cm Vol. Wter Content 11% 10% 9% 8% 7% 6% No-Burn Precipittion (mm dy -1 ) 5% 90 4% 100 3% 09/08 12/08 03/09 06/09 09/09 12/09 03/10 06/ Dte (mm/yy) Figure 7.2: Weekly volumetric soil moisture content t 15 cm depth in the stnding crop (SC), single residue (No-Burn) nd burned residue (Burn) tretments. Dshed lines represent () clerfelling; (b) residue burning; (c) plnting nd (d) cnopy closure. Rinfll events re shown s verticl lines scling to n inverted secondry Y xis. Single stndrd devitions re given s verticl I brs Minerlistion, nitrifiction nd immobilistion Minerlistion nd immobilistion of NH 4 -N followed similr ptterns cross ll tretments throughout the study period (Figure 7.3A). Although p vlues cnnot be shown for ech time point in Figure 7.3A nd B, significnce differences re given t p < 0.05, (LSD 5% ). Tretment differences were vrible over time. The NH 4 -N fluxes in the No-Burn nd Burn tretments were similr while differing slightly from the stnding crop tretment. Minerlistion nd NH 4 - N immobilistion tended to be significntly greter in the stnding crop tretment for most incubtion periods (p < 0.03 in ech cse). NO 3 -N fluxes remined consistently less in the stnding crop tretment thn in the Burn nd No-Burn tretments (p < 0.04 in ech cse), lthough nitrifiction or immobilised NO 3 -N gve similr ptterns nd chnged order of significnce cross the three tretments over time (Figure 7.3B). NO 3 -N fluxes in the No-Burn tretment remined slightly higher thn in the Burn tretment from just prior to plnting, coinciding with the erly onset of rinfll (Figure 7.2). Periods of NH 4 -N nd NO 3 -N immobilistion were correlted with soil moisture content (R = 0.406, p < 0.01), immobilistion occurring during periods of low soil moisture content. Immobilistion of NH 4 -N nd nitrifiction ws significntly higher t 0-5 cm throughout the study period thn t 5 15 cm or cm. Net N minerlistion in the stnding crop tretment (Figure 7.3C) ws significntly below the clerfelled tretments on number of occsions prior to plnting (p < 0.04). The Burn tretment lso peked significntly in the lst summer prior to cnopy closure (p = 0.037). 126

157 Minerlistion (kg NH 4 -N h -1 dy -1 ) /08 12/08 03/09 06/09 09/09 12/09 03/10 06/ () (b) (c) SC Burn No-Burn (d) (A) -2.5 Dte (mm/yy) Nitrifiction (kg NO 3 -N h -1 dy -1 ) () (b) SC Burn No-Burn /08 12/08 03/09 06/09 09/09 12/09 03/10 06/10 (c) (d) (B) -0.4 Dte (mm/yy) Net minerlistion (kg N h -1 dy -1 ) /08 12/08 03/09 06/09 09/09 12/09 03/10 06/ () (b) (c) SC Burn No-Burn (d) (C) -3.0 Dte (mm/yy) Figure 7.3(A-C): N minerlistion fluxes in the 0-30 cm soil lyer fter in situ incubtion of closed cores: (A) minerlistion to NH 4 -N (B) nitrifiction to NO 3 -N nd (C) net N minerlistion. Tretments re stnding crop (SC), residue retention (No-Burn) nd burned residue (Burn). Dshed lines represent the times of () clerfelling; (b) residue burning; (c) plnting nd (d) cnopy closure. I-brs represent lest significnt differences (LSD 5% ). 127

158 Leching nd tmospheric inputs NH 4 -N (Figure 7.4A) ws gined by the open cores in the stnding crop tretment during most incubtion periods, losses only occurring fter high rinfll events. Gins in the stnding crop tretment occurred primrily during the dry seson, beginning t the onset of the second wet seson (October 2009). NH 4 -N ws lost from the open cores in both felled tretments fter clerfelling. Differences between Burn nd No-Burn were often significnt (p < 0.03), but interchnged over time. One lrge loss occurred from the Burn tretment (during 12/09) tht coincided with loss from the stnding crop tretments nd gin in the No-Burn tretment. NO 3 -N (Figure 7.4B) ws lso gined in the open cores of the stnding crop tretment for much of the study period. A loss of NO 3 -N occurred through leching in the No-Burn nd Burn tretments from three months fter clerfelling. These losses decresed in intensity fter the trees were six months old. After six months losses continued to smll extent in the No-Burn tretments, but remined ner zero in the Burn tretment. More NO 3 -N ws initilly lost during the second incubtion period fter burning, but losses becme lger in the No-Burn tretment therefter (p < 0.04). Lrgest losses NO 3 -N losses coincided with high rinfll events. A net N gin occurred with throughfll in the stnding crop tretment for most incubtion periods (Figure 7.4C), leching losses occurring only fter high rinfll events. Net N loss through leching occurred from both felled tretments from round two months fter clerfelling, persisting to six months fter plnting. Losses becme smller tretments, with some smll gins with rinfll. Losses from the No-Burn between the time of burning nd up to six months fter plnting were significntly lrger thn from the Burn tretment for incubtion periods where lrge NO 3 -N losses occurred (p < 0.02). 128

159 NH 4 -N deposition\leching (kg h -1 dy -1 ) 0.3 () (b) (c) SC (d) Burn 0.2 No-Burn /08 12/08 03/09 06/09 09/09 12/09 03/10 06/ (A) -0.4 Dte (mm/yy) NO 3 -N deposition\leching (kg h -1 dy -1 ) 0.3 () (b) (c) (d) / /08 03/09 06/09 09/09 12/09 03/10 06/ SC Burn No-Burn (B) -0.9 Dte (mm/yy) Net N deposition\leching (kg h -1 dy -1 ) /08 12/08 03/09 06/09 09/09 12/09 03/10 06/ () (b) (c) (C) Dte (mm/yy) Figure 7.4(A-C): Minerl N dded to the 0-30 cm soil lyer through tmospheric deposition (positive vlues) or lost through leching (negtive vlues), estimted by the sequentil soil coring method for (A) NH 4 -N, (B) NO 3 -N nd (C) net N. Tretments re stnding crop (SC), residue retention (No-Burn) nd burned residue (Burn). Dshed lines represent the times of () clerfelling; (b) residue burning; (c) plnting nd (d) cnopy closure. I-brs represent lest significnt differences (LSD 5% ). 129 SC Burn No-Burn (d)

160 Root uptke Root uptke in Figure 7.5 included N leched or dded from decomposition from the litter lyer of the stnding crop tretment nd residues of the felled tretments. Some minor weed nd coppice growth occurred for the first few months fter clerfelling despite rigorous control mesures. Net N uptke occurred in the stnding crop throughout the study (Figure 7.5), which remined reltively consistent from the time of clerfelling to cnopy closure of the new crop. A reltively lrge gin occurred in the stnding crop tretment just prior to burning. A loss occurred in the felled tretments fter clerfelling, during three incubtions prior to burning. Some smll gins occurred in the felled tretments from just prior to burning to plnting. Losses begn fter plnting s the new crop grew nd with the onset of the second wet seson. These losses continued to the end of the study. Differences between Burn nd No-Burn tretments were interchngeble nd significnt for few incubtion periods. Net N uptke (kg h -1 dy -1 ) / /08 03/09 06/09 09/09 12/09 03/10 06/ () (b) Dte (mm/yy) SC Burn No-Burn Figure 7.5: Chnges in minerl N in the 0-30 cm soil lyer through root ctivity (negtive vlues) nd residue/litter dditions (positive vlues). Tretments re stnding crop (SC), residue retention (No-Burn) nd burned residue (Burn). Dshed lines represent the times of () clerfelling; (b) residue burning; (c) plnting nd (d) cnopy closure. I-brs represent lest significnt differences (LSD 5% ) (c) (d) Atmospheric deposition Addition of N through tmospheric inputs (Chpter 5 nd Figure 7.6) followed similr trends to minerlistion (Figure 7.3) nd core mesured tmospheric inputs (Figure 7.4). A spike in tmospheric N deposition resulted in spike in N detected in the cores, prticulrly orgnic-n deposition. Correltions here reflect tmospheric inputs prior to ech incubtion. Soil NO 3 -N 130

161 concentrtions were positively correlted with the quntity of NH 4 -N dded by throughfll in the stnding crop tretment (R=0.438; p < 0.01) nd rinfll in the clerfelled tretments (R=0.486; p < 0.01). This ws most notble for NH 4 -N nd orgnic-n. Although rinfll is not shown in (Figure 7.6), rinfll dditions were greter thn throughfll, throughfll 89% of NO 3 -N in rinfll nd 96% of NH 4 -N in rinfll. Uptke of N occurred through cnopy exchnge processes. The quntity of throughfll orgnic-n deposition ws positively correlted with nitrifiction rte (R=0.633; p < 0.01). Throughfll orgnic-n deposition ws lso correlted with NO 3 -N (R=0.399, p < 0.01). The quntity of throughfll NH 4 -N deposition ws negtively correlted with nitrifiction rte (R=-0.502; p < 0.01). Nitrifiction rtes were negtively correlted with the quntity of NH 4 -N dded with rinfll (R=-0.325; p < 0.01). N ddition with rinfll (kg h-1) () (b) (c) (d) Orgnic-N NH4-N NO3-N 0 09/08 12/08 03/09 06/09 09/09 12/09 03/10 06/10 Dte (mm/yy) Figure 7.6: NO 3 -N, NH 4 -N nd Orgnic-N dded throughfll (weekly). Dshed lines represent the times of () clerfelling; (b) residue burning; (c) plnting nd (d) cnopy closure using dt from CHAPTER Cumultive net nitrogen fluxes Cumultive nd nnulised net nitrogen fluxes estimted from core smples re shown in Tble 7.2. The period between clerfelling nd plnting exhibited net N minerlistion in ll tretments, with sttisticlly similr levels in the No-Burn nd Burn tretments, but with significntly lower levels in the stnding crop tretment. Plnting to cnopy closure gve cumultive net immobilistion of N in the Burn nd stnding crop tretments, which were sttisticlly similr. The No-Burn tretment yielded net N minerlistion, sttisticlly different to the other tretments. All tretments were significntly different for cumultive net N 131

