The Potential of Carbon Sequestration to mitigate against climate change in forests and agro ecosystems of Zimbabwe

Transcription

1 The Potentil of Crbon Sequestrtion to mitigte ginst climte chnge in forests nd gro ecosystems of Zimbbwe L. Mujuru i

2 Thesis committee Promotor Prof. Dr R. Leemns Professor of Environmentl System Anlysis nd Erth System Science Wgeningen University Co-Promotor Dr M.R. Hoosbeek Associte professor, Erth System Science Group Wgeningen University Other members Dr R. Merckx, K.U. Leuven, Belgium Dr S.D. Keesstr, Wgeningen University Prof. Dr K.E. Giller, Wgeningen University Prof. Dr T.W.M. Kuijper, Wgeningen University This reserch ws conducted under the uspices of Resource School for Socio-Economic nd Nturl Sciences of the Environment (SENSE) nd Wgeningen Institute for Environment nd Climte Reserch (WIMEK). ii

3 The Potentil of Crbon Sequestrtion to mitigte ginst climte chnge in forests nd gro ecosystems of Zimbbwe Lizzie Mujuru Thesis submitted in fulfilment of the requirements for the degree of doctor t Wgeningen University by the uthority of the Rector Mgnificus Prof. Dr M.J. Kropff, in the presence of the Thesis Committee ppointed by the Acdemic Bord to be defended in public on Tuesdy 20 My 2014 t 1.30 p.m. in the Aul. iii

4 Lizzie Mujuru The Potentil of Crbon Sequestrtion to mitigte ginst climte chnge in forests nd gro ecosystems of Zimbbwe, 220 pges. PhD thesis, Wgeningen University, Wgeningen, NL (2014) With references, with summries in Dutch nd English ISBN iv

5 DEDICATION To my husbnd,freddy, my mother, Estery nd in loving memory of my son Simbrshe. v

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7 TABLE OF CONTENTS CHAPTER 1 1 Introduction Bckground Crbon nd Nitrogen Cycling in Agriculturl Systems Effects of lnd mngement prctices on C nd N storge Fertilistion nd SOC storge in gro ecosystems Chronosequence pproch for studying chnges in soil C stock Soil crbon nd nitrogen frctions nd their stbilistion Modelling Soil orgnic crbon in gro ecosystems The profile for Zimbbwen ecosystems Scope nd Objectives of this thesis Outline of the thesis 24 CHAPTER Introduction Mteril nd methods Study site Surfce litter nd soil smpling Soil density frctiontion Results Surfce litter in lnd mngement systems Soil bulk density, ph, C nd N in lnd mngement systems Crbon nd N in density frctions Discussion Surfce litter retention in griculturl nd forest systems Soil bulk density, C nd N in different lnd mngement systems Soil C nd N dynmics in density frctions Conclusion 52 vii

8 CHAPTER Introduction Mterils nd methods The study site Chrcteristion of experimentl sites Soil smpling nd nlysis Soil density frctiontion Soil C sequestrtion potentil Sttisticl nlysis Results Soil crbon nd nitrogen contents Crbon nd nitrogen in density frctions Soil C sequestrtion potentil Discussion Crbon nd nitrogen in bulk soil Crbon nd nitrogen in density frctions Soil C sequestrtion potentil Conclusions 81 CHAPTER Introduction Study site nd methodology Experimentl design nd dt collection Lbortory nlysis Soil orgnic mtter frctions nd chrcteristion Sttisticl nd Dt Anlysis Results Forest floor C nd N distribution Forest stnd nd soil chrcteristics Crbon nd nitrogen in density frctions Discussion Forest floor C nd N Forest stnd nd soil chrcteristic 106 viii

9 4.4.3 Crbon nd nitrogen in soil orgnic mtter frctions Conclusion 110 CHAPTER Introduction Mterils nd Methods Study sites Modelling of crbon sequestrtion potentil Comprison of equilibrium SOC levels from RothC nd Lngmuir eqution Dt nlysis Results nd Discussion Reltionship between mesured nd modelled SOC stocks Reltionship between mesured SOC pools nd conceptul RothC SOC pools Long term prediction of C storge in different systems Comprison of equilibrium level estimtion by Lngmuir eqution vs RothC model Conclusion 133 CHAPTER Introduction Potentil of tillge mngement prctices for SOC sequestrtion Effects of fertilistion on SOC nd TON storge C:N rtios in different lnd mngement prctices Potentil of plnttion forests to mitigte climte chnge Modelling soil C stocks in Zimbbwen gro- ecosystems Conclusion 157 REFERENCES 162 SUMMARY 193 SAMENVATTING 197 ACKNOWLEDGEMENTS 202 ABOUT THE AUTHOR 204 FUNDING 209 ix

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11 Introduction Chpter 1 Introduction 1

12 Chpter Bckground Developing countries re more vulnerble to climte chnge becuse lrge proportion of their popultion directly depend on griculturl nd nturl ecosystems for their livelihoods. Agriculturl systems should overcome three simultneous chllenges of: ensuring dequte food for growing popultion, incresing smll holder frmer s resilience to chnging climte nd reduction of greenhouse gs emissions. Globlly, gro-ecosystems cn compenste C emissions by one third through the doption of mitigtion strtegies such s ddition of biomss to soils, miniml soil disturbnce nd soil conservtion (Cole et l., 1997). Incresing soil C storge hs therefore rendered soils mjor focl point becuse of their bility to ccumulte significnt quntities of orgnic C (Bnger et l., 2009; Food nd Agriculture Orgnistion (FAO), 2010). Soil mngement strtegies therefore become n importnt C mitigtion pproch through mitigtion mesures involving both CO 2 emissions reduction nd incresing C sinks. Globl climte chnge hs been linked to incresed concentrtions of greenhouse gsses (GHGs); crbon dioxide (CO 2 ), methne (CH4) nd nitrous oxide (N 2 O) (IPCC, 2007), the most importnt GHG being CO 2. The concentrtions of the Erth s tmospheric CO 2 is controlled by interction, the structure nd dynmics of terrestril ecosystems, climte, ocens nd nthropogenic CO 2 emissions (Foley et l., 2003). The mount of CO 2 in the tmosphere depends on its interctions with the Erth s surfce s CO 2 diffuses into ocens or is bsorbed by plnt photosynthesis. Some of the C bsorbed by plnts returns to the tmosphere through utotrophic respirtion, while the rest is converted to biomss, which eventully becomes litter nd converts into soil orgnic crbon (SOC). SOC is heterogeneous mixture of simple nd complex orgnic C compounds derived from biomss nd is lso linked to soil qulity nd productivity (Ll, 1986). SOC sequestrtion is mjor sink for tmospheric CO 2 (IPCC., 2000) with the globl soil C pool (1550 Pg) being three times greter thn the biotic pools (560 Pg) nd twice the tmospheric C pool (760 Pg) (Johnson & Curtis, 2001; Ll, 2008). SOC is lso slowly returned to the tmosphere through 2

13 Introduction heterotrophic (microbil) respirtion. Photosynthesis nd respirtion processes thus enble terrestril ecosystems to influence the mount of CO 2 in the tmosphere. During these processes, soils ply key role in globl C nd nitrogen (N) 1 cycling (Food nd Agriculture Orgnistion (FAO), 2010) resulting in storge of pproximtely 75% nd 95% of terrestril C nd N respectively (Eswrn et l., 1993; Schlesinger, 1997). CO 2 emissions cn thus prtly be compensted by creting or promoting crbon (C) sinks, such s incresing biosphere sinks (IPCC, 2007). Agriculturl soils cn contribute pproximtely 89% of GHG mitigtion potentil through C sequestrtion with n dditionl 2% nd 9% mitigtion potentil for N 2 O nd CH 4 respectively (Smith et l., 2007). This results in estimted emissions reduction of 5-14% over 5-10 decdes (Chn et l., 2008) with the griculturl systems potentilly storing bout Mt CO 2 eq nnully. However, mitigting GHG emissions from griculture without compromising food security remins mjor societl nd scientific chllenge. Mximistion of the mitigtion potentil requires dequte knowledge of lnd use nd mngement prctices ssocited with improved crop production nd reducing degrdtion (Ll, 1997; Ll et l., 1998; Singh & Ll, 2005). There is need to complement this with incresing knowledge nd understnding of how soils respond to chnging environmentl conditions (Powlson et l., 2010) nd lnd mngement prctices. The role of plnting trees is lso interesting, s tree growth sequesters C from the tmosphere into biomss nd soil (Hrmon et l., 1990). There is, unfortuntely insufficient informtion on chnges in soil C storge fter estblishing plnttion forests (Scott et l., 1999) yet the Kyoto Protocol of the United Ntions frmework Convention on Climte Chnge (UNFCCC) llows mitigtion of GHG emissions through reducing deforesttion nd stimulting fforesttion nd reforesttion. Afforesttion is plnting of new forests on lnd tht hs not supported forests in the lst fifty yers wheres reforesttion is plnting trees on lnd tht supported forests in the pst fifty yers but tht hs 1 Nitrogen is n importnt nutrient tht is essentil for plnt growth nd soil microorgnisms ffecting photosynthesis, decomposition, C nd nutrient cycling. 3

14 Chpter 1 been converted to non-forested lnd (Brown et l., 1986). Soil C chnges occurring during fforesttion, reforesttion or deforesttion re considered under Article 3.3 of the Kyoto Protocol, while soil C sequestrtion in croplnds, grzing lnds, mnged forests nd lnd subjected to revegettion re considered under Kyoto Article 3.4. As C is stored under those circumstnces, C sequestrtion processes occur nd these processes cn be considered s climte chnge mitigtion strtegy. The potentil of REDD+ in woodlnds nd svnnhs cn be chieved by the recovery of the woodlnds fter clering since most of the woodlnd species hve extensive rooting systems tht fcilitte recovery fter cutting (Mistry, 2000). For exmple, dt on primry production nd soil crbon storge indicte tht miombo woodlnds cn sequester g m -2 yr -1 (Frost, 1996). In ddition, re-growth stnds re highly productive ecosystems with higher growth rte ( mm) thn uncut stnds ( mm) hving high rtes of photosynthetic processes nd therefore high uptke of crbon dioxide. C emissions s well s offsets through C sequestrtion in ll systems hve to be reported to the UNFCCC s prt of ntionl greenhouse gs inventories (IPCC, 2006). Furthermore, the contribution of forests to climte chnge hs been recognized s cornerstone of the post climte chnge gend with the decision on the reduction of emissions from REDD+ in COP-16 in Cncun. REDD+ includes policy pproches nd positive incentives on issues relting to reducing emissions from deforesttion nd forest degrdtion in developing countries nd recognises the contribution of conservtion, sustinble mngement of forests nd enhncement of forest crbon stocks in chieving REDD+ objectives. The importnt role of soil s C source or sink nd the role in offsetting tmospheric CO 2 concentrtions cretes need for ccurte evlution of the effects of griculturl mngement prctices on soil C nd N storge. Furthermore, understnding the soil crbon dynmics under forest plnttions (minly pine) nd evluting the C nd N storge potentil over n ge sequence is importnt. There is cler need for reserch into the consequences of ny lndmngement ctivities in order to determine nd understnd their role in globl nutrient cycles. This chpter outlines: the importnce of C nd N in gro ecosystems nd the effects of lnd mngement prctices on C nd N storge 4

15 Introduction (section 1.2), soil C nd N frctiontion nd stbilistion (section 1.3), importnce of modelling SOC in gro ecosystems (section 1.4), Zimbbwen lnd use informtion (section 1.5) nd the scope, objectives nd outline of the thesis (sections 1.6 nd 1.7). 1.2 Crbon nd Nitrogen Cycling in Agriculturl Systems Soil orgnic mtter (SOM) comprise ll ded orgnic mteril within the soil or its surfce (Bldock & Skjemstd, 2000) nd plys fundmentl role in soil processes. Soil orgnic mtter dynmiclly determines soil productivity nd ffects essentil nutrient fluxes of C, N, phosphorous (P), sulphur (S) nd other nutrients fcilitted by microbil decomposition. The qulity nd quntity of SOM nd its position in the soil determines its resistnce to microbil decomposition. Next to being beneficil to soil biologicl, chemicl nd physicl properties, SOM is sensitive nd responsive to (chnges in) lnd mngement ctivities. Above nd belowground plnt residues (i.e. ded or decying biomss nd root exudtes) constitute the mjor source of SOM. Soil orgnic C nd N re mjor components of SOM tht ffect soil qulity very much linked to soil physicl structure, wter nd nutrient holding cpcity (Russell, 1973; Ll, 1986; Hu et l., 1997; Jimenez et l., 2002b) nd provision of energy for soil orgnisms (Jimenez et l., 2002b). SOC is the C content of SOM, which is pproximtely 50% nd is lrgest in litter nd orgnic lyers. Generlly, soils hve SOC rich topsoil followed by decresing SOC content with incresing soil depth. SOC is either reported s concentrtion (i.e. mss of C per unit mss of soil; g kg -1 ) or s stock or density expressed on re bsis (Kg m -2 or Mg h -1 ). Estimtion of C density requires dt on soil bulk density, stone content nd depth of smpling. Btjes nd Sombroek, (1997) showed tht 23% of the plnt residues convert into SOC for ntive pristine pririe while de Mores Sà nd Séguy (2008) found rtes of 15-26% conversion in griculturl systems. In semi-rid res, the conversion rte of plnt residues into SOC is 29% under continuously cropped systems but only 9% for systems tht re regulrly left under fllow (Cmpbell et l., 2000). The conversion rte is thus ffected by fctors such s moisture, 5

16 Chpter 1 temperture, crop species nd/type nd the mount of residues. Removl of crop residues generlly decreses SOC levels in cropping systems (Unger et l., 1990). In gro-ecosystems, the cycles of C nd N re linked through pools in crops, crop residues nd in SOM (Figure 1.1). The ccumultion of plnt biomss is in turn Figure 1.1 Greenhouse gs emission sources/removls nd processes in gro ecosystems (IPCC, 2006). relted to the mount of soil vilble N (Vlek et l., 1981). The dynmics of soil N reserves is continuously mintined by processes of minerlistion nd immobilistion. Atmospheric N is found in the form of N 2 tht cnnot be utilised by most orgnisms nd only enters the soil through processes of dry/wet N deposition, N fixtion or through the ddition of fertilisers nd/or mnures. Nitrogen fixtion is done by N-fixing bcteri, which either live s free bcteri surviving on their own in the soil crrying out chemicl rections tht chnge N 2 to mmonium (NH 4+ ) (Nitrogen Fixtion), or s symbiotic root nodule bcteri tht supply N to legumes/pod-producing plnts in the fmily Fbcee/Leguminose. 6

17 Introduction Ammonium nd nitrte re forms of N tht cn be used by most plnts nd re found in mino cids. However, orgniclly bound N cn be converted microbiologiclly into inorgnic minerl forms (minerliztion), leding first to formtion of mmonium (NH + 4) nd possibly ending up s nitrtes (nitrifiction) (Fowler et l., 2013). Nitrifiction nd denitrifiction processes in griculturl systems contribute bout 0.53 Pg C equivlents of nnul nitrous oxide fluxes which ccounts for more thn 50% of the globl nthropogenic N 2 O flux (Robertson, 2004). Processes such s mmoni (NH 3 ) voltilistion nd the uptke of N by plnts cuses depletion of soil N wheres the return of crop residues increses the soil orgnic N pool (Vlek et l., 1981). In ddition, nimls lso fcilitte C nd N cycles fter they die s their biomss is trnsformed into mmonium nd other constituents, thus continuing the cycle. The process of SOM trnsformtion occurs fter colonistion of ded orgnic mtter by rnge of soil orgnisms, s they obtin nourishment for growth. During decomposition, bout hlf of the C in SOM is minerlised nd relesed s CO 2 (White, 2006) while the other is either stbilised through humifiction or is complexed with soil minerls (Gregorich & Jnzen, 1996). The C nd N sequestrtion cpcity of soil is therefore determined by its bility to protect the SOC nd N from degrdtion (i.e. stbility) (Six et l., 2002). The mounts of C stbilised in ech soil under specific mngement conditions is importnt in determining the extent to which soil cn operte s C sink. However, the bility of soil to stbilise the sequestered C vries with physicl (e.g. bulk density nd cly%) nd chemicl properties (e.g. ph) (Borken et l., 2002) of the minerl soil mtrix nd the structure of the SOC (Singh & Ll, 2005; Murillo et l., 2006). Soils cn become sturted with C resulting in no further stbilistion even with dditionl inputs (Six et l., 2002).The efflux of soil CO 2 lso differs between fine-textured soils nd corse textured soils (Zk et l., 1994). In order for C sequestrtion to be considered importnt for climte chnge mitigtion, the net trnsfer of CO 2 from the tmosphere into the soil or biomss should be in form tht is not immeditely reemitted (Powlson et l., 2011). Soil orgnic C levels respond linerly to C inputs (Yin et l., 2005; Yin & Ci, 2006; Cmpbell et l., 2007) with the slopes differing with qulity of inputs nd soil type. Theoreticlly, the C equilibrium level cn be restored in soil when 7