162 minerlistion between clerfelling nd cnopy closure, which showed the lrgest net N minerlistion in the No-Burn, followed by the Burn, with net immobilistion occurring in the stnding crop tretment. The nine month period between clerfelling nd plnting ws dominted by immobilistion of NH 4 -N, nd nitrifiction to NO 3 -N (Figure 7.3A nd B). Tretment nitrifiction differences were significnt during ech period of the study t p< No-Burn tretment immobilistion of NH 4 -N ws significntly lrger thn in the Burn tretment, which ws significntly lrger thn the stnding crop tretment (p<0.001). Immobilistion of NH 4 -N did not chnge significntly with depth between felling nd plnting (p >0.08) nd plnting nd cnopy closure (p=0.232). Nitrifiction however tended to be greter t the surfce thn t depth between felling nd plnting (p<0.001) nd plnting nd cnopy closure (p <0.001). Overll nitrifiction ws 2.7 fold greter t 0 to 5 cm thn 5 to 15 cm depths nd 7.0 fold greter t 5 to 15 cm thn 15 to 30 cm depths (p<0.001). The combined effect of N deposition nd leching could lso be estimted from the core smple dt (Tble 7-2). A net N gin occurred in the stnding crop tretment throughout the study through tmospheric deposition (Tble 7-2) with slightly higher deposition rte during the first nine months of the study. Although deposition lso occurred in the felled tretments, it ws significntly outweighed by leching losses. An overll net N loss occurred in the No-Burn nd Burn tretments (Tble 7-2). These were sttisticlly similr for the first period (felling to plnting) nd lrger in the No-Burn during the second period (plnting to cnopy closure). Differences between the felled tretments were not significnt over the entire period (felling to cnopy closure). Differences between tretments in Tble 7-2 were ttributed to tretments differences in NO 3 -N during the first, second nd full period p<0.001in ech cse. Deposition of NH 4 -N ws not significntly different between tretments during ny period (p = 0.24, 0.06, 0.09 respectively). Leching of NO 3 -N decresed significntly with depth for ll periods (p = 0.01, 0.04, 0.01 respectively), wheres NH 4 -N ws only significnt with depth during the first period (p = 0.00, 0.21, 0.11 respectively). This significnt difference occurred s greter leching t 15 cm thn t 30 cm depth. A totl of 4.1 kg h -1 yr -1 of NH 4 -N nd 41.8 kg h -1 yr -1 NO 3 -N ws gined with rinfll in the stnding crop during the clerfelling to cnopy closure period. A totl of 3.6 nd 19.1 kg h -1 yr -1 of NH 4 -N nd 73.6 nd 39.6 kg h -1 yr -1 NO 3 -N ws leched during the clerfelling to cnopy closure period in the No-Burn nd Burn tretments respectively. Less N ws recorded s throughfll bove ground during ech period thn ws recorded by open core clcultions. 132

163 A lrge quntity of N ws lost from the soil surfce of the stnding crop with root uptke (Tble 7-2) cross both periods. Root uptke ws significntly lrger in the stnding crop tretment thn in the felled tretments throughout the study. Clculted root uptke ws smll prior to plnting, with no significnt differences between felled tretments. Post plnting root uptke ws lrger, but differences were not significnt between felled tretments. Root uptke prior to plnting occurred s loss of NO 3 -N in the felled tretments, occurring primrily from 0 to 5 cm depth. A smll gin in NH 4 -N occurred in the felled tretments t the 0 to 5 cm soil depth during this period. Root uptke of NO 3 -N nd NH 4 -N ws lrgest in the stnding crop tretment, but not significnt different between felled tretments. More NO 3 -N ws tken up thn NH 4 -N throughout the entire study period. Uptke did not chnge with depth prior to plnting (p=0.25), but decresed with depth fter plnting (p<0.001). These differences occurred through significnt differences in NO 3 -N uptke cross ech period, p<0.001 in ech cse. Although differences in NH 4 -N uptke did occur, these were s smller uptke from the 0 to 5 cm depth thn from the other two depths (p<0.001 for ech period). Root uptke mounted to 42.8 kg h -1 yr -1 of NH 4 -N nd 76.5 kg h -1 yr -1 NO 3 -N in the stnding crop tretment for the entire study period. A totl of 8.6 nd 4.6 kg h -1 yr -1 of NH 4 -N nd 35.2 nd 44.1 kg h -1 yr -1 NO 3 -N ws leched during the clerfelling to cnopy closure period in the No-Burn nd Burn tretments respectively. Aboveground N ccumultion (ccretion estimted from biomss studies) ws lrger thn 0 to 30 cm root uptke clculted using core methods. However, stnding crop boveground N ccumultion (including litterfll) ws of similr order of mgnitude to core clculted uptke. Loss of N from the litter nd residues ws lrge. This loss could not be directly relted to soil core fluxes s fine root growth occurred ner the surfce (visul observtion) in direct contct with the humus lyer component of the residues nd litter lyer. 133

164 Tble 7-2: Cumultive (kg h -1 ) nd nnulised (kg h -1 yer -1 ) net nitrogen fluxes in-field cores of 0 30 cm soil from clerfelling to cnopy closure of the new crop. Cumultive (kg h -1 ) Annulised (kg h -1 yer -1 ) No-Burn Burn Stnding Stnding p No-Burn Burn crop crop Clerfelling to Net Minerlistion b plnting Deposition - leching b < (9 months) Uptke b < Plnting to cnopy closure (11 months) Clerfelling to cnopy closure (20.1 months) Rinfll/Throughfll N ccretion * Net Minerlistion b b Deposition - leching b 21.3 c < Uptke b < Rinfll or Throughfll N ccretion * Net Minerlistion b -5.7 c Deposition - leching b < Uptke b < Rinfll/Throughfll N ccretion * Residue/litter mss loss * Minerlistion, nitrifiction nd deposition gins re positive; immobilistion nd net loss re negtive. Different, b, c superscripts denote significnt differences (LSD5%). * includes litterfll, but excludes cnopy exchnge 7.5. DISCUSSION Denitrifiction ws likely to hve been zero or negligible t our study site due to the well drined nture of the soil (sndy texture) nd the wter contents mesured over the course of the tril CHAPTER 3, (Dovey et l. 2011) remining t or well below field cpcity (Groffmn 1995; Færge nd Mgid 2004). Net minerl N content in cores ws therefore ffected by combintion of some or ll of the following processes: microbilly medited N minerlistion nd nitrifiction fluxes, N ddition with rinfll/throughfll, litter decomposition nd N leching. It is therefore importnt to relise tht the methodology used in the study cn hve pronounced influence on N dditions, losses nd trnsformtions in the soil. In the open cores, inputs of wter, N nd other nutrients nd leching continued throughout the incubtion period, but wter uptke by plnt roots ws excluded. This my led to higher soil moisture contents thn tht experienced under mbient field conditions. Closed-top cores receive no tmospheric N inputs nd leching is negligible, but cores cn become somewht drier thn mbient field conditions, which cn ffect microbil ctivity negtively. These fctors, when compred to the closed 134

165 cores nd the bulk soil in situ incubtion fluxes my hve ltered the comprtive bsl minerlistion rtes between the cores, possibly violting the ssumptions given in the methodology (Smethurst nd Nmbir 1989; Jussy et l. 2004). This my hve incresed soil N turnover rtes (Wienhold et l. 2009) possibly incresing the N minerlised nd nitrified (nd subsequently leched out), of the open-topped cores nd bulk soil, prticulrly in the clerfelled tretments. Despite these drwbcks, the methods give lrge mount of informtion bout N fluxes in the surfce soil where the mjority of fine root grow occurs (Smethurst nd Nmbir 1989). This is the most importnt N supply region of the soil; therefore leching out of this zone constitutes displcement of N from this primry supply zone. Tken over the two study periods, dt in Tble 7-2 show net minerlistion for the clerfelled tretments, but smll immobiliztion effect in the stnding crop. The most plusible explntion for this observtion is tht lower tempertures throughout the incubtion period (Figure 7.1), coupled to significntly lower soil moisture contents (Figure 7.2) nd significntly lower minerl N contents t the onset of most incubtion periods, ll contributed to lowered N minerlistion rtes in the stnding crop tretments. Leching process could hve been most pronounced in the two felled tretments becuse the soil ws wetter thn the stnding crop t the strt of ech incubtion period (Figure 7.2), nd N minerlistion nd nitrifiction process would hve been fvoured by higher tempertures in the felled tretments (Figure 7.1) nd higher ph in the topsoil of the burnt tretment. ph (KCl) t the end of the study (0 to 15 cm) verged t 4.42, 6.37 nd 5.21 in the stnding crop, Burn nd No-Burn tretments respectively. Furthermore, despite cnopy cpture nd uptke of NH 4 -N nd NO 3 -N, the stnding crop received more net N (through orgnic-n inputs in throughfll) thn N inputs received through tmospheric deposition in the two clerfelled tretments (CHAPTER 5). Higher moisture content, coupled to higher nitrifiction nd leching rtes ws possibly n overriding mechnism in the open-top cores of the cler-felled tretments. Similrly, different conditions in the bulk soil fluxes my hve resulted in flse N uptke. Immobilistion, higher leching nd some limited weed nd coppice growth my hve ccounted for positive root uptke vlues prior to plnting. 135

166 Stnding crop N fluxes Net N minerlistion (Tble 7-2) remined reltively stble in the stnding crop throughout the study period lternting between N relese nd N immobilistion (or consumption). The reltively stble soil tempertures nd consistently low soil moisture contents (between rinfll events) gve little opportunity for rpid minerlistion to occur. The low levels of soil moisture (Figure 7.2) nd the negtive number for net N minerlistion over the study period (Tble 7-2) shows tht the stnding crop ws sub-optimlly supplied with both wter nd soil derived N. This implies potentil growth limittion through both fctors. However, N ccretion in the stnding crop (Tble 7-1) continued despite being limited by negtive soil minerlistion N fluxes. As root uptke my be overestimted using this technique (Smethurst nd Nmbir 1989), lrge proportion of bove-ground tree N ccretion ws most likely supplied from tmospheric inputs (throughfll), litter decomposition (Tble 7-1) nd deeper soil lyers. Throughfll nd stemflow recorded t this study site (CHAPTER 5) contributed lrge mount of N between clerfelling nd cnopy closure of the new crop. The difference between ddition nd ccretion resulted in more N dded to the soil surfce thn ws utilised nd immobilised (Tble 7-2) Felled crop N fluxes Post clerfelling N dynmics were different from tht of the stnding crop (Tble 7-2). Chnges in N-fluxes becme more pprent t round three months fter clerfelling, coinciding with the time tht rinfll induced soil moisture contents begn to diverge. Soil moisture nd dringe fluxes presented in (CHAPTER 4; Dovey et l. (2011) show these differences to grdully diminished with growth nd incresing wter demnd of the new crop. The low N minerlistion rtes under rpidly drying soils in the stnding crop (Figure 7.2) nd the incresed N minerlistion rtes with soil moisture rechrge fter clerfelling show the relince of N minerlising soil micro-orgnisms on stble nd wet soil moisture regime. A lck of significnt temperture differences between the stnding crop nd the felled tretments (nd lter No-Burn) occurred through shding of the soil by the hrvest residues while the open cnopy rchitecture of the stnding crop permitted high levels of solr penetrtion between individul tree cnopies, thereby incresing surfce tempertures. Leching nd root uptke estimtes were not sttisticlly different between tretments despite lrger minerlistion rtes in the No-Burn tretment. This implies residue burning to hve miniml impct on soil leching processes on 136