18 Chpter 1 mngement systems remin unltered nd is ffected by fctors such s soil texture, climte, vegettion type nd time. These fctors hve bering on the bility of soil to rech equilibrium level. Difference between equilibrium SOC levels nd the current C stocks indictes the SOC sequestrtion potentil of system. In some cses, SOC sequestrtion in excess of the equilibrium level, cn occur becuse of incresed productivity owing to the removl of inherent soil constrints limiting plnt growth (Russel & Willims, 1982). This demonstrtes tht chnges in SOC storge over time cn be mnipulted by mngement prctices through regultion of inputs nd outputs. Smith et l. (2008) reviewed studies on potentil of SOC sequestrtion nd estimted verge nnul mitigtion potentil of ll GHGs, in griculturl systems for wrm-dry climtes ( to 0.88 Mg C h -1 yr -1 ) nd wrm-moist climtes (0.26 to 1.30 Mg C h -1 yr -1 ) with differences in ech zone ttributed to initil SOC levels, time periods nd site productivity fctors Effects of lnd mngement prctices on C nd N storge Some mngement prctices hve been identified s suitble for orgnic C nd N sequestrtion (Dll & Myer, 1987; Jnzen, 1987; Smith et l., 2007) including plnt residue mngement, tillge, mnure nd fertiliser ppliction nd tree plnting (FAO, 2004). Severl studies hve shown tht crop rottion nd diversity, fertilistion, residue mngement, conservtion tillge, groforestry nd the mintennce or restortion of degrded lnds cn increse soil C sequestrtion (Ll, 2004; Smith et l., 2007; Purkysth et l., 2008; Gong et l., 2009). The extent of the influence of ech prctice on soil C sequestrtion however, depends on lnd-use type, crop species, socil nd ecologicl dynmics nd intensity of the mngement prctices. Biophysicl components lso ffect griculturl systems nd their potentil for C sequestrtion (Ll et l., 1999; Whlen & Chng, 2002). Socio-economic nd demogrphic fctors similrly ffect decisions on choice of griculturl mngement prctice (Sebrook et l., 2008). In some cses, soil C storge in prticulr frmer s field is ffected by socio-economic fctors such s the frmer s welth ctegory nd/or sources of income (Zingore et l., 8

19 Introduction 2007). In smll holder gro ecosystems, CO 2 emissions re not only from decomposition nd plnt respirtion but lso indirect emissions come from the production of some griculturl inputs such s fertilisers, pesticides nd herbicides. Furthermore, griculturl systems lso emit CH 4, through ruminnt fermenttion (52%), biomss burning (19%) nd niml wste tretments (8%) (Robertson, 2004). Conventionl tillge, which is n ncient prctice is beneficil in controlling weeds, reducing soil compction nd prepring fvourble crop seedbed (Aziz, 2010). On the other hnd, lthough impcts vry with site chrcteristics, conventionl tillge is seen s the mjor cuse of SOM degrdtion (Buschizzo et l., 2001). Decresing soil C stocks in griculturl lnds by exposing SOM to microbil decomposition nd thus enhncing its decomposition nd loss into the tmosphere (Ll et l., 1998); (Six et l., 2000; Zotrelli et l., 2005). Tillge lso modifies soil physicl conditions, such s ertion, porosity, temperture nd moisture nd this fvours decomposition of plnt residues nd SOC (Drees et l., 1994; Verhulst et l., 2010). In ddition, tillge ccelertes soil erosion, which promotes trnsfer of soil nutrients. Furthermore, tillge lters the qulity nd quntities of soil orgnic inputs through its effects on crop growth (Dorn, 1987). Conservtion tillge (i.e. no, reduced or/ minimum tillge), on the other hnd, cn enhnce soil C sequestrtion nd cn reduce C loss nd is now considered n importnt griculturl mngement prctice (Six et l., 2000b; Food nd Agriculture Orgnistion (FAO), 2010). In comprison with conventionl tillge, reduced or no tillge prctices in which crop residues re left on the soil surfce reduce soil loss due to decresed erosion (Lee et l., 1993), increse surfce soil C concentrtion (Blnco et l., 2009) nd improved wter use efficiency Fertilistion nd SOC storge in gro ecosystems Fertilisers hve been successfully used to enhnce C sequestrtion (Ll, 1999) lthough, the ppliction of N fertilisers cn result in nitrous oxide (N 2 O) emissions (Robertson et l., 2000) nd lters the soil C nd N dynmics. The min 9

20 Chpter 1 purpose of fertiliser ppliction is to increse crop production. This is ccompnied by subsequent increse in biomss nd thus indirectly elevtes soil C sequestrtion. Better results cn be obtined using combintion of inorgnic fertilisers nd orgnic mnure which cn mintin nd even improve SOC storge (Su et l., 2006). In smll holder frming communities, the mount nd frequency of inorgnic fertiliser ppliction depends on frmer s possibilities. For exmple, welthy frmers pply more inorgnic fertilisers thn the poor frmers who sometimes do not pply ny fertilisers (Ncube et l., 2009). Moreover, the eduction level plys decisive role in the mount of fertiliser used. In smllholder frmlnds, livestock re n importnt sset nd their dung is mixed with litter nd used s the min source of soil mendment (Frmyrd Mnure (FYM) referred s mnure in this study). Mnure is therefore redily vilble nutrient source lthough the quntities re often not enough. Appliction of mnure cn sequester more C thn other fertility tretments over rnge of soils nd climtic conditions, lthough fctors such s crop species nd soil texture re lso importnt (World Bnk, 2012). Mnure ppliction shows greter C gins in clyey soils thn nitrogen fertiliser lone nd hs the dvntge of hving both nutritive vlue being source of orgnic C for improving soil tilth. In ddition, mnure is more resistnt to microbil decomposition when compred with plnt residues, resulting in greter C storge given the sme quntity of C input (Jenkinson et l., 1990). Furthermore, mnure ppliction increses development of prticulte orgnic mtter (Kpkiyi et l., 1999) nd the formtion nd stbilistion of soil mcro-ggregtes (Whlen & Chng, 2002) Chronosequence pproch for studying chnges in soil C stock Understnding C nd N dynmics in plnttion forest soils is needed in order to determine the role of trees in climte chnge mitigtion nd the long-term impcts of mngement ctivities. Studies of soil C dynmics in plnttion forests often involve sptil temporl substitution methods, such s the chronosequence pproch (Covington, 1981). The chronosequence pproch fcilittes long-term studies of vegettion succession nd soil dynmics. The length of time required to 10

21 Introduction mke direct investigtion of forest succession mkes the chronosequence pproch more fvourble for determintion of long-term chnges in soil C nd N stocks due to fforesttion (Covington, 1981; Wllce & Freedmn, 1986; Kloptek, 2002). The chronosequence pproch ssumes tht the expected pttern of temporl development tht would be exhibited by prticulr stnd is reflected by the pttern observed mong stnds of different ges (Wllce & Freedmn, 1986). Chronosequence pproches to study soils therefore fcilitte the nlysis of C nd N vrition over stnd-ge using spce-for-time substitution nd llows investigtion of temporl shifts of the C nd N mount over stnd ge. Forest plnttions re either exotic or indigenous forest stnds rtificilly estblished covering minimum re of 0.5 h, with tree crown cover of t lest 10% nd totl height of mture trees bove five metres (FAO, 2001). Conifer nd brodlef tree plnttion species hve different strtegies for belowground lloction of ssimilted C (Guo & Gifford, 2002). The storge of C in forest soils is lso ffected by site qulity nd lnd use prctice (Ll, 1997). Furthermore, ntive nd exotic trees re ble to sequester nd stbilise C from the tmosphere nd potentilly contribute to countercting the greenhouse effect. Preferbly, fst growing trees re recommended s excellent options for mitigting CO 2 emissions through soil nd biomss C sequestrtion (Montgnini & Porrs, 1998). Plnting the fst growing exotic tree species substitutes requirements for vrious indigenous wood requirements nd cn fcilitte the regenertion of ntive species. In ddition, trees protect soil by mens of the litter lyer nd lef cnopy, thereby decresing runoff nd erosion nd incresing wter infiltrtion rtes. There is lso reduction of soil tempertures nd improved wter holding cpcity under the tree cnopy (Snchez et l., 1997). Root ctivities nd orgnic mtter inputs improve soil structure (Grdner et l., 1999). In forested systems, SOM pools include ll the components tht cn be prtitioned in pools of forest floor, bove ground biomss, belowground biomss constituents, wter soluble orgnics, light frction mteril nd stble humus (Stevenson & Cole, 1999). In ech forest, the ccumultion of C nd N vries within different soil depths, hving, litter-fll s the min input of C nd nutrients to the forest floor, nd is importnt for both the forest-soil system nd nutrient cycling (Strr et l., 2005). Forest floor consists of three lyers: 1) Litter lyer (L), 11

22 Chpter 1 which consists of fresh nd recently fllen, non-decomposed mteril. This mteril is identifible by the nked eye s plnt residues. The litter lyer usully contins less thn 10% fine orgnic mtter; 2) Frgmented lyer (F), where orgnic mteril is frgmented nd prtly decomposed with plnt residues being mcroscopiclly recognizble; nd 3) Humus lyer (H), which consists of decomposed orgnic mtter, originting from litter fll from decdes go nd root turnover of vrible ge. The H lyer mterils hve more thn 70% is fine orgnic mtter decomposed beyond recognition of their origin. The build-up of H lyer is dependent on the inflow of plnt litter nd the ctivity of the decomposer community. In ddition the H lyer is composed of the formtion nd ccumultion of reclcitrnt or very slowly decomposing litter nd microbil residues (Yni et l., 2003; Keith et l., 2010). 1.3 Soil crbon nd nitrogen frctions nd their stbilistion The trnsformtion of SOM gives rise to pools rnging from very ctive (lbile) to stble/inert/pssive (non - lbile) tht re differentited from ech other by their degree of decomposition, reclcitrnce, nd turnover rte (Gregorich et l., 1994). The constituents of ech pool hve different functionl roles in the dynmics of SOM nd nutrient cycling. The lbile C pool consists minly of soil orgnisms, polyscchrides, celluloses nd hemi-celluloses with turnover time vrying from weeks to months, while the reclcitrnt pool consists of lignin, lipid polymers, resins, suberins, wxes nd fts with turnover time vrying from yers to decdes. The inert pool consists of chrcol nd pyrolysed C with turnover time of centuries to millenni. The mount of orgnic C in ech frction/pool depends on the qulity nd quntity of the orgnic mtter dded to the soil nd types of decomposition products from biologicl ctivities. Stbility of C in ech pool is relted to the ggregtion of soil prticles fcilitted by different types of binding gents. Tisdll nd Odes (1980) clssified binding gents into () Trnsient binding gents - comprised of microbil nd plnt derived polyscchrides nd my be rpidly decomposed by microbes; (b) temporry binding gents - includes roots nd fungl hyphe (especilly 12

23 Introduction mycorrhizl hyphe) nd (c) persistent binding gents - consists of romtic humic mteril ssocited with morphous Fe nd Al compounds, nd polyvlent metl ctions. Odes (1984) nd Six et l., (2000) showed tht orgnic mterils within soil ggregtes hd lower decomposition rtes thn those outside of ggregtes. The SOM pools cn be seprted by chemicl or physicl extrction methods. Chemicl methods hve, however, proved difficult by ffecting the extrction process (Duxbury et l., 1989) wheres, physicl seprtion of SOM reduces chemicl ltertion of the orgnic mterils (Golchin et l., 1994; Gregorich & Jnzen, 1996; Swnston et l., 2005; Liu et l., 2013). The dvntge of physicl frctiontion techniques is the isoltion nd detection of SOC storge nd sensitivity of soil to lnd mngement nd climte of prticulr site. Frctiontion of SOM by density cn be chieved using hevy liquids such s Sodium Polytungstte whose density rnges between 1.6 nd 2.0 g cm -3. Density frctiontion seprtes SOM reltive to turnover rte, structure nd function (Christensen, 2001; Echeverrí et l., 2004; Crow et l., 2007). SOM is seprted into three bsic functionl pools of: free light frction (flf), occluded light frction (olf), nd minerl ssocited frctions (MHF) (Golchin et l., 1994b; Six et l., 2002; Norris et l., 2011; Sequeir et l., 2011) which vry in C nd N concentrtion, C:N rtios nd ᵟ13 C over the soil profile (Figure 1.2). Crow (2011) nlysed the men residence time of the three soil C pools nd found them with turnover rtes of 3.5 yers (flf), 10 yers (olf), nd yers (MHF). The light frction is usully mde up of two distinct frctions of flf nd the olf, with differences between the two lrgely explined by the positions they occupy in soil ggregtes. Though these frctions re different, most studies do not seprte them nd usully clssify them together s the Light frction (Six et l., 2002) especilly when comprbility is limited by diversity of methods used to seprte the two (Cerli et l., 2012). Compred with the orgnic C stock of whole soil smple, the flf the olf hve higher C nd N concentrtions nd wider C:N rtios (Golchin et l., 1994; Schrumpf et l., 2013). The light frction orgnic C, prticulte orgnic C nd microbil biomss C form the bsis of the flf or inter-ggregte orgnic mtter (Yng et l., 2009; Sequeir et l., 2011). The flf consists of prtilly decomposed plnt remins, root frgments, spores, seeds, fecl pellets, fungl hyphe, funl crcsses, 13

24 Chpter 1 microbes nd microbil remins t vrious stges of decomposition (Poirier et l., 2005; Gregorich et l., 2006). The light frction serves s n esily decomposble Figure 1.2 Schemtic presenttion of soil orgnic mtter frctions (Adopted with permission from Schrumpf et l. (2013). substrte for soil micro-orgnisms nd short-term reservoir of plnt nutrients (Gregorich et l., 1994), the structure is determined by the qulity of orgnic input nd soil type (Hu et l., 1997). The flf therefore, plys mjor role in controlling short term ecosystem productivity hving turnover rte of few dys to few yers (Yng et l., 2009; Sequeir et l., 2011). Other ttributes of the flf prt from the high decomposbility include the high sensitivity to mngement prctises nd high C/N rtios (Echeverrí et l., 2004). The flf is more susceptible to chnges in temperture nd moisture. In ecosystems tht re usully burnt, 14

25 Introduction chrcol cn ccount for considerble proportion of the flf (Skjemstd et l., 1990; Cdisch et l., 1996). Accumultion of light frction is much higher nd fster in upper soil lyers nd lso higher in forests thn griculturl systems. The flf hs significnt contribution to totl ctive orgnic C in the upper soil depths of most soils (Yng et l., 2009; Mtos et l., 2011) except soils contining blck C (von Lützow et l., 2002). In the tropics the light frction is lrgely confined to the top 22 cm (Trumbore & Zheng, 1996; Echeverrí et l., 2004) nd the mss, qulity nd contribution of light frction to totl orgnic C decresing with soil depth (Tn et l., 2007). Additions to the light frction come from plnt biomss, excretion of root exudtes, root residues nd other orgnic mtter ccumulting on the soil surfce nd subsequent mixing through bioturbtion (Gle et l., 2000). In cropping systems, tillge mixes nd breks down soil ggregtes exposing previously occluded SOC to decomposers resulting in loss of C nd some complexed SOC from intr ggregte light frction (Post & Kwon, 2000). Turnover of light frction is sometimes linked to mcro-ggregte formtion nd therefore ffected by ny form of soil disturbnce (Bere et l., 1994; Bremer et l., 1994). Although totl SOC declines with tillge, the decline in the mount of lbile C frctions indictes instbility of soil s structure (Prton et l., 1987). This frction is usully ignored in terms of C stbilistion yet it is the most sensitive to disturbnce. The olf is obtined from soil complexes fter ppliction of ultrsonic energy which disrupts the stble soil-ggregtes (Golchin et l., 1994; Sequeir et l., 2011) to relese mterils tht re more decomposed, trpped nd physiclly protected within soil ggregtes (Six et l., 2000). This frction is more biologiclly resistnt nd often ssocited with soil minerl colloids thus distinguishing it from flf lthough it is usully found mixed with the flf. The olf cn be relesed from lrge ggregtes by microorgnisms when the vilble substrte in the lbile pool is depleted (Six et l., 2001). The frction comprises humified C compounds t vrious stges of decomposition (Mrín-Spiott et l., 2008) including romtic structures such s polyphenols which hve slow decomposition rtes limited by low levels of N, P nd S (Ll, 2008). The olf is therefore less decomposble thn the flf nd lso hs higher humifiction index 15

26 Chpter 1 (i.e. higher lkyl nd lower O-lkyl C) (Golchin et l., 1994; Gregorich et l., 2006; Mrín-Spiott et l., 2008). The turnover rte of olf rnges from few decdes to hundreds of yers (Six et l., 2000b). Totl orgnic C in the olf frction is usully higher in soils tht hve more cly becuse of incresed ggregte formtion. The C:N rtio of olf is lower thn flf due to both loss of C nd incorportion of soil N. In ecosystems, where orgnic inputs re of comprtively similr qulity, the olf cn indicte differences in mgnitude, rte nd nture of decomposition (Golchin et l., 1994b). The MHF contins the oldest, most stble nd biologiclly resistnt C pool (Kögel-Knbner et l., 2008) consisting of reclcitrnt C which is dsorbed onto silt nd cly minerl surfces hving turnover time of decdes to centuries (Six et l., 2002; Gregorich et l., 2006; von Lützow et l., 2006; Schrumpf et l., 2013). Totl orgnic C in the MHF is usully higher in soils tht hve more cly becuse fine texture induces high dsorption onto chrged surfces of ggregtes nd mkes the C less susceptible to microbil ttck (Kölbl & Kögel -Knbner, 2004). The C in this frction is mde of finer mixture of mterils more heterogeneous in nture, whose origin cnnot be esily distinguished nd hs orgnic C loding of minerls decresing with soil depth (Schrumpf et l., 2013). An understnding of the dynmics of this frction is importnt in climte chnge mitigtion due to storge of more decomposed nd more reclcitrnt C (Alvrez & Alvrez, 2000). Concentrtion of C in MHF nd its contribution to totl SOC increses with soil depth (Tn et l., 2007). 1.4 Modelling Soil orgnic crbon in gro ecosystems Lnd mngement prctices, nd especilly prctices tht increse SOC, contribute to climte chnge mitigtion. The long-term impcts of such ctivities cn be studied with simultion models vlidted using locl dt. Models cn be used s decision support tool for simulting different climte nd mngement scenrios to ssess long term impcts of ech prctice. For exmple the Climte Rpid Overview And Decision Support simultor (C ROADS), which provides visul nd numericl projections, helps to understnd the gp between plnned policies 16