167 this site. It lso implies, given the initil growth improvement fter burning tht fctors other thn N were more limiting to growth. The long led time between clerfelling nd burning my hve reduced initil differences between the Burn nd No-Burn plots by llowing N nd orgnic compounds to enter the soil from residue decomposition prior to burning. Immobilistion of N (Tble 7-2) in the burnt plots fter burning indictes soil depletion of orgnic substrtes required for N minerlistion nd though unfvourble soil surfce moisture nd temperture conditions (high diurnl vrition). A substntil quntity of N ws relesed through N minerlistion nd potentilly through burning, but this occurred before plnting. Owing to this dely, N ws not vilble to the new crop, but lost through leching nd wind erosion prior to being beneficil. The lrge loss of N during burning nd lter incresed leching will not be economiclly beneficil if the cost of N replcement is greter thn tree growth gins relised fter burning. The burn tretment in Tble 7-2 lso shows the lrgest chnge between periods (from clerfelling to plnting) nd (from plnting to cnopy closure): Nitrogen minerliztion is lrge nd positive in the former period (28.8 kg h -1 ) nd becomes negtive (immobilized) in the second period. These fctors my hve compromised growth lter in the rottion while potentilly negtively impcting the sustinbility of future N pools. Higher levels of net N minerlistion in the No-Burn tretments were likely relted to the lrger mounts of orgnic substrte on the soil surfce nd more stble soil surfce temperture nd moisture regimes. As result, N minerlistion remined positive (lthough less) fter plnting, contrsting with the Burn tretment. While erly growth ws less in the No-Burn tretment thn in the Burn tretment, continued N supply will stisfy lrger proportion of tree growth N demnd lter in the rottion, given the reduced N supply in the Burn tretment. Net minerlistion rtes my pek gin in the No-Burn tretments lter in the rottion s substrte provision (lrge levels of residue remining on the No-Burn s opposed to the Burn tretment), soil moisture retention nd temperture stbilistion continue enhnce net N minerlistion. This ws found to occur in similr study on snds (Nzil et l. 2002). Tretment effects on cumultive net minerlistion (Tble 7-2) were similr to those in comprble study in Brzil (Gonclves et l. 2007), where higher rtes of N minerlistion 137

168 occurred fter miniml disturbnce (residue retention) thn fter residue burning. Similr quntities of N were minerlised in the Brzil study, 58 kg h -1 in the minimum disturbnce tretment nd 28 kg h -1 in the burned tretment over 21 month period, to our study (Tble 7-2). Minerlistion in the stnding crop tretment of the Brzil study ws different, 77 kg h -1 ; nd ws explined by solr penetrtion from the edges of the stnding crop plots from the djcent felled plots nd higher rinfll of the Brzil study. Solr penetrtion did not occur from the edges in our study. Men monthly net minerlistion rtes (Tble 7-2) were lso within similr order of mgnitude to those in studies reported in Nzil et l. (2002) under similr crop, climte nd soil conditions to our study Consequence of tmospheric N inputs This study suggests lrge mount of tmospheric N input to the site. High levels of tmospheric N ddition my hve incresed both the rte nd quntity of nitrifiction nd minerlistion. Atmospheric deposition my ffect the rte nd quntity of N relesed from N minerlistion through chnges in C: N rtios, soil ph nd litter qulity (Månsson nd Flkengren-Grerup 2003; Ro et l. 2009). Nitrogen minerlistion rtes were incresed fter N fertilistion in certin studies (Arnio nd Mrtikinen 1992; Prescott et l. 1995; Fox 2004), lthough lrge N dditions reduced orgnic mtter qulity nd minerlistion rtes in one study s result of incresed nitrifiction inducing bse ction stripping (Fox 2004). The ddition of orgnic nd inorgnic compounds from the decomposing residues nd from tmospheric deposition in prticulr, my hve ltered the bsl minerlistion rtes by chnging the quntity nd qulity of minerlisble soil orgnic compounds. This my lso be constrint in some N minerlistion models tht ssume bsl minerlistion rte tht is ltered by wter nd temperture regimes lone (Pul et l. 2002). Such models do not llow N to become immobilised in the soil nd do not ccount for the effects of N dditions from bove the soil surfce either. This my be evidence towrds N minerlistion fluxes being overridden by tmospheric orgnic N inputs. Although literture does not report on the interction between tmosphericlly derived orgnic-n nd N-minerlistion, it is suggested in our study to hve contributed to N-fluxes. A lck of lrge differences between the tretment extremes during the period under study (Figure 7.3) my lso indicte net N minerlistion to be lrgely dependent on intrinsic site fctors rther thn upon soil nd site mngement differences. Intrinsic site 138

169 differences overriding tretment effects hs been observed in number of similr studies, reviewed in Crlyle et l. (1998b). The role tht the quntity nd form of tmospheric N inputs plys in N minerlistion processes needs further investigtion s this my drive the bility of site to immobilise nd conserve N during the inter-rottion CONCLUSION Our study shows reltively blnced N fluxes in the undisturbed stnding crop where relese nd consumption of N occurs t low levels, incresing during high soil moisture conditions. Nitrogen gined with rinfll under the cnopy (throughfll) ppered to be rpidly minerlised, nitrified nd possibly removed with tree uptke. Site disturbnce through clerfelling incresed the rte of N relese through minerlistion processes through n improvement in soil moisture nd temperture regimes nd the provision of dditionl substrte. Incresed soil moisture nd NO 3 -N concentrtion in the soil of the felled plots induced n increse in NO 3 -N leching, displcing N from upper to lower soil lyers. Burning of residues in this low-n system reduced soil net N minerlistion by nerly 53% over the 20 month period. The first six months of tree growth ws initilly improved on the burnt plots, which suggest tht fctors other thn N supply were more limiting to growth during this phse of crop growth. Burning ws lso less N conservtive. The loss of N from the Burn tretment through burning nd reduced minerlistion my limit the vilbility of N to the trees in the burnt plots lter in the rottion. Evidence of lrge reduction in N minerlistion in the burnt tretment from clerfelling until cnopy closure cn be seen in Tble 7-2. This loss of N during burning nd the lrge quntities of more mobile N in the sh fter burning cn increse the risk of N loss through erosion (wind nd wter). This will reduce future N vilbility to the current crop nd negtively impct on soil N pools nd fluxes in the long-term (O'Connell et l. 2004; Smill et l. 2010). The higher N minerlistion rtes, remining t net positive fter residue retention will enble more sustined supply of N to the new crop throughout the rottion. Conservtion of the residues nd the subsequent slower relese of N in the No-Burn tretment is more sustinble prctice on this site thn residue burning s this llows more time for N tht minerlised under residue retention to be tken up nd stored in the tree biomss. Losses of N in this low-n system cn lso be substntilly off-set by tmospheric deposition. Despite residue retention being 139

170 more N conservtive prctice, N is still lost from the system during hrvesting nd potentil deep dringe leching losses. Further residue mngement strtegies imed t N retention in the soil nd rpid recovery need to be investigted for N conservtion on such sites. 140

171 7.7. APPENDIX Temperture ( C) () (b) (c) SC Burn No-Burn (d) /08 12/08 03/09 06/09 09/09 12/09 03/10 06/10 Dte (mm/yy) Appendix 7-1: Men dily ir nd soil (2.5 cm depth) tempertures for the stnding crop (SC), single residue (No-Burn) nd burned residue (Burn) tretments. Dshed lines represent () clerfelling; (b) residue burning; (c) plnting nd (d) cnopy closure () (b) (c) SC Burn No-Burn (d) Temperture ( C) /08 12/08 03/09 06/09 09/09 12/09 03/10 06/10 Dte (mm/yy) Appendix 7-2: Men dily ir nd soil (22.5 cm depth) tempertures for the stnding crop (SC), single residue (No-Burn) nd burned residue (Burn) tretments. Dshed lines represent () clerfelling; (b) residue burning; (c) plnting nd (d) cnopy closure. 141

172 120 () (b) (c) (d) Extrctble minerl NH 4 -N (kg h -1 ) SC Burn No-Burn 0 09/08 12/08 03/09 06/09 09/09 12/09 03/10 06/10 Dte (mm/yy) Appendix 7-3: Minerl NH 4 -N in the 0-30 cm soil lyer. Tretments re stnding crop (SC), residue retention (No-Burn) nd burned residue (Burn). Dshed lines represent the times of () clerfelling; (b) residue burning; (c) plnting nd (d) cnopy closure. I-brs represent lest significnt differences (LSD 5% ). 40 Extrctble minerl NO 3 -N (kg h -1 ) SC Burn No-Burn /08 12/08 03/09 06/09 09/09 12/09 03/10 06/10 Dte (mm/yy) Appendix 7-4: Minerl NO 3 -N in the 0-30 cm soil lyer. Tretments re stnding crop (SC), residue retention (No-Burn) nd burned residue (Burn). Dshed lines represent the times of () clerfelling; (b) residue burning; (c) plnting nd (d) cnopy closure. I-brs represent lest significnt differences (LSD 5% ). 142

173 Appendix 7-5: Cumultive (kg h -1 ) mmoni, nitrte nd net nitrogen fluxes in-field cores t 0-5, 5-15 nd cm soil depths from clerfelling to cnopy closure of the new crop. Flux Tretment Depth Net Minerlistion No-Burn Burn Stnding crop Clerfelling to plnting Plnting to cnopy closure Clerfelling to cnopy closure NO 3 -N NH 4 -N Net-N NO 3 -N NH 4 -N Net-N NO 3 -N NH 4 -N Net-N Deposition minus leching No-Burn Burn Stnding crop Root Uptke No-Burn Burn Stnding crop

174 Appendix 7-6: Cumultive (kg h -1 ) mmoni nd nitrte fluxes in-field cores t 0-30 cm soil depth from clerfelling to cnopy closure of the new crop. Clerfelling to Plnting to Clerfelling to Flux Tretment plnting cnopy closure cnopy closure NO 3 -N NH 4 -N NO 3 -N NH 4 -N NO 3 -N NH 4 -N No-Burn Net Burn Minerlistion Stnding crop Deposition minus leching Root Uptke No-Burn Burn Stnding crop No-Burn Burn Stnding crop Appendix 7-7: Ammoni fluxes (kg h -1 ) in-field cores t 0-30 cm soil depth for ech mesured dte. Net Minerlistion Deposition minus leching Root Uptke Stnding Stnding Stnding Dte crop No-Burn Burn crop No-Burn Burn crop No-Burn Burn 13/02/ /03/ /05/ /10/ /11/ /12/ /01/ /02/ /03/ /04/ /05/ /06/ /07/ /09/ /10/ /11/ /12/ /01/ /02/ /03/ /05/ /06/

175 Appendix 7-8: Nitrte fluxes (kg h -1 ) in-field cores t 0-30 cm soil depth for ech mesured dte. Net Minerlistion Deposition minus leching Root Uptke Stnding Stnding Stnding No-Burn Burn No-Burn Burn Dte crop crop crop No-Burn Burn 13/02/ /03/ /05/ /10/ /11/ /12/ /01/ /02/ /03/ /04/ /05/ /06/ /07/ /09/ /10/ /11/ /12/ /01/ /02/ /03/ /05/ /06/

176 CHAPTER 8: CONTRIBUTION OF GRAVITATIONAL LEACHING TO INTER-ROTATION NUTRIENT FLUXES ABSTRACT Little is known bout the fte nd sustinbility of nutrients during the intensively mnged inter-rottion period of highly productive short rottion clonl Euclyptus grown on the sndy soils of the Zululnd costl ecosystem in South Afric. A study ws designed to compre grvittionl nutrient leching within the top meter of soil in n undisturbed stnding crop of mture clonl Euclyptus with djcent clerfelled res subjected either to prescribed residue burning or to residue retention (brodcsting). Leching in the undisturbed crop decrese strongly with depth s the mture trees utilised most of the nutrients nd wter resulting in smll losses beyond 100 cm depth. Clerfelling overll induced short-lived spike in nitrogen nd ction leching compred with the low leching losses in the undisturbed crop. The incresed felled tretment leching beyond 100 cm ws only significnt during the period fter clerfelling through the temporry unplnted period to six months fter the new crop ws plnted. Soil wter nd nutrient uptke with new crop growth grdully reduced soil moisture content nd soil solution nutrient concentrtions reducing leching in the felled plots to levels similr to the undisturbed crop. Residue burning incresed the pools of C nd Mg in the soil, but did not significntly lter their leching. Although nitrte leching incresed during the first few months fter burning nd potssium leching decresed, these chnges were smll compred to the effect of clerfelling on leching. Losses of nutrients ssocited with biomss loss during clerfelling nd residue burning in ddition to clerfelling induced leching loss my increse the risk of nutrient depletion t our site. These high nutrient losses, reltive to smll pools sizes t this site my ct s severe limittion to future tree productivity. The durtion of incresed post-felling leching losses my however be reduced through nutrient conservtion strtegies, such s erly reestblishment nd fertilistion. This study my indicte similr soils fforested cross the Zululnd region to be t risk of nutrient depletion if not mnged to conserve nutrients during the clerfelling nd erly estblishment phse. 5 Submitted for publiction 146