27 Introduction nd further ctions needed for stbilistion of tmospheric GHG concentrtions. C ROADS scenrios show the risks of climte chnge impcts (Findeling et l., 2003). Modelling hs further quntified different fctors controlling SOM dynmics nd helped to project the long-term effects of different mngement ctivities under chnging climte. Models re lso useful in predicting loss or storge of SOC under different lnd-use systems (Ll, 2009). However, most of the informtion used to understnd the impcts of lnd mngement on the dynmics of soil orgnic crbon is obtined from long-term experiments nd short-term lbortory experiments. In ddition, modelling SOC stbilistion provides mens of ssessing lnd-use nd mngement impcts reltive to the effects of climte chnge (Prton et l., 1994). Models, such s CENTURY nd RothC, evlute SOC dynmics bsed on environmentl nd climtic dt (Jenkinson et l., 1992; Woomer, 1993; Colemn & Jenkinson, 1996; Flloon & Smith, 2002; Cerri et l., 2007) nd help to improve understnding the consequences of soil mngement. However, only few of the model-bsed projections pply to tropicl dry lnds (FAO, 2004). RothC, for exmple, hs been successfully used in Zmbi s woodlnd nd improved fllow systems (Jenkinson et l., 1999; Kong & Colemn, 2008) nd testing the model for griculturl nd plnttion-forest soils is importnt. Thus, models re importnt for the determintion prtitioning of totl system C in different lnd mngement prctices. Other models such s FARMSIM hs been used in Zimbbwe to quntify the effects of interctions between different frm ctivities (Tittonell et l., 2007; Rufino et l., 2011). 1.5 The profile for Zimbbwen ecosystems Zimbbwe is lndlocked country locted in tropicl Southern Afric. It flls between ltitudes nd S. The ltitude rnges between 162 m nd 1,592 m bove se level in the southern nd estern prts respectively. In the est is series of mountin rnges with peks up to 1, 592 m bove se level (World Fctbook, 2013). The totl lnd re is bout 39.0 Mh, of which Mh re under frgmented indigenous forests (minly miombo woodlnds nd svnnhs 17

28 Chpter 1 nd bout 5.85 Mh being protected forest under ntionl prks, wildlife reserves nd forest reserves) nd 0.12 Mh is under commercil forest plnttions. A further 5.07 Mh is covered by bush lnds. About M h is griculturl lnd while bout 0.65 Mh re wter bodies nd urbn settlements (World Bnk, 2009). Figure 1.3 shows lnd tenure distribution in Zimbbwe. The lnd re is divided into five nturl regions bsed minly on rinfll, temperture nd soil type (Figure 1.4). Regions I, II, nd III hve nnul rinfll between 650 nd 1050 mm nd re suitble for intensive niml nd crop production wheres regions IV nd V re mostly suitble for extensive livestock production becuse of nnul rinfll below 650 mm. Agriculture contributes bout 15% to nnul GDP. Zimbbwen soils re inherently infertile (Nympfene, 1991). This, when coupled with long period of cultivtion nd erosion, contributes to low SOC stocks. Despite the low fertility, the mjority of smll holder frming ctivities re dependent on rin fed griculture which is dominted by conventionl tillge nd continuous mize cropping. Approprite soil mngement is thus key to mintining griculture productivity nd reducing degrdtion (Ll et l., 1998; Singh & Ll, 2005). Therefore, doption of conservtion frming my ply n importnt role of soil C sequestrtion in Zimbbwe to boost the low OC levels (<1%) nd the generlly deficient plnt nutrients (Nympfene, 1991; Mugwir et l., 1992). Zimbbwe, like mny of the southern Africn countries, hs been experiencing rpid forest decline. Woodlnds nd forests in communl nd resettlement res re hevily frgmented nd degrded due to the clering for griculture nd hrvesting for vrious wood nd non-wood forest products. The mjor woodlnd type is the miombo whose crown cover cn vry between 20 nd 60 % (Wlker & Desnker, 2004). Miombo is the vernculr term for the sesonlly dry, deciduous woodlnds, semi deciduous, semi evergreen or drought deciduous woodlnds hving some species with pre-rin lef flush (Frost, 1996). The woodlnds re dominted by Brchystegi, Julbernrdi nd/or Isoberlini, spp. extending cross 2.7 million km 2 of some of the world s poorest countries (Cmpbell et l., 2007). The diverse woodlnd uses include firewood, 18

29 Introduction timber, poles, wtershed protection, provision of soil fertility (lef-litter), grzing nd browsing, edible fruits, mushrooms nd cterpillrs. The lnd use chnge to cultivtion hs n effect on C stocks (1069 Mt C). The extensive woodlnd cover mkes Zimbbwen forest cover potentil C sink though thretened by deforesttion nd degrdtion. The forests/woodlnds hve potentil to ct s C sinks provided forest mngement regimes re properly designed to reduce vulnerbility to conversion for griculturl crop production nd severe wild fires. Figure 1.3 Lnd tenure distribution in Zimbbwe. SSCF = smll scle commercil frms, LSCF= lrge scle commercil frms, Forest lnd = gzetted forests/woodlnds nd commercil plnttions (Moyo, 2000; Zimbbwe s Fourth Ntionl Report to the Convention on Biologicl Diversity, 2010). The Zimbbwen lnd reform progrmme hs shifted >31% of the country s woodlnds from privte tenure to stte control, with usufruct rights for communities nd individuls (Mtose, 2006). This ctivity cused forest nd woodlnds clernce to become the mjor driver of lnd use chnge resulting in n nnul loss of bout 1.5% (bout h) (UNDP, 1997; FAO, 2005). Ntionl woodlnd cover declined from 53% in 1992 to 42.34% in 2008 wheres 19

30 Chpter 1 crop lnd incresed from 27.48% in 1992 to 41.24% in 2008 (Zimbbwe s Fourth Ntionl Report to the Convention on Biologicl Diversity, 2010). Incresed droughts nd floods hve been cited s the min climtechnge effects in Sub-Shr Afric (FAO 2010). Despite Zimbbwe s reltively tiny contribution to globl GHG emissions, droughts nd cyclones re being experienced. The 1980s nd erly 1990s witnessed Zimbbwe s driest periods for the twentieth century. Ntionl verge rinfll declined by bout 5% between 1900 nd 2000, notwithstnding the episodes of wetter thn verge conditions in the 1920s, 1950s, nd 1970s (MDG report, 2010). Agriculturl sesons hve shifted s well, s evidenced by lte onset nd sometimes lte cesstion of riny.seson s. Longer-term rinfll predictions for Zimbbwe remin uncertin s climte chnge cuses more climtic extremes. The highest monthly dily mximum tempertures for most prts of the country, for exmple, re incresing Figure 1.4: Mp showing gro ecologicl regions I-V of Zimbbwe. (Vincent & Thoms, 1961; FAO, 2006). 20

31 Introduction by pproximtely 2ºC per century nd the percentge of dys with low tempertures decresing t rte of 15 dys per century. Assuming tht GHG emissions continue long the projected trjectories, tempertures will rise by between 0.5 C nd 2.0 C by 2030, nd 1.0 C nd 3.5 C by Vrious climte models project tht rinfll ptterns will chnge nd tht frequency of drought nd flood events will increse s well nd projected tht n dditionl 10 to 20% rinfll decline will be experienced in Zimbbwe by 2050 (MDG report, 2010). In ddition to climte chnge, lnd degrdtion hs become n importnt environmentl issue in Zimbbwe. There re synergies in simultneously ddressing climte chnge nd this lnd degrdtion. Thus, lnd degrdtion nd climte chnge will both be mitigted when effective wys to sequester C in soils re implemented. This not only promotes soil restortion but lso chieves winwin scenrio (FAO, 2004). There is therefore need for informtion on pproprite locl soil mngement prctices tht cn be used to chieve this win win gol in Zimbbwen gro ecosystems. 1.6 Scope nd Objectives of this thesis Considering the high rte of deforesttion in Africn countries, there is potentil for mngement of forests nd woodlnds for C sequestrtion through coppice or regenertion mngement (Chidumyo, 1991; Chidumyo, 1993) while incresing productivity of lnd through SOC sequestrtion. However, in most countries (including Zimbbwe) the current policy frmework is not sufficient to ensure the reflection of interntionl environmentl nd socil sfegurds nd stndrds for sustinble forest resource utilistion e.g. REDD+. The potentil for climte chnge mitigtion through SOC sequestrtion under griculturl ctivities in tropicl soils is poorly understood. Most griculturl bsed studies focused on soil fertility nd crop yields iming t ensuring crop productivity but without considertion of the implictions of the ctivities on climte chnge mitigtion. Assessment of SOC dynmics under different lnd mngement prctices cn therefore help to drw meningful conclusions bout SOC s contribution to globl C stocks (i.e. s sources or sinks) thus, enbling the 21