177 8.2. INTRODUCTION The residues nd litter lyer remining fter clerfelling plnttion forest contins lrge quntities of nutrients, their retention viewed s necessry to ensure present nd future nutrient supply (Crlyle et l. 1998b; Pitek nd Allen 1999). While number of residue mngement options re vilble for Euclyptus plnttions, including retention, displcement nd removl (du Toit et l. 2004; Smith nd du Toit 2005) residue burning fter clerfelling is often prcticed s mens of creting clen site tht hs lower wildfire risk, nd is esier to plnt nd mnge. However, concerns re being rised within the South Africn forestry industry bout the impcts of residue burning on tree productivity in current nd future rottions. Lrge losses of plnt nutrients ssocited with residue burning, prticulrly nitrogen (N) nd crbon, cn be detrimentl to nutrient poor ecosystems (Morris 1986; du Toit nd Scholes 2002; Ndel 2005), while beneficilly reducing N in polluted ecosystems (Gomez-Rey et l. 2007). Furthermore, wind nd wter displcement of sh from burning followed by incresed soil erosion nd leching due to loss of soil surfce cover nd incresed soil hydrophobicity, my excerbte nutrient losses nd eventully deplete soil nutrients (Pilkington et l. 2007). Residue retention is sometimes viewed in negtive light s it increses the fuel-lod of site, incresing the risk of wildfire. This risk is gretest on cooler, drier sites where residues persist through slow brekdown. The fllow inter-rottion period fter clerfelling my lso be time of incresed nutrient leching or loss with dringe, becuse lrge quntities of nutrients re relesed through residue leching nd more rpid residue decomposition, long with incresed soil moisture nd lck of plnt uptke (Fisher nd Binkley 2000; Gonclves et l. 2007; O'Hehir nd Nmbir 2010). Losses through leching or displcement of nutrients my be greter fter burning s the dditionl mobile nutrients relesed in the sh dd to nutrients relesed during decomposition of the old crop root system (Powers et l. 2005). Residue nd litter stores of crbon nd nutrients re more importnt on sndy soils tht re chrcteristiclly low in cly, orgnic crbon nd fertility (Rosenstruch 1938; Noble et l. 2005). The retention, displcement or loss of residues during the inter-rottion phse cn significntly impct the nutrients dynmics of site (O'Hehir nd Nmbir 2010). Impcts of such losses cn be lrge on sndy soils where soil nutrient pools re smll within the region where fine-root density is gretest (top 50 cm of the soil (Smethurst nd Nmbir 1990; Crlyle nd Nmbir 2001)) nd the bility of soil to retin nutrients is low. The sndy soils of the 147

178 Zululnd costl ecosystem in South Afric re chrcterised by low cly nd orgnic crbon content, low nutrient nd wter storge cpcity nd high dringe rtes (Hrtemink nd Hutting 2005). These re chrcteristics tht my ultimtely limit nutrient vilbility nd consequently reduce stnd productivity. Such soils hve limited bility to buffer chemicl, physicl nd orgnic (biologicl) chnges nd re t risk of becoming degrded. In ddition, the Zululnd cost is lso chrcterised by high tempertures nd rinfll nd low vpour pressure deficit tht results in rpid growth of trees, short rottion lengths (six to seven yers), high nutrient demnd nd rpid litter nd residue decomposition. This my led to ccelerted loss of orgnic mtter nd nutrients, s nutrients tht re not tken up fter mobilistion my be displced through leching to greter depths in the soil profile or completely lost to deep dringe. This study imed t compring displcement of nutrients due to leching with grvittionl soil wter dringe within the top meter of the soil between stnding crop of mture clonl Euclyptus nd djcent clerfelled res subjected to burning nd residue retention (brodcsting slsh). This study drws from number of component studies within lrger experiment tht imed to determine the impct of site mngement on long-term sustinble productivity of clonl Euclyptus, with respect to nturl nd mngement induced nutrient losses nd gins MATERIALS AND METHODS Mterils nd methods for this chpter tht re common to other chpters re given in CHAPTER RESULTS Rinfll nd dringe volumes Wter collected s rinfll nd cnopy dringe (throughfll nd stemflow) (Appendix 8-1) with predicted soil wter dringe for ech tretment nd ech depth (CHAPTER 4) Dt ws cumulted cross the time periods felling to burning, burning to six months fter plnting (i.e. stnd ge 0.5 yers), from stnd ge six months to one yer (cnopy closure) nd from stnd ge 148

179 one yer to 18 months (1.5 yers). The durtion of these 4 monitoring periods were 4, 11, 6 nd 6 months respectively, thus totlling to 27 months for the project. Only one dringe event occurred between burning nd plnting, which hs been included s prt of the burning to 0.5 yers cumultive totl. Due to interception under the tree cnopy, cnopy dringe ws slightly lower thn rinfll. Totl dringe predicted t ech depth ws initilly lower in the stnding crop tretment t ech depth, but s tree growth commenced dringe in the felled tretments becme similr to dringe in the stnding crop tretment. The cumultive dringe in the stnding crop tretment ws considerbly lower t depth thn tht in the felled tretments between felling nd cnopy closure Nutrient concentrtions All 15 cm stnding crop tretment N concentrtions tended to follow cnopy dringe N concentrtions (Figure 8.1). Soil solution NH + 4 concentrtions (Figure 8.1A nd B) were generlly higher in the stnding crop tretment thn in the felled tretments t ll depths, for the + period between felling nd cnopy closure of the new crop (1 yer old). Soil solution NH 4 concentrtions were similr between the Burn nd No-Burn tretments t ll depths. Concentrtions decresed with depth cross ll tretments. Soil solution mmonium nd nitrte, s well s volume weighted NH + 4 nd NO - 3 in rinfll nd cnopy dringe hd prticulrly low concentrtions during Jn-Feb NO - 3 concentrtions tended to be lower in the stnding crop thn the felled tretments t ll depths (Figure 8.1C nd D). A rise in NO - 3 concentrtions t ll depths fter felling ws mintined until round six months fter felling. Concentrtions of NO - 3 were lowest t 100 cm depth in the Burn nd No-Burn tretments. NO - 3 ws higher in the Burn tretment between plnting nd 0.5 yers fter plnting t ll depths. After this, NO - 3 concentrtions were similr in ll tretments. Orgnic-N concentrtions (Figure 8.1E nd F) generlly decresed with depth, with concentrtions in the No-Burn tretment tending to be higher on verge thn the Burn tretment t ll depths. Orgnic-N levels were initilly low in the felled tretments round the time of plnting, but incresed to levels similr to the stnding crop tretment towrds the end of the study. 149

180 NH 4 -N (mg L -1 ) () (b) (c) (d) Rinfll Cnopy SC No-Burn Burn NH 4 -N (mg L -1 ) () (b) (c) (d) 50 cm SC No-Burn Burn 100 cm SC No-Burn Burn (A) 0 (B) 0 8 () (b) (c) (d) 8 () (b) (c) (d) 7 7 NO 3 -N (mg L -1 ) (C) /08 12/08 03/09 06/09 09/09 12/09 03/10 06/10 09/10 12/10 Dte (mm/yy) NO 3 -N (mg L -1 ) (D) /08 12/08 03/09 06/09 09/09 12/09 03/10 06/10 09/10 12/10 Dte (mm/yy) 150

181 Orgnic-N (mg L -1 ) () (b) (c) Rinfll (d) Cnopy SC No-Burn Burn Orgnic-N (mg L -1 ) () (b) (c) (d) 50 cm SC No-Burn Burn 100 cm SC No-Burn Burn /08 12/08 03/09 06/09 09/09 12/09 03/10 06/10 09/10 12/ /08 12/08 03/09 06/09 09/09 12/09 03/10 06/10 09/10 12/10 (E) Dte (mm/yy) (F) Dte (mm/yy) Figure 8.1(A-F): Soil solution nd volume weighted rinfll nd cnopy dringe NH 4 -N (), NO 3 -N (c) nd orgnic-n (e) concentrtions mesured t 15 cm depth nd NH 4 -N (b), NO 3 -N (d) nd orgnic-n (f) concentrtions mesured t 50 cm nd 100 cm depths for felled nd stnding crop (SC) res of the study site. Dshed verticl lines represent () felling; (b) residue burning; (c) plnting nd (d) cnopy closure. 151

182 Rinfll ws enriched with bse ctions K +, C 2+, Mg 2+ nd N + in pssing through the tree cnopy (Figure 8.2), but ws mrkedly more concentrted in the soil solution t ll depths thn in cnopy dringe. Soil solution bse ction concentrtions were ltered fter felling nd gin fter residue burning. With few exceptions, ction concentrtions were two to eight-fold higher in the soil solution t ll depths thn in the cnopy dringe nd rinfll. Soil solution K + tended to be lower in the stnding crop tretment thn in the felled tretments t ech depth (Figure 8.2A nd B). On verge, differences in K + concentrtion between the stnding crop nd Burn tretment were smll t the 15 cm depth, while K + concentrtion incresed with depth for the felled tretments. Concentrtions of K + were, on verge, higher in the Burn thn in the No-Burn tretment t 15 cm depth, becoming lower in the Burn t 50 nd 100 cm depth. K + concentrtions tended to decrese from plnting towrds the end of the study period in ll tretments, decresing in the felled tretments to below the stnding crop tretment concentrtions. Soil solution C 2+ concentrtions (Figure 8.2Cnd D) were mostly lower in the stnding crop thn the felled tretments cross ll depths. Clcium concentrtion decresed with depth in the stnding crop tretment, but incresed with depth in both felled tretments. The concentrtion of C 2+ peked t 15 nd 50 cm depths fter burning nd remined higher thn the No-Burn until six months fter plnting. Concentrtions in the felled tretments becme similr to those in the stnding crop tretment from round six months fter plnting. Soil solution Mg 2+ concentrtion (Figure 8.2E) incresed t 15 cm depth in the Burn tretment fter burning, while remining similr between the stnding crop nd No-Burn tretments t 15 cm depth. Soil solution Mg 2+ concentrtion incresed with depth in the felled tretments (Figure 8.2F). These concentrtions peked t 100 cm depth in both felled tretments prior to burning, but grdully reduced to lower thn the stnding crop tretment few months fter plnting. Soil solution N + concentrtions decresed reltive to the stnding crop tretment from few months fter felling (Figure 8.2G nd H). Concentrtion of N + incresed with depth in ll tretments, prticulrly in the stnding crop tretment. Concentrtions were similr in the Burn nd No-Burn tretments throughout. 152