32 Chpter 1 ssessment of potentil roles of griculture nd forestry to mitigte climte chnge. The study of C storge potentil provides importnt informtion for shping policies nd incentives for better mngement of both griculturl nd forest systems. Informtion cn be useful when designing projects tht provide opportunities for communities to improve their livelihoods in ccessing crbon mrkets. Dt will be useful when responding to C demnd in future voluntry nd regultory greenhouse gs emissions mrkets. SOC cn become trdble mrket commodity under the Kyoto protocol (c.f. Article 3.4) where C credits cn be ssigned for griculturl C sequestrtion. The involvement of smll holder frmers in soil C sequestrtion, simultneously dvnces food security, mitigte climte chnge nd improve the environment thereby, trnsforming people from stte of production to tht of lnd resource stewrdship. The prticiption in C mrkets requires informtion on the potentil of lnd-use systems nd this study, in turn, fcilittes identifiction of possible C sequestrtion options. My reserch will help to clrify some of these issues by nlysing SOC nd TON nd their distribution in density frctions t different depths under selected griculturl prctices. Results provide importnt bseline informtion for griculture nd forestry opertions nd enbles comprisons between current nd projected future soil C sequestrtion nd emission reductions levels. This reserch estimtes C nd N sequestrtion potentil of selected Zimbbwen forests nd gro-ecosystems. The focus is C becuse crbon dioxide is the most bundnt greenhouse gs tht cn be sequestered in groecosystems. The study exmines effects of tillge prctices nd fertilistion on SOC nd TON pools in different smll holder frming systems reltive to ntive forests. Additionlly, the C sequestrtion potentil under tree frming nd its impcts on SOM prtitioning ws studied using chronosequence pproch. The impcts of different tillge nd fertility tretments on stbilistion of C nd N nd the prtitioning of SOM frctions s described for cropping nd forested systems remins uncler. The C nd N stocks were nlysed in bulk soil nd in three density seprted frctions t different soil depths. Lnd mngement prctices selected 22

33 Introduction for the study included conservtion tillge (i.e. reduced tillge (RP) nd no tillge (DS)), conventionl tillge (CT), fertility mendments (fertiliser nd mnure) nd plnttion forestry prctices. The min reserch hypotheses re: Long term fertiliser nd mnure ppliction will increse SOC in whole soil nd eventully in stbilised C frctions; Accumultion of different SOC frctions is influenced by the long-term ppliction of N fertiliser lone nd the combined ppliction of N fertiliser nd mnure under continuous mize cropping nd conventionl tillge; The light frction would be responsive indictor of short nd long term chnges in SOC; Tillge effects on C nd N dynmics vry with soil type; Lnd mngement nd soil properties (texture) ffect SOM frction contents; The lnd use legcy ffects prtitioning of SOC nd N in homefields nd outfields; nd The plnting of trees significntly ffects soil C storge t different depths over rottion with higher C sequestrtion rtes in younger stnds thn older stnds. These hypotheses result in the following specific reserch objectives: To ssess the effects of tillge nd fertilistion on SOC nd TON stbilistion in selected griculture nd forest systems on soils of contrsting texture; To ssess verticl distribution of SOC nd TON nd their frctions s ffected by different tillge prctices (CT, RP nd DS) reltive nturl forests; To ssess verticl distribution of SOC nd TON nd their frctions s ffected by N fertiliser nd combintion of N fertiliser nd mnure ppliction; To quntify C storge in the totl soil orgnic mtter nd SOM frctions in sndy nd clyey soils under different tillge nd fertility mngement; 23

34 Chpter 1 To determine orgnic C nd N dynmics in Pinus ptul chronosequence through ssessment of prtitioning of C nd N stocks in minerl soils nd forest floor reltive to ntive forests; nd To simulte the chnging SOC pool through time nd to predict the future pools nd impcts under lnd use chnge nd chnging climte scenrios. 1.7 Outline of the thesis In order to stisfy the hypotheses nd objectives outlined in 1.6, this thesis hs been structured into five chpters: Chpter 2 gives n ccount of the impcts of tillge prctices on C nd N stocks in bulk soil nd in three density seprted SOM frctions t 0-10 nd cm depths. The chpter compres C nd N storge in three tillge systems on frmer mnged experiments nd djcent nturl forests in sndy Hplic Arenosols nd clyey Rhodic Ferrlsols of Zimbbwe. The C nd N storge in tillge systems is compred to bseline mesurements tken in 2005 (only t 0-10 cm) for whole soil. Chpter 3 investigtes the effects of long term fertilistion on 1) SOC nd TON nd 2) density frctions for soil up to 50 cm on Luvisols nd Arenosol in Murew district of Zimbbwe. Soils collected from two frms on fields with contrsting soil types were nlysed. The chpter explores the potentil of C sequestrtion under continuous conventionlly tilled fields with inorgnic fertiliser nd cttle mnure ppliction reltive to distnce from homested. C nd N in whole soil nd density frctions ws determined by dry combustion. Chpter 4 exmines C nd N storge in pine plnttions locted in the estern highlnds of Zimbbwe long n ge series. Plnttions of exotic pine trees were estblished fter clering nturl forests. These nturl forests cn either be ptches of moist forests or ptches of Miombo woodlnds depending on soil type nd depth. Forests ply mjor role in regulting the rte of increse of globl tmospheric crbon dioxide (CO 2 ) concentrtions creting need to investigte the bility of exotic plnttions to sequester CO 2 from the tmosphere. C nd N storge ws ssessed from rndomly selected replicted 24

35 Introduction plots in Pinus ptul stnd ges of 1, 10, 20, 25, nd 30 yers plus two nturl forests. Annul litter fll nd C nd N in forest floor were ssessed. Furthermore, comprisons were mde between C nd N storge in older pine ge clsses (25 nd 30 yers) with the two nturl forests (moist forest nd Miombo woodlnd). Chpter 5 shows the impcts of lnd mngement prctices on future C stocks using the dynmic RothC model. This model is used to ssess the impct of lnd mngement on SOC storge nd helps to investigte scenrios nd hypotheses tht re beyond the relm of current ssessments. The running of the model requires informtion on soil type, plnt cover nd monthly climte input dt. In this chpter, experimentl dt from Chpters 2 nd 3 re used to ssess performnce nd bility of RothC to simulte long-term SOM chnges in Zimbbwen soils. The results confirm the bility of RothC to simulte short nd long-term soil C dynmics. The reltionship between conceptul model pools nd density seprted frctions ws determined. Furthermore the equilibrium levels estimted by the RothC model were compred with equilibrium levels by the Lngmuir eqution. Chpter 6 summrises the min conclusions derived from the integrtion of the entire work. It lso highlights proposls for mngement nd reserch to ensure sustinble utilistion of forest nd griculturl lnd resources. 25

36 Chpter 1 26

37 Effects of tillge on soil orgnic crbon nd nitrogen storge CHAPTER 2 Lnd use nd mngement effects on soil orgnic mtter frctions in Rhodic Ferrlsols nd Hplic Arenosols in Bindur nd Shmv districts of Zimbbwe Published s: Mujuru, L., Murev A., Velthorst E nd Hoosbeek M Lnd use nd mngement effects on soil 0rgnic mtter frctions in Rhodic Ferrlsols nd Hplic Arenosols in Bindur nd Shmv districts of Zimbbwe, Geoderm