183 K (mg L -1 ) () (b) (c) (d) Rinfll Cnopy SC No-Burn Burn K (mg L -1 ) () (b) (c) (d) (A) () (b) (c) (d) (B) cm SC No-Burn Burn 100 cm SC No-Burn Burn () (b) (c) (d) C (mg L -1 ) C (mg L -1 ) (C) 0 09/08 12/08 03/09 06/09 09/09 12/09 03/10 06/10 09/10 12/10 Dte (mm/yy) (D) /08 12/08 03/09 06/09 09/09 12/09 03/10 06/10 09/10 12/10 Axis Title 153

184 25 () (b) (c) (d) 25 () (b) (c) (d) Mg (mg L -1 ) Rinfll Cnopy SC No-Burn Burn Mg (mg L -1 ) cm SC No-Burn Burn 100 cm SC No-Burn Burn 5 5 (E) 0 70 () (b) (c) (d) (F) 0 70 () (b) (c) (d) N (mg L -1 ) N (mg L -1 ) /08 12/08 03/09 06/09 09/09 12/09 03/10 06/10 09/10 12/ /08 12/08 03/09 06/09 09/09 12/09 03/10 06/10 09/10 12/10 (G) Dte (mm/yy) (H) Dte (mm/yy) Figure 8.2(A-H): Soil solution nd volume weighted rinfll nd cnopy dringe K + (), C 2+ (c), Mg 2+ (e) nd N + (g) concentrtions recorded t 15 cm depth nd K + (b), C 2+ (d), Mg 2+ (f) nd N + (h) concentrtions recorded t 50 nd 100 cm depths for felled nd stnding crop (SC) res of the study site. Dshed verticl lines represent () felling; (b) residue burning; (c) plnting nd (d) cnopy closure. 154

185 Soil solution mss flux NH 4 -N leching t 15 cm in the stnding crop tretment ws more thn tht dded with cnopy dringe (Appendix 8-2), decresing strongly with depth (Appendix 8-2:). In contrst, less NH 4 -N leched to 15 cm depth in the felled tretments thn dded by rinfll. Significntly less NH 4 -N leched to 15 cm depth under the felled tretments thn under the stnding crop tretment. From felling to six months fter plnting more NH 4 -N leched to 50 cm in both felled tretments thn did in the stnding crop tretment. This lrger NH 4 -N leching persisted to cnopy closure t 50 nd 100 cm in the No-Burn tretment. Fluxes therefter becme similr in felled tretments t 15 nd 50 cm depth nd cross ll tretments t 100 cm. Leching of NO 3 -N (Appendix 8-2b) t the pltes contrsted NH 4 -N leching nd NO 3 -N leching vlues were much lrger in mgnitude thn NH 4 -N leching (Appendix 8-2). Cnopy dringe NO 3 -N ws similr to 15 cm stnding crop leching, nd this ws significntly lower thn leching under the felled tretments. Fr more NO 3 -N ws leched in the felled tretments thn ws dded with rinfll. Leching of NO 3 -N tended to decrese with depth cross ll tretments. Nitrte leching in burnt nd unburnt plots were sttisticlly similr cross ll times nd depths with one notble exception: the vlues t ll depths for the period between burning nd six months fter plnting, where burnt plots showed significntly greter NO 3 -N leching. Nitrte leching lso become similr between tretments t ech depth from round six months fter plnting. More orgnic-n ws leched pst 15 cm thn dded by cnopy dringe to the stnding crop nd rinfll in the felled tretments. Orgnic-N decresed with depth cross ll tretments. It ws significntly less t depth in the stnding crop tretment thn in both felled tretments (Appendix 8-2c). Orgnic-N leching ws initilly lower in the felled tretments thn the stnding crop tretment t 15 cm depth. Significntly less orgnic-n ws leched to 50 nd 100 cm in the Burn tretment thn the No-Burn tretment fter burning nd up to six months fter plnting. The quntity of bse ctions K (Appendix 8-3), C (Appendix 8-3b) nd Mg (Appendix 8-3c) leched to 15 cm in the stnding crop tretment ws more thn tht dded by cnopy dringe, nd more t 15 cm in the felled tretments thn tht dded by rinfll. Cnopy ction dringe ws lrger thn rinfll ction ddition. Felled tretments recorded highest levels of potssium leching beyond 100 cm, most of the significnt differences reltive to the stnding crop 155

186 occurring during the period from burning to 6 months. Leching of K decresed strongly with depth in the stnding crop tretment, in contrst to the felled tretments. More C ws leched t ll depths in the felled tretments thn the stnding crop tretment from felling to six months fter plnting. Similr quntities of C were leched in the felled tretments for most periods nd depths, with the totl for burnt tretments being slightly greter thn the unburnt tretment. Leching of C took plce in similr quntities t ech depth in the felled tretments, but in strong contrst, diminished with depth under the stnding crop tretment. Mgnesium leched in the felled tretments ws only higher thn tht leched in the stnding crop tretment t 15 cm depth between burning nd six months fter plnting, but lower for ll other periods. Mgnesium leching strongly decresed with depth in the stnding crop tretment. Mgnesium leching incresed with depth in the felled tretments with lrge increses during the burning to six months period. Leching fluxes in Burn nd No-Burn tretments t 50 nd 100 cm depth were similr. Nitrogen specition chnged from being lrgely comprised of NH + 4 ions in the stnding crop tretment to NO - 3 ions in the felled tretments between felling nd six months fter plnting (Tble 8-1). This specition in the felled tretments reverted bck to become similr to the stnding crop tretment fter the trees reched six months of ge. In totl, significntly greter quntity of N ws leched beyond 100 cm from the felled tretments thn from the stnding crop tretment (Tble 8-1). Burning resulted in more NO - 3 ions nd mrginlly less NH + 4 ions leching beyond 100 cm in thn in the No-Burn tretment. Significntly more N ws leched beyond 100 cm in the Burn tretment thn in the No-Burn tretment between felling nd six months fter plnting (Tble 8-1). Leching of K +, C 2+, Mg 2+ nd N + ions ws significntly lrger in the felled tretments thn in the stnding crop tretment t ech depth between felling nd six months fter plnting (Tble 8-1). Significntly more K + ions nd less C 2+ ions were leched beyond 100 cm in the No-Burn nd thn in the No-Burn tretment between felling nd six months fter plnting (Tble 8-1). These differences becme less pprent fter the trees were six months of ge, however, differences in C 2+ nd N + ion leching remined significnt. Reltive to the Stnding crop, C 2+ leching ws mrginlly higher nd N + lower in the felled tretments from six to 18 months of tree ge. 156

187 Tble 8-1: Leching (kg h -1 ) beyond 100 cm depth for felled nd stnding crop (SC) res of the study site over the felling to 0.5 yers fter plnting period (15-months), nd the 0.5 to 1.5 yers fter plnting period (12-months, fter cnopy closure). Different, b, c superscripts denote significnce difference between tretments t LSD 5% Stnding crop Burn No-Burn Tretment F. probbility Felling to 0.5 yers fter plnting Stnding crop Burn No- Burn Tretment F. probbility 0.5 to 1.5 yers fter plnting Orgnic-N b NH 4 -N b 2.4c < NO 3 -N b K b 32.2c C b 39.4b < <.001 Mg b 85.6b N b 126.4b < b 11.9b <.001 Accumultion of nutrients into the bove ground biomss nd nutrient storge in the litter lyer remined lrge in the stnding crop tretment (Tble 8-2). Nutrient uptke into the trees of the stnding crop tretment underwent rpid turnover through high rtes of litterfll. Litterfll hd not commenced by cnopy closure in the felled tretments. Residue burning resulted in lrge losses of nutrients (prticulrly N) from the residues (Tble 8-2). Very little sh ws present in the Burn tretment t cnopy closure (visul observtion), ll nutrients remining in the sh were ssumed to hve entered the soil with rinfll or lost through wind erosion. The No-Burn tretment hd lrge boveground N, C nd Mg nutrient pools t the end of the study (Tble 8-2). Nitrogen nd C bove-ground biomss pools t one yer fter plnting in the No-Burn plots were 60 nd 80% of the stnding crop pools respectively. Residue retention lso mintined lrge mounts of P nd Mg in the bove-ground biomss reltive to the stnding crop tretment. Burn tretment N nd C bove-ground biomss pools were 20% nd 7% of the stnding crop pools respectively t cnopy closure, consisting of live tree biomss lone. Limited K retention in the residues of the No-Burn tretment yielded similr K bove-ground biomss pools to the Burn tretment (Tble 8-2). 157

188 Tble 8-2: Above -ground nutrient pools t felling nd cnopy closure nd litterfll, burning loss nd tree biomss nutrient content occurring between felling nd cnopy closure of the new crop. Pool or Flux Tretment N P K C Mg N kg h -1 Pools t felling Stnding crop Felled plots Stnding crop Litter fll Tree biomss Burning loss Burn Tree biomss No-Burn Tree biomss Pools t Cnopy closure Stnding crop Burn* No-Burn *Surfce sh ws wshed into the soil nd no longer present nd remnnts were too smll nd vrible for ccurte smpling Soil nutrient pools nd fluxes Concentrtions of totl N, extrctble P nd exchngeble bse ctions decresed with depth nd exchngeble N concentrtion ws highest t ech depth in the stnding crop plots. A reduction in soil totl N pool sizes (Tble 8-3) occurred fter felling t ll depths, the totl N pool size in the felled tretments being significntly smller thn in the stnding crop tretment. The totl N pool size in the pre-felling ssessment seemed lower thn lter on in the stnding crop tretment. Exchngeble K pools were significntly lrger in the felled tretments thn the stnding crop tretment, ccounted for by lrge differences in the surfce to 15 cm soil lyer. These pools were significntly lrger in the No-Burn tretment thn the Burn tretment. Potssium levels in No-Burn tretment t the end of the study were however similr to those recorded t the strt. Clcium pool sizes were only significntly lrger in the Burn tretment compred to the No-Burn nd stnding crop tretments. The lrger totl C pool in the Burn tretment thn the other two tretments ws solely due to very lrge pool in the surfce 15 cm lyer. Differences between Burn nd No-Burn were not significnt. Pre-felling C nd Mg levels remined reltively constnt in the stnding crop tretment. The N pool ws significntly smller in the felled tretments t the end of the study thn in the stnding crop tretment. The reduced N pool size in the felled tretments compred with the stnding crop tretments ws due to pronounced decrese in N concentrtions t depth, prticulrly in the 50 to 100 cm lyer. Sodium concentrtions in the 50 to 100 cm lyer of the No-Burn nd the Burn tretments were round 9% nd 31% of stnding crop tretment vlues respectively t the end of the study. 158

189 Tble 8-3: Soil nutrient pools t incrementl depths between 0 nd 100 cm soil depth t the strt nd end of the study (kg h -1 ). Significnt differences (LSD 5% ) re shown between totl pool sizes (bold text) t the end of the study. Depth Bry 2 Slt-Exchngeble bse ction rnge (cm) Totl N extrctble P K C Mg N Pre-Felling Post cnopy closure Stnding crop Burn b b 1411 b b No-Burn b c b Fprob 0.3% 83% 1.5% 4.2% 53% < DISCUSSION Leching in n undisturbed stnding crop Nutrient leching, occurring to smll extent in the undisturbed stnding crop, ws dependnt on rinfll volume nd intensity, with dringe volumes constrined by high tree wter use (CHAPTER 4). An nnulised nutrient leching beyond 100 cm (felling to 1 yer) mounted to 4.2 kg h -1 N (1.3 kg h -1 of NH 4 -N, 0.5 kg h -1 NO 3 -N, 2.3 kg h -1 orgnic-n), 7.8 kg h -1 K, 4.0 kg h -1 C, 9.0 kg h -1 Mg nd 36.3 kg h -1 of N. Rpid wter use under the mture Euclyptus crop reduced dringe loss (CHAPTER 4) nd the possibility of deeper soil leching losses. A study in Congo on site with sndy soils similr to our study site showed rpid nutrient uptke by Euclyptus tree roots to limit dringe volumes under mture Euclyptus (Lclu et l. 2005), resulting in reltively smll frction of nutrients leched below 6 m reltive to those entering the soil. Nutrient leching t 15 cm ws fr higher thn nutrients dded by cnopy throughfll. Mobile nions to fcilitte the leching of bse ctions would hve been present in reltive bundnce in the form of sulphtes nd chlorides (not monitored in our study) due to proximity of the study 159