38 Chpter 2 Lnd use nd mngement effects on soil 0rgnic mtter frctions in Rhodic Ferrlsols nd Hplic Arenosols in Bindur nd Shmv districts of Zimbbwe Abstrct Soil orgnic crbon (SOC) is mjor ttribute of soil qulity tht responds to lnd mngement ctivities nd is lso importnt in the regultion of globl crbon (C) cycling. This study evluted bulk soil C nd nitrogen (N) contents nd C nd N dynmics in three density seprted soil orgnic mtter (SOM) frctions. The study ws bsed on three tillge systems on frmer mnged experiments; (conventionl tillge (CT), ripping (RP), direct seeding (DS)) nd djcent nturl forest (NF) in Hplic Arenosols (sndy) nd Rhodic Ferrlsols (clyey) of Zimbbwe. Crbon stocks were significntly lrger in forests thn tillge systems, being significntly lower in sndy soils (15 nd 14 Mg C h -1 ) thn clyey soils (23 nd 21 Mg C h -1 ) t 0-10 nd cm respectively. Nitrogen content followed the sme trend. At the 0-10 cm depth SOC stocks incresed under CT, RP nd DS by 0.10, 0.24, 0.36 Mg h -1 yr -1 nd 0.76, 0.54, 0.10 Mg h -1 yr -1 on sndy nd clyey soils respectively over four yer period while N stocks decresed by 0.55, 0.40, 0.56 Mg h -1 nd 0.63, 0.65, 0.55 Mg h -1 respectively. SOM frctions were dominted by minerl ssocited hevy frction (MHF) which ccounted for 86-93% nd 94-98% on sndy nd clyey soils respectively. Tillge systems on sndy soils hd smllest verge free light frction (flf) nd occluded light frction (olf) C stocks (25.3± 1.3 g m -2 nd 7.3 ± 1.2 g m -2 ) t 0 30 cm when compred with corresponding NF (58.4± 4 g m 2 nd 18.5 ±1.0 g m -2 ). Clyey soils, hd the opposite, hving ll flf C nd N in tillge systems being higher (80.9±12 g C m -2 nd 2.7±0.4 g N m -2 ) thn NF (57.4± 2.0 g C m -2 nd 2.4 ± 0.3 g N m -2 ). Results suggest tht olf nd MHF C nd N re better protected under DS nd RP where they re less vulnerble to minerlistion while flf contributes more in CT. Thus, DS nd RP cn be importnt in mintining nd improving soil qulity lthough their prcticbility cn be hmpered by unsupportive institutionl frmeworks. Under previling climtic nd mngement conditions, improvement of residue retention could be mjor fctor tht cn distinguish potentil of different 28

39 Effects of tillge on soil orgnic crbon nd nitrogen storge mngement prctices for C sequestrtion. The exploittion of the benefits of RP or DS nd corresponding sustinbility of systems need support for surfce cover retention which should lso be extended to conventionl tillge. Key words: Soil density frctiontion; soil mngement; soil orgnic mtter; soil crbon sequestrtion; soil nitrogen; tillge. 2.1 Introduction Soil orgnic mtter (SOM) represents lrge, dynmic nd complex terrestril reservoir of crbon (C) in the form of orgnic compounds derived from plnt, niml or microbil biomss (Bldock, 2009). Lnd use prctices in gro ecosystems ffect the storge of soil orgnic crbon (SOC) nd nitrogen (N). The rtes of C relese from the soil vries with lnd use type, climte nd the soil mtrix. Severl studies hve shown tht significnt mounts of C were lost from soil s crbon dioxide (CO 2 ) when forests were converted to griculture (IPCC., 2000; Food nd Agriculture Orgnistion (FAO), 2010), through the relese of physiclly protected soil C (Six et l., 1999; McConkey et l., 2003; Denef et l., 2007). Despite this loss, griculture is inevitbly required to enhnce food security in the developing countries which re thretened by food shortges (FAO, 1996). In this regrd, conservtion griculture (CA) hs been recommended s mens of C ccumultion nd soil qulity enhncement (Ken & Johnson, 1993; Chivenge et l., 2007b; Álvro-Fuentes et l., 2008; Dercon et l., 2010) nd hs been proposed s mens of sustinble lnd use mngement (Food nd Agriculture Orgnistion (FAO), 2010). In southern Afric, the resons for the success of conservtion frming hve been clerly outlined (Mrongwe et l., 2011; Andersson & Giller, 2012). However, the success of some conservtion frming prctices such s no-tillge in C storge depends on qulity nd quntity of orgnic residue inputs nd the degree of soil disturbnce (de Mores Sá et l., 2011). 29

40 Chpter 2 No tillge or miniml tillge prctices might ffect SOC stocks, lthough some studies hve shown tht no-tillge increses SOC concentrtions only in the upper lyers of some soils hving no significnt differences with conventionl tillge over the whole profile (Blnco-Cnqui & Ll, 2008; Poirier et l., 2009). Soil orgnic C my be present in the soil s either 1) reltively fresh (lbile) SOM not protected by the soil mtrix, s 2) SOM physiclly protected in ggregtes (occluded), or s 3) SOM dsorbed onto minerl surfces (chemiclly stbilised) (Dll & Myer, 1986). Although lnd mngement systems such s tillge my not hve n effect on bulk SOC nd N, they my hve n effect on individul SOM frctions (Von Lützow & Kӧgel-Knbner, 2009). Moreover, chnges in SOM frctions my provide insight into the effects of tillge prctices on SOC stbilistion (von Lützow et l., 2007 ; Mrín-Spiott et l., 2008). Isoltion of these functionl pools cn be done by density or size seprtion. Density frctiontion is often coupled with ultrsonic dispersion to give three mutully exclusive frctions: free light frction (flf) extrcted before the brekdown of ggregtes; occluded light frction (olf) isolted fter ultrsonic disruption nd minerl ssocited frction (MHF) recovered in the remining hevy precipitte which is considered s stble (Poirier et l., 2005; Gregorich et l., 2006). Evidence of differences in residence time of flf nd olf hs been shown (Golchin et l., 1994b; Gregorich et l., 2006) nd confirmed by rdio C dting (Bisden et l., 2002; Swnston et l., 2005). The minerlssocited hevy frction contins more processed mteril with slower turnover rte nd higher degree of chemicl protection (Hssink, 1995) thn olf. Soil orgnic C nd N in density frctions re importnt ttributes of the qulity of soil nd ssocited impcts of lnd mngement systems. The pplicbility nd fesibility of density frctiontion hs not been fully exploited in Zimbbwen soils. Some studies in Zimbbwe focused on crop production nd soil orgnic mtter frctions in ggregte sizes nd hve reveled tht physiclly seprted SOM frctions chnge with ggregte size (Chivenge et l., 2007b; Nymdzwo et l., 2009). Despite the importnce of SOC storge in the light frction, there is little informtion bout size, composition nd stbility of free 30

41 Effects of tillge on soil orgnic crbon nd nitrogen storge nd occluded light frctions in sndy nd clyey soils of Zimbbwe. In ddition, reserch on experimentl sttions hs not provided the required informtion on C sequestrtion potentil since it does not represent frmer s conditions (Giller et l., 2011). In this study, ctive collbortion with frmers in the reserch plots provided true picture of smll holder frmers condition which cn result in the trnsfer of technologies to their own fields. The ntive forests in the re re minly miombo woodlnds which re subjected to nnul wild fires nd re utilised by locl communities for timber nd non -timber forest products. We therefore im to evlute the effects of lnd mngement systems on: 1) bulk soil C nd N contents nd 2) the C nd N dynmics of three frctions, (i.e. the free light frction (flf), occluded light frction (olf) nd minerl ssocited hevy frction (MHF)) on frmer mnged tillge experiments on sndy nd clyey soils. In order to ssess the effect of griculturl lnd use, irrespective of tretment, on SOC nd N contents, smples were lso collected from djcent nturl forests to show the C benefits of not clering more lnd for griculture. Agriculturl lnd is mostly product of deforesttion of these ntive forests. 2.2 Mteril nd methods Study site This reserch ws crried out t two experimentl loctions; Hereford in Bindur district (17 42 S; E) nd Nyrukund in Shmv district (17 00 S; E) (Figure 2.1) which were estblished in frmers fields in The two res trnscend zone with ltitudinl rnges between 1000 nd 1800 m..s.l. nd nnul unimodl rinfll of mm. Sndy soils, mostly derived from corse grnite cover nerly 70% of Zimbbwe (Thompson & Purves, 1981) mostly of the Kolinitic order, Fersillitic group under the Zimbbwen soil clssifiction, corresponding to Alfisols in soil Txonomy (Nympfene, 1991). 31