190 site to the ocen. Although leching ws most notble for the ctions, it ws the lrgest for Mg 2+ followed by K + nd C 2+. High levels of litterfll nd rpid decomposition t the soil surfce (dt submitted for publiction) dded to nutrients in the soil solution, incresing leching t 15 cm. Further dditions my hve occurred through rpid fine root turnover, lthough not recorded in our study. Nitrogen leching ws lso lrger t 15 cm depth thn in the throughfll, but not to the sme extent s N ws immobilised through soil microbil ctivity under the mture trees (dt submitted for publiction). Lclu et l. (2003) demonstrted tht lrge proportion of nutrient uptke by clonl Euclyptus re derived from litterfll nd root turnover. These nutrients, depending on their mobility in the soil, my lech through the upper soil lyers prior to uptke. Despite high leching t the soil surfce under the stnding crop, leching decresed strongly with depth (Appendix 8-2, Appendix 8-3). This demonstrtes displcement of nutrients occurring under the mture trees from the surfce to reltively deeper lyers. These nutrients re not lost from the soil, but re either tken up by the trees, ccumulted in the soil t depth or immobilised in orgnic complexes Post felling Soil nutrient leching incresed few months fter clerfelling s lck of wter uptke fforded soil moisture rechrge nd more rpid dringe with rinfll. This leching (nd high ction loss) beyond 100 cm in felled tretments ws fr greter thn rinfll input. This ws brought bout through boveground biomss dditions, ccelerted minerlistion processes, increses in mobile ccompnying nions such s NO - 3 nd lck of uptke further incresing soil solution concentrtions. Increses in soil temperture nd moisture following hrvesting triggers more rpid minerlistion nd nitrifiction of soil orgnic mtter (Crlyle 1993), process tht - occurred t our site nd cn be inferred from the bundnce of NO 3 fter felling in Figure 8.1..The more mobile K + nd NO - 3 ions were rpidly leched wheres C 2+ nd NH + 4 ions leched slower, being less mobile in the soil. This lower mobility reduced C 2+ leching losses fter burning, s ws evident by higher clcium concentrtions in the upper soil lyers of the burnt plots. Felled tretments hd high leching of N, K, C, Mg nd N beyond 100 cm, which ws most significnt during the period fter felling nd burning to 6 months fter the new crop ws plnted. The incresed leching during the fllow nd erly growth period occurred when tree uptke ws not sufficient to reduce dringe nd utilise the concentrted soil solution. 160

191 Nitrogen leching beyond 100 cm soil depth incresed during the first few months fter burning through n increse in NO - 3 ions in the soil solution (Figure 8.1b). Although K + leching ws reltively lower in the Burn thn the No-Burn, C + nd Mg 2+ leching were not substntilly ltered following burning. A reduction in K + leching loss beyond 100 cm in the Burnt plots my hve been s result of more rpid erly tree growth in the Burn tretment. Leching t 100 cm in the felled plots however differed strongly from stnding crop plots (Tble 8-1) s incresed soil wter dringe crried nutrients from shllower lyers to deeper lyers. In ddition, nutrients ccumulted in the soil t depth were relesed during incresed dringe nd cesstion of root uptke. Nutrient nd wter uptke grdully becme lrge enough to reduce leching during the rpid tree cnopy development phse of ll felled plots fter plnting (Tble 8-1) Atmospheric inputs Atmospheric inputs (CHAPTER 5) plyed mjor role in soil solution chemistry under the stnding crop. High NH 4 -N concentrtions in cnopy dringe trnslted into high NH 4 -N concentrtion in the soil lechte of the stnding crop tretment. This is typicl of N rich sites (fertile soils) tht receive lrge N inputs (Slesk et l. 2009). Nitrogen minerlistion (formtion of NH 4 -N) ws dominnt in the stnding crop throughout the study period (dt submitted for publiction). High litterfll nd litter lyer decomposition lso plyed role. In contrst with the stnding crop, peks in rinfll NH 4 -N corresponded with peks in soil solution NO 3 -N of the felled tretments. Incresed N minerlistion followed by rpid nitrifiction my hve been responsible for the lrge increses in NO 3 -N leching in felled plots Soil ction ccumultion The increse in soil ction concentrtions with depth (prticulrly N) in the stnding crop tretment of our study (Tble 8-3) my indicte n ccumultion of ctions in the deeper soil through limited dringe. These ctions gined through tmospheric deposition, cnopy exchnge nd litterfll my explin the high N concentrtions in the soil nd soil solution (Myer et l. 2000; de Vries et l. 2003). Ction deposition in our study ws less thn tht lost through leching beyond 100cm in the mture crop. This ws likely through combintion of wter nd nutrient uptke tht limited the movement of these nutrients through the soil profile. Lower rinfll (dry periods) immobilise soil nutrients nd result in n ccumultion of nutrients 161

192 in the soil, wheres higher rinfll periods promote leching of mobile nutrients, incresing losses nd reducing soil nutrient pools (Johnson et l. 2002). A portion of the ctions in the soil solution under the felled tretments my therefore hve originted from the relese of ctions ccumulted in the soil under the previous crop. This is shown s n initil increse in the leching of these elements (prticulrly N) s consequence of incresed dringe fter felling. It is lso likely tht relese of nutrients from the previous crop tree root system ws delyed, incresing soil solution concentrtions few months fter clerfelling. A short dely in decomposition of old tree roots my therefore hve contributed to delyed increse in soil solution concentrtions (Stevens et l. 1993; Powers et l. 2005) Study outcome The increse in leching ws short lived nd constrined to the period directly following clerfelling, decresing in mgnitude fter plnting to become similr to the mture crop by the cnopy closure stge. Assuming men stnding crop dt to represent potentil future leching nd tmospheric deposition, Figure 8.3, hypotheticl scenrio bsed on K dt demonstrtes this short lived yet reltively substntil post-hrvest leching effect. A similr pttern emerges - for NO 3 nd ctions recorded in this study, except for the incresed NO - 3 nd decresed K + - leching fter burning. The dditionl increse in NO 3 fter felling nd burning ws smller thn the increse cused through clerfelling lone nd lso short-lived. Atmospheric ddition of ll nutrients ws then lrger thn leching losses under the mture crop t our site, which cn be relted to n ccumultion of nutrients in the soil tht re not tken up or immobilised in the mture crop. It is pprent from the dt in this study tht the effect of clerfelling on leching ws more substntil thn the effect of residue mngement on leching. A combintion of hrvesting nutrient losses, oxidtive nd prticulte losses during burning, when dded to the smll increse in leching losses fter burning my rpidly reduce nutrient pools on our site if ccumulted over successive rottions. These losses will primrily occur from orgnic or biomss nutrient pools, which re crucil to mintining nutrient supply to trees on such infertile sites (Lclu et l. 2003). Further to this, loss of nutrients from the top 100 cm of soil is criticl fter clerfelling nd during the estblishment phse of the new crop, given the smll soil nutrient pools, rpid decomposition of residue fter clerfelling, incresed leching losses nd lrge erly tree growth 162

193 demnds. Nutrient leching is therefore concern since it occurs during smll portion of the rottion nd my be further reduced through minimising the durtion of the temporry unplnted period fter felling nd through ccelerting erly tree growth Leching (kg h -1 yr -1 ) Deposition Burn No-Burn Stnd ge (yers) Figure 8.3: A hypotheticl projection of nnul leching nd tmospheric inputs to rottion end using mture crop dt for projected yers (bsed on K dt). Although root growth my extend beyond 3m depth fter the first yer (Bouillet et l. 2002) this soil lyer crries the bulk of nutrient scvenging fine roots (Lclu et l. 2001). Deeper explortion to depths of round 85% of the tree height under non-limiting soil conditions (Christin et l. 2011) my imply rooting depth in the new crop of round 10 m in our study. Nutrients lost to deeper lyers my therefore be extrcted by deep roots nd redistributed to shllower soil lyers through litterfll nd root turnover (McCulley et l. 2004). d Silv et l. described reltive uptke of nitrte trcers nd C nd K nlogue trcers, giving evidence of significnt nutrient uptke t 3 m depth. Additionl increses in N losses from the system s whole my lso occur s consequence of orgnic mtter lost during burning diminishing the soils bility to store N (Pilkington et l. 2007). Conservtion of residue N pools with reduced N leching loss beyond 100 cm my help to conserve N in the long term. Residue retention ws shown to reduce N leching in Pinus rdit grown on sndy soils (Crlyle et l. 1998) nd under Euclyptus (Gómez-Rey et l. 2008), nd ws further improved with residues brek-up nd soil mixing. This ws s result of n improved soil crbon sttus. 163

194 8.6. CONCLUSION Leching losses in generl were reltively smll under the undisturbed mture crop nd my be of little consequence to the nutrient sttus of the site if such levels occur throughout the rottion. Dt presented in this study demonstrted lrge, short durtion increse in leching s result of clerfelling which ws rpidly reduced to pre-felling levels by the newly plnted crop. Residue mngement hd little effect on leching. Prescribed burning, n extreme residue mngement prctice resulted in temporry increse in nitrte leching which ws smll compred to losses ssocited with clerfelling lone. The most substntil nutrient losses in this study were incurred through hrvesting biomss removl, residue burning (oxidtion) nd clerfelling induced leching. Given the size of the soil nutrient pools reltive to nutrient losses t our study site it is likely tht incresed nutrient losses will ccelerte decline in these site nutrient pools. Residue burning on such nutrient poor soils should be voided, not so much for the increse in leching, but for the oxidtive nd prticulte loss during burning in ddition to clerfelling losses. This my result in long-term site nutrient decline nd reduce the inherent productivity potentil of the site. Incresed leching of N nd ctions fter clerfelling my be of concern to plnttion mngers s this will decrese short-term nutrient vilbility to the new crop. Given the high nutrient demnd during erly tree growth nd the high risk of dditionl nutrient loss during the temporry unplnted period, mngement prctices tht reduce losses nd promote nutrient retention need to be encourged (Nzil et l. 2002). Rpid re-estblishment nd rpid erly growth my fcilitte the conservtion of soil nutrients during this high loss period. Hrvesting nd biomss removl will however, eventully deplete the nutrients on these infertile soils, unless nutrients re replced through fertilistion (minerl, orgnic or N-fixtion) or rottion lengths re extended to llow for nturl replenishment. 164