42 Chpter 2 The dominnt soils in the Nyrukund re cn be clssified s corse grined sndy to sndy lom soils, corresponding to Ferric Luvisols (Thompson & Purves, 1981; Scoones, 2001; FAO, 2006) but the IPCC defult clsses derived from the hrmonised world soils dtbse (Btjes, 2010) clssified them s Arenosols, (>70% snd nd <8% cly) nd re brodly referred s sndy soils. Hereford soils re red clys vrying from silty cly lom to cly, with chrcteristics corresponding to Rhodic Ferrlsols (Nympfene, 1991; FAO/IIASA/ISRIC/ISSCAS/JRC, 2012) nd flling into the ctegory of low ctivity clys (Btjes, 2010). The two res hve the sme climte (Region II) nd vegettion type of miombo woodlnd dominted by Brchystegi spiciformis nd Julbernrdi globiflor to mixed woodlnd (dominted by Acci polycnth) nd bush lnd. They, however, differ in lnd use history nd soil chrcteristics. The forest on clyey soils ws prt of commercil frming re nd ws not s intensively utilised s the one on sndy soils nd most of the griculturl lnd ws creted fter deforesttion of nturl forest res. Shmv s Nyrukund wrd, is communl re where frmers hve been prcticing conservtion frming on smll pieces of lnd for environmentl nd socio-economic gins (Belder et l., 2007; Thompson, 2010). Mjor crops grown in the re re: mize (Ze mys L.), cow-pes (Vign unguicult L. Wlp), sugr bens (Phseolus vulgris L.), groundnuts (Archis hypoge L.), sorghum (Sorghum bicolor, L.) nd sunflower (Helinthus nnuus, L.). Additionlly, frmers in Hereford grow commercil crops such s soybens (Glycine mx. L. Merr) nd tobcco (Nicotin tbcum L.). In Hereford mechnised tillge with disc nd hrrow ws used before the frm ws designted s new resettlement re in Within ech re, four frmer s field were chosen, ech being 0.3 h, subdivided into three equl portions (0.1 h) where the three min tretments were estblished: 1) conventionl frming (CT) consists of n ox drwn plough to depth of cm once before plnting. Residues were removed nd the remining biomss incorported into the soil during ploughing in the next seson; 2) minimum tillge with n ox drwn ripper (RP) cm followed by mnul plnting nd fertiliser ppliction. Mize stover ws supplemented by thtching 32

43 Effects of tillge on soil orgnic crbon nd nitrogen storge grss in the first seson. In subsequent yers, crop residues were to be retined in the field fter hrvesting. Ground cover of Mg h -1 ws required in ech RP plot; nd 3) no tillge using n ox drwn direct seeder (DS) synchronised seeding nd fertiliser ppliction. Residues were lso retined or supplemented to chieve the Mg h -1 ground cover. Figure 2.1: Mp of Zimbbwe showing loction of Shmv nd Bindur districts. Ech tillge system ws divided in hlf, where during the first two yers, mize (Z. mize. L) ws grown on one side of the field nd cowpes (V. unguicult L. Wlp) t Nyrukund or soy ben (G. mx L. Merr) t Hereford grown on the other. During the next two yers the sides were switched. Ech tretment received nnul bsl fertiliser of 165 kg h -1 compound D (i.e. 11 kg h -1 N, 10 kg h -1 P, 10 kg h -1 K), which ws followed by 69 kg h -1 N pplied s mmonium nitrte in splits t 4 nd 7 weeks fter germintion. The soy ben 33

44 Chpter 2 received no top dressing but ws inoculted with commercil rhizobium before sowing. Weed control ws by herbicide in RP nd DS while in CT it ws done conventionlly by cultivtor or plough. All fields in ech soil type were plnted with similr mize nd legume vrieties ech yer. In ddition to the griculturl sites in ech re, n indigenous forest site in close proximity nd on the sme soil type ws included s they re trget for griculturl expnsion nd cn be useful reference for C storge cpcity. Initil C nd N stocks s recorded during the 2005/2006 seson (Interntionl Mize nd Whet Improvement Centre (CIMMYT), unpublished results -2005/2006) hd n verge C concentrtion rnging between 3 nd 5 g kg - 1 on sndy soils compred to 13 nd 15 g kg -1 for the clyey soils. Men C stocks t 0-10 cm under CT, RP nd DS were 4.79, 6.38, 5.45 Mg h -1 nd 6.08, 16.08, Mg h -1 on sndy nd clyey soils respectively (Tble 2.1). Nitrogen stocks were 0.93, 0.93, 1.06 nd 1.92, 2.04, 1.92 Mg h -1 on sndy nd clyey soils respectively. The ph ws higher in clyey soils thn in sndy soils. All vlues of soil chrcteristics were higher on clyey soils thn sndy soils except the mount of course nd medium snd which were obviously lrger in sndy soils Surfce litter nd soil smpling At ech smple point, surfce litter ws collected by inserting 30 cm dimeter ring into the litter on top of the minerl soil nd the litter ws collected from inside the ring (g litter per surfce re). Three composited soil smples (ech bsed on 4 sub-smples) were collected from ech field, (i.e. per tretment, for two depth intervls of 0-10 nd cm). A totl of 144 soil smples were tken from the griculturl plots, (i.e. 2 res 4 sites 3 tretments 2 depths 3 replictes). The four sites in ech soil type were treted s replictes reltive to tillge systems. Forest soil smples totlled 24 soil smples (2 res 6 smple points 2 depths) plus six litter smples. An dditionl soil smple ws collected, t ech depth by inserting metl ring of 100 cm 3 into the soil for bulk density mesurements. The soils were ir-dried nd sieved (< 2 mm) before C nd N nlysis nd physicl frctiontion. 34

45 Effects of tillge on soil orgnic crbon nd nitrogen storge Tble 2.1: Bulk soil chemicl nd physicl soil properties t 0 10 cm depth before estblishment of three tillge systems t sites selected for the study t Nyrukund nd Hereford study res. Chrcteristic Shmv Nyrukund (Sndy soils) Bindur- Hereford (Clyey soils) Tillge CT RP DS CT RP DS ph (C Cl 2 ) BD (g cm -3 ) SOC (Mg h -1 ) TON (Mg h -1 ) C:N rtio P (mg kg -1 ) ex K (cmol/kg) ex C (cmol/kg ex Mg (cmol/kg TEB (cmol/kg BASE SAT % Cly % Silt % snd % CT CT = = conventionl conventionl tillge, tillge, RP= RP= ripping, ripping, DS= direct DS= direct seeding. seeding. Source: Interntionl Mize nd Whet Improvement Centre (2005/2006). Source: (Interntionl Mize nd Whet Improvement Centre (CIMMYT), unpublished results -2005/2006) Soil density frctiontion Soil smples were subjected to physicl frctiontion to obtin three orgnic mtter frctions (free light frction (flf), occluded light frction (olf) nd minerl ssocited hevy frction (MHF)) using the method described by Roscoe et l. (2000). The frctions were recovered by density seprtion using sodium polytungstte (3N 2 WO 4.9WO 3.xH 2 O; SPT). Briefly, 10g ir dried soil <2 mm ws 35

46 Chpter 2 shken in centrifuge tube with 50 ml SPT solution of density 1.6 g cm -3 nd gently mixed by inverting 5 times nd llowed to stnd for t lest 1 hour to fully wet the smple. Next, the smples were centrifuged for 23 minutes t 4500 rpm. The superntnt ws poured over Büchner funnel with 0.45 µm Whtmn GF/F glss filter using Millipore vcuum filtrtion unit (Millipore, Bedford, MA). The light frction (LF) lso referred to s the free light (flf) or lbile frction ws rinsed off the filter into glss beker nd oven dried t 40 ºC. SPT solution with density 1.6 g cm -3 ws dded to the residul soil mteril nd shken by hnd to bring the precipitte into solution. An ultrsonic probe ws used for 5 minutes to disperse the occluded light frction fter which the suspension ws left to stnd for 30 minutes. The suspension ws centrifuged for 23 minutes t 4500 rpm. The centrifuged tubes were decnted over Büchner funnel with 0.45 µm Whtmn GF/F glss filter. The occluded light (olf) or physiclly protected light frction ws rinsed off the filter into glss beker nd oven dried t 40 ºC. The remining precipitte, the minerl-ssocited (MHF) frction, needed to be rinsed thoroughly,(i.e. t lest 5 times for clyey soils nd three times for sndy soils, or until conductivity of the filtrte ws <50µS cm -1 ) in order to remove ll STP. Totl C nd N contents of flf, olf nd MHF nd whole soil were determined by dry combustion using CN nlyser. Bulk soil C nd N were estimted using eqution [1] nd the mount of SOC in frctions ws expressed s concentrtion (g C kg 1 of soil) nd multiplied by depth nd bulk density to obtin stocks of ech frction. Orgnic C (Mg h -1 ) = Depth (cm) Bulk density (g/cm 3 ) Crbon content (%) [1] Dt were tested for homoscedsticity using Levenes s test nd it showed tht error vrinces of dependent vribles were not equl cross groups except C:N rtios. Sttisticl comprisons of C nd N mong lnd mngement systems were therefore done using multivrite nlysis of vrince (MANOVA) which is generlistion of nlysis of vrince tht permits testing for men differences on severl dependent mesures simultneously. Wilk s Lmbd ws used s the multivrite mesure. Crbon nd N were the dependent vribles whilst soil 36

47 Effects of tillge on soil orgnic crbon nd nitrogen storge type, mngement system nd depth were the fixed fctors. Tukey s HSD