195 8.7. Appendix Predicted Dringe (mm) yr to 18 months 0.5 yrs to 1.0 yr Burning to 0.5 yrs Felling to Burning 0 SC Burn 15 cm No-Burn SC Burn 50 cm No-Burn SC Burn 100 cm Depth (cm) & Tretment No-Burn Rinfll Cnopy Dringe Appendix 8-1: Rinfll, cnopy dringe nd Hydrus predicted dringe volumes t 15, 50 nd 100 cm for felled nd stnding crop (SC) res of the study site. Ech portion of the brs represents dringe predicted for ech period. 165

196 NH4-N (kg h -1 ) () NO3-N (kg h -1 ) SC b b b b Burn 15 b b b c b b No- Burn b c SC b b Burn 50 b b b b b b No- Burn c b b SC Burn 100 Depth (cm) & Tretment b c 1.0 yr to 18 months 0.5 yrs to 1.0 yr Burning to 0.5 yrs Felling to Burning b b No- Burn b c LSD (5%) Cnopy Rinfll 1.0 yr to 18 months 0.5 yrs to 1.0 yr Burning to 0.5 yrs Felling to Burning (b) SC b Burn 15 b No- Burn SC b Burn 50 b No- Burn SC b Burn 100 Depth (cm) & Tretment b No- Burn LSD (5%) Cnopy Rinfll 0rgnic-N (kg h -1 ) (c) b SC b b b Burn 15 b b b No- Burn b b b SC Burn 50 No- Burn Appendix 8-2: Totl N leched during ech time period t 15, 50 nd 100 cm nd dded with rinfll nd cnopy dringe for felled nd stnding crop (SC) res of the study site. Different, b, c superscripts denote significnce difference between tretments t ech depth for ech time period t LSD 5%. A LSD 5% stcked br is given to show significnt differences. b b b b SC Burn No- 100 Burn Depth (cm) & Tretment 1.0 yr to 18 months 0.5 yrs to 1.0 yr Burning to 0.5 yrs Felling to Burning LSD (5%) Cnopy Rinfll 166

197 K (kg h -1 ) () b b c SC Burn 15 No- Burn SC b b b b Burn 50 c b c b No- Burn SC b b b b Burn 100 Depth (cm) & Tretment b c c b No- Burn LSD (5%) 1.0 yr to 18 months 0.5 yrs to 1.0 yr Burning to 0.5 yrs Felling to Burning Cnopy Rinfll C (kg h -1 ) b b b b b b b c b b b c b 1.0 yr to 18 months 0.5 yrs to 1.0 yr Burning to 0.5 yrs Felling to Burning (b) 20 0 SC b Burn 15 b No- Burn SC b Burn 50 b No- Burn SC b Burn 100 Depth (cm) & Tretment b No- Burn LSD (5%) Cnopy Rinfll Mg (kg h -1 ) b b b b b c b b b b b b b b b b 1.0 yr to 18 months 0.5 yrs to 1.0 yr Burning to 0.5 yrs Felling to Burning (c) 20 0 SC b Burn 15 b No- Burn SC b Burn 50 b No- Burn Appendix 8-3: () K, (b) C, nd (c) Mg leched during ech time period t 15, 50 nd 100 cm nd dded with rinfll nd cnopy dringe for felled nd stnding crop (SC) res of the study site. Different, b, c superscripts denote significnce difference between tretments t ech depth for ech time period t LSD 5%. A LSD 5% stcked br is given to show significnt differences. SC b Burn 100 Depth (cm) & Tretment b No- Burn LSD (5%) Cnopy Rinfll 167

198 Appendix 8-4: Ammonium, nitrte nd orgnic nitrogen deposition nd leching t 15, 50 nd 100 cm soil depths cumulted for ech key period. Lest significnt difference (LSD) nd ANOVA F probbility given for leching differences between tretments (kg h -1 ). Stnding Stnding Stnding Stnding Time period Felled Burn No-Burn Burn No-Burn Burn No-Burn LSD F pr. crop crop crop crop (yers) Cnopy Rinfll 15 cm 50 cm 100 cm NH 4 -N Felling to Burning Burning to to < to Felling to to NO 3 -N Felling to Burning <.001 Burning to < to to Felling to < to Orgnic N Felling to Burning Burning to to to Felling to to

199 Appendix 8-5: Potssium, clcium nd mgnesium deposition nd leching t 15, 50 nd 100 cm soil depths cumulted for ech key period. Lest significnt difference (LSD) nd ANOVA F probbility given for leching differences between tretments (kg h -1 ). Stnding Stnding Stnding Time period Felled Burn No-Burn Burn No-Burn Burn No-Burn LSD F pr. crop crop crop crop (yers) Cnopy Rinfll 15 cm 50 cm 100 cm K Felling to Burning <.001 Burning to < to < to Felling to < to Stnding C Felling to Burning <.001 Burning to to < to Felling to < to Mg Felling to Burning <.001 Burning to < to to <.001 Felling to < to <.001 N Felling to Burning <.001 Burning to to < to <.001 Felling to < to <.001

200 CHAPTER 9: GENERAL DISCUSSION AND CONCLUSION 9.1. ABSTRACT The risk of nutrient depletion nd yield decline of successive rottions ws ssessed on the low nutrient nd orgnic mtter Aeolin snd derived soils in the subtropicl environment of the Zululnd costl ecosystem. The effects of hrvesting intensity, residue mngement nd fertilistion on pools nd fluxes of nutrients in the residues nd soil nd in new crop were mesured. Residue mngement included retention, burning nd doubling of residues nd whole tree removl. Plots of the old crop were left undisturbed to enble comprison of nutrient pools nd fluxes. Nutrient recycling nd tmospheric dditions (prticulrly N nd K) provided substntil nutrient supply to the undisturbed crop, while miniml leching losses beyond 1 m soil depth enbled nutrients to ccumulte in the biomss nd first meter of soil. Substntil nutrient (prticulrly N) nd biomss losses occurred during hrvesting nd residue burning. Soil pools of C nd Mg incresed fter burning, but soil N minerlistion ws reduced by 46%. Leching beyond 1 m incresed drmticlly fter clerfelling then decresed when trees were 6 months old, but ws not ffected by residue burning. Nutrient losses due to hrvesting nd burning were more substntil thn leching losses. Nutrient uptke during the erly rpid growth phse ws met primrily through residue nd litter decomposition. Erly growth ws not ltered by hrvesting intensity nd residue mngement, but incresed fter fertilistion. The risk of nutritionl productivity decline (projected over seven yer rottion) ws countered in this system through tmospheric inputs tht replced lrge proportion N, K, nd C lost through stem wood removl. However, Mg my be t risk s deposition ws too smll to replce losses. While not reflected in subsequent growth, evidence from nutrient flux studies suggested tht incresed nutrient removl through more intense hrvesting nd residue mngement cn increse the risk of subsequent rottion productivity decline. 170

201 9.2. INTRODUCTION The Zululnd costl plin is productive nd rpid timber growing re of gret vlue to the South Africn forestry industry (Louw 1997; du Toit et l. 2001). High productivity over consecutive six to seven yer rottions hs been relised on the nutrient poor (dystrophic) soils cross this region due to intensive mngement. An pprent lck of productivity decline on these low nutrient sites hs continued despite incresed losses induced through high productivity. This hs been questioned nd discussed by plnttion mngers nd reserchers for mny yers. Evidence linking hrvesting intensities nd residue mngement prctices with productivity decline in the subsequent rottion nd in the long-term hs been presented cross number of interntionl study sites with low litter nd soil N (Sint-André et l. 2008) (CHAPTER 2). These productivity declines were dependent on climtic (fst growth sites) nd edphic properties linked with soil orgnic mtter nd low nutrient vilbility t ech site (nutrient poor sites) (Corbeels et l. 2005). Evidence of productivity responses to soil crbon nd nutrient losses on nutrient poor (dystrophic) South Africn sites hs been limited to fertilistion t plnting (prticulrly N) studies. Substntil positive responses occurred on number of infertile sites cross Zululnd tht were degrded through intensive griculturl prctices or on sites tht possessed physiogrphicl constrints (du Toit et l. 2001; du Toit nd Oscroft 2003). These pst responses (discussed in CHAPTER 6) re perhps necdotl evidence tht sites cross this region were t one time nd still my be t vrious levels of griculturlly induced nutrient degrdtion. It lso demonstrtes the potentil for these sites to become degrded through intensive griculture The Study The study presented in this thesis ws therefore commissioned by members of the South Africn forestry industry who were interested in understnding the mjor processes driving the sustinble supply of nutrients to the trees grown in the Zululnd costl region on nutrient poor soils. This included the effects of hrvesting nd residue mngement prctices on these processes. Describing ech component of the biogeochemicl nutrient cycling processes t high level of resolution is complex nd requires substntil humn nd finncil resources. This study therefore considered the effects of extremes in mngement prctices on the bulk processes driving nutritionl sustinbility t the site. The effects of clerfelling nd residue mngement on the biomss nd pools nd fluxes of nutrients N, P, K, C nd Mg in the new tree crop, residues nd soil were compred between clerfelled stnd nd n undisturbed stnd. The clerfelled stnd hd number of mngement opertions pplied tht resulted in tretments with vrious quntities of residues nd 171

202 nutrient lods on the soil surfce. Residues were brodcst cross most tretments t clerfelling, the exception being removl of ll residues through whole tree hrvesting (Whole-Tree), leving the old litter lyer intct. The residues from the Whole-Tree tretment were used to crete residue (nd nutrient lod) doubling tretment (Double). A burning tretment (Burn) ws pplied nd contrsted with residue retention (No-Burn) nd the undisturbed crop (stnding crop). The Burn tretment, No-Burn nd stnding crop were ssumed to be t the extremes of nutrient flux processes by virtue of their impct on crbon nd nutrients. A further tretment (Fert) used fertiliser (pplied t plnting) to replce nutrients lost through stemwood hrvesting with residue retention. These tretments were designed to enble n in-depth understnding of chnges in growth nd nutrient fluxes under vrious levels of nutrient vilbility. An dditionl tmospheric deposition monitoring site ws lso included within the region (CHAPTER 5), ginst which tmospheric inputs were compred. The study ws crried out from prior to clerfelling nd ws completed t cnopy closure of the new crop (one yer fter plnting). However number of mesurements were continued beyond cnopy closure. In completion this study hs given mny insights into the interctions between site nutrient dynmics, mngement nd productivity. The study ws seprted into the following sub-studies tht determined: Describing tretment effects on soil moisture nd prediction of dringe fluxes (CHAPTER 4) The mgnitude of tmospheric ddition in rinfll nd cnopy dringe (CHAPTER 5CHAPTER 5: bove). Chnges in residue, litter, tree growth nd biomss nutrient pools nd fluxes (CHAPTER 6). Chnges in top-soil N minerlistion fluxes (CHAPTER 7). The impct on nutrient leching to one meter soil depth using predicted dringe fluxes nd collection of free dringe soil solution (CHAPTER 8). This finl chpter is summry of key findings nd synthesis of the dt presented in the previous chpters. This chpter will give insights into the work, offer some specultion nd suggest future reserch to refine nd expnd the findings of this study. 172

203 9.3. METHODS OF DATA SUMMARY: Nutrient flux dt derived from ech thesis chpter is summrised between felling nd cnopy closure ( 20.1 month period) using grphicl representtions of the Stnding crop, Burn nd No- Burn tretments. Soil nd biomss pools re given t the time of cnopy closure of the new crop, totlled to one meter depth. Annulised fluxes nd growth bsed nutrient ccretion of the Stnding Crop re clculted using the sme 20.1 month period. Nutrient ccretion is clculted s the sum of tree growth nd litterfll. Decomposition is clculted s the sum of litterfll nd nutrients lost from the litter lyer through decomposition. Where dt ws not vilble for vrious tretments, it ws estimted from literture or other tretments. Further to this, blnce of nutrients entering nd leving the soil ws clculted. A nutrient budget ws lso clculted for the full rottion, projecting ll mesured dditions nd losses to seven yers fter plnting Short-term nutrient blnce The blnce of short term (clerfelling to cnopy closure) nutrient fluxes ws clculted s the sum of nutrient ddition t the soil surfce nd losses to the tmosphere or below 1 m soil depth for ech tretment. This clcultion ssumes the blnce of ll nutrient ddition nd losses to the soil to contribute to soil nutrient supply (nd pools) during the 20 month clerfelling to cnopy closure period. Additions considered included tmospheric inputs, residue nd litter decomposition, fertilistion nd post burning sh (tht indirectly ccounts for burning losses). Losses considered were leching nd nutrient uptke through tree growth. These were ssumed to be the only fctors reducing soil nutrient supply, essentilly reducing short term soil nutrient vilbility. Other losses (runoff, erosion nd denitrifiction) were ssumed to be negligible. Aboveground nutrient uptke through growth ws clculted s the sum of boveground growth ccretion nd litterfll. Residue decomposition prior to burning ws dded to sh lyer nutrient content fter burning s n estimte of short term soil nutrient supply in the Burn tretment. Fluxes not recorded in the other tretments were estimted. Leching in the Fert. tretment ws ssumed to be equl to tht in the No-Burn tretment. Whole-Tree tretment leching ws estimted ssuming the rtio between No-Burn leching loss : residue decomposition to be similr to Whole-Tree tretment leching loss : residue decomposition. This did not include the contribution of belowground old root system decomposition to leching. Atmospheric inputs nd leching of P were not included in the shortterm blnce (not ssessed t the site). 173

204 These clcultions were done to determine the blnce of nutrient dditions nd losses to the soil so tht the contribution of the blnce of these nutrient fluxes to soil nutrient supply could be determined. This ws contrsted with growth uptke during the rpid erly growth phse of the new crop. A negtive blnce implies n incresed uptke of redily plnt-vilble below ground nutrient reserves during the erly tree growth phse, while positive blnce suggests nutrient surplus. A true reflection of nutrient vilbility is more complex nd needs to tke into ccount, for exmple, concurrent processes of minerlistion nd immobilistion in the cse of N nd P, s well s sorption-desorption nd redox rections in the cse of P. However, despite these shortcomings, this estimtion does give n indiction of mgnitude nd role tht vrious boveground nutrient sources ply in supplying or supplementing erly tree growth nutrient demnds. The belowground nutrient reserves comprise soil nutrients held on the soil exchnge, in the soil solution nd relesed from the old tree root system. As the old root system ws not mesured, it ws ssumed to form prt of the soil nutrient pool. Decomposition of the old root system ws ssumed to contribute similr quntities of nutrients to the soil solution cross ll tretments Projected long-term nutrient budget A seven yer forwrd projected budget ws estimted s the cumultive sum of felling to cnopy closure fluxes by combining the 20 months clerfelling to cnopy closure fluxes with stnding crop fluxes therefter. This generted n lgebric blnce of ll nutrient dditions nd losses occurring in ech tretment. Similr ssumptions were used s bove (Section 9.3.1), while ll other fluxes (including minerl wethering nd runoff) were lso ssumed to be negligible. Phosphorus deposition ws estimted from literture derived vlues in CHAPTER 5 s 0.5 kg h -1 yr -1. Nutrient budgets were contrsted with soil pool sizes to 1 m soil depth using totl N, extrctble P nd slt extrctble bse ctions. Although these pools reflect close pproximtion of soil nutrient reserves given the smll ction exchnge of these soils nd the negligible wethering dditions tht cn occur, it cn only be tken s n index of soil nutrient supply. Totl N includes frction of reclcitrnt pools, smll frction of which becomes vilble through N minerlistion. Site nutrient pools were tken s the sum of soil, litter nd tree biomss pools. The nutrient budget ws clculted s dditions nd losses to site nutrient pools, so only included tmospheric dditions, leching losses, burning losses, fertiliser dditions nd biomss removl from the site. Exchnge nd cycling between the trees, litter lyer nd soil ws therefore excluded. 174

205 9.4. RESULTS AND DISCUSSION The short term nutrient blnce (defined in section 9.3.1) is discussed in sections to Min findings: The undisturbed stnding crop Accumultion of nutrients into the bove ground biomss nd nutrient storge in the litter lyer remined lrge in the stnding crop tretment during the 20.1 month intensive monitoring period. Lrge mounts of nutrients (prticulrly N nd K) were dded through tmospheric deposition over the 20 month study period (Figure 9.1). Orgnic sources were noted s potentil mjor contributor to N deposition nd noted s requiring further investigtion (CHAPTER 5). Leching in the undisturbed crop decresed mrkedly with incresing depth with smll losses beyond 1 m depth. This ws ttributed to high wter use in the undisturbed crop. More N nd less K, C nd Mg were supplied to the soil in the stnding crop during the 20 month period thn ws tken up in boveground tree growth (Tble 9-1). The quntity of N supplied through deposition ws lrger thn the combined loss through leching pst 1 m soil depth, boveground ccretion nd N immobilistion. Lrger ction losses reltive to dditions occurred primrily through leching pst 1m soil depth. It is however, unlikely tht leching loss through deep dringe occurred under the stnding crop. Model estimtes indicted dringe only to occur to depth of three meters under the conditions recorded during the study period (CHAPTER 4). This cn limit leching loss s tree roots would hve ccess to nutrients t this soil depth. A zero leching loss my therefore be ssumed under the stnding crop under norml climtic conditions nd result in positive ction blnce in the undisturbed crop. Recycling of nutrients (internl trnsloction nd litterfll decomposition), tmospheric ddition nd high wter use (low nutrient mobility in dry soil) limited tree growth demnd on soil nutrient reserves (Figure 9.1). The undisturbed crop therefore mintined more positive nutrient blnce thn the post felling crop, ccumulting nutrients in the tree biomss nd soil Min findings: Clerfelled nd replnted crop Growth up to 2.5 yers fter plnting ws not ltered by ny level of hrvesting intensity or residue mngement. Replcement of stem wood nutrient losses through fertilistion improved erly growth to 2.5 yers fter plnting. Vector nlysis t 6 months suggested tht this occurred through responses to N nd P supply nd t one yer fter plnting to Mg nd C supply. 175

206 Biomss removl during hrvesting nd residue burning ws responsible for lrge proportion of the lrge nutrient losses observed. (Figure 9.1). Residue burning left lrge quntities of C nd Mg on the soil surfce. Clcium pool size in prticulr ws significntly incresed in the top soil lyer, persisting beyond cnopy closure (Figure 9.1, Tble 9-1). Burning induced the lrgest N (nd orgnic mtter) loss of ll tretments. Ction losses were however gretest fter whole tree removl. Net N minerlistion in the top 30 cm of the soil ws reduced by 46% reltive to the No-burn tretment during the 20.1 months following burning. Rpid soil moisture rechrge tht occurred fter clerfelling drmticlly incresing leching of N nd the ctions beyond 1 m. Losses of N were incresed primrily through more rpid nitrifiction rising soil nitrte concentrtions following clerfelling. Soil moisture content nd leching losses were however, not significntly ltered by residue burning compred to residue retention. A lrge proportion of Mg leching (42%) occurred prior to plnting. A mjority of K (79%) nd C (67%) nd similr Mg (54%) leching occurring between plnting nd six months fter plnting (CHAPTER 8). The post clerfelling increse in leching ws therefore short-lived s new crop growth rpidly reduced soil moisture content nd soil solution concentrtions to levels similr to the undisturbed crop by round six months fter plnting Nutrient blnce Decomposition nd tmospheric inputs over the felling to cnopy closure period resulted in n excess of N in the No-Burn tretment reltive to boveground ccretion (Tble 9-1). In contrst, K, C nd Mg were dded to the soil in quntities fr smller thn boveground tree ccretion. Lrge losses of Mg with leching beyond 1 m resulted in net loss of Mg from the soil during the 20 month period. Similrly, leching of K in ddition to growth uptke resulted in lrge net K deficit. This deficit lso occurred for C, but ws due to rpid C uptke during erly growth rther thn leching loss. Short term soil blnce ws most negtive for K nd lest for C, mening tht stnd nutrient uptke of prticulrly K hd to be supplied from redily vilble soil storge pools. Phosphorus seemed to remin in excess of boveground growth uptke, but leching nd deposition were not specificlly recorded. Above ground orgnic nutrient pools in the Burn tretment comprised live tree biomss lone s no residue pools ws present on the soil surfce. Differences between the soil N nd Mg blnces nd 176

207 tree uptke were more negtive for the Burn tretment thn for the No-Burn tretment (Tble 9-1). A positive C blnce occurred in the Burn tretment s result of the sh lyer on the soil surfce, but this ssumed ll the sh remined (no wind or wter erosion) nd nutrients it contined in it were in plnt vilble minerl form. Fertiliser ddition (ssuming similr leching to No-Burn) creted positive nutrient blnce for ll elements except Mg. Whole-Tree boveground nutrient pools comprised the prtilly decomposed previous rottion litter lyer nd residul tree biomss. Although these pools were mrginlly smller thn No-Burn pools, except for C pools tht were much smller (48% of pre-clerfelling stnding crop pools). Whole tree removl resulted in P, K, C nd Mg deficit ssuming similr leching losses. No growth depression occurred in the Whole-Tree tretment s nutrients relesed from the litter lyer in ddition to the soil nd the old root system were most likely sufficient to supply erly growth demnds. Only soil P nd K were significntly lower in the Whole-Tree tretment thn the No-Burn tretment t cnopy closure. Vector nlysis indicted tht these elements were in decline (CHAPTER 6). Pools of K were similr cross ll tretments due to low K retention in the soil nd residues which ws leched out shortly fter clerfelling. Residue retention mintined lrge mounts of nutrients in the boveground biomss pools to cnopy closure reltive to residue burning nd whole tree removl. Nutrients remining in residues crete more fvourble condition s these cn become vilble lter in the rottion nd dd to supply for lter tree growth demnd. This my, however, hve less benefit to tree growth s nutrient demnd is usully smll in helthy mture stnds nd semi-mture stnds (Lclu et l. 2003). Due to residue nd litter lyer retention boveground biomss N, C nd Mg pools t the end of the study were lrger in the No-Burn tretment thn in the Burn tretment. The No-Burn tretment hd ttined 57% nd 76% of the stnding crop preclerfelling N nd C biomss pools respectively by one yer fter plnting. In contrst Burn tretment N nd C biomss pools ttined only 18% nd 7% of the pre-clerfelling stnding crop biomss nutrient pools by cnopy closure. 177

208 Stnding crop (Annulised vlues in brckets) No-Burn Burn Figure 9.1: Nitrogen nd phosphorus fluxes (kg h -1 ) between clerfelling nd new crop cnopy closure, nd pools t cnopy closure s defined bove. (nd = not determined) 178

209 Stnding crop (Annulised vlues in brckets) No-Burn Burn Ction fluxes (kg h -1 ) between clerfelling nd new crop cnopy closure, nd pools t cnopy closure s defined bove. (nd = not determined) 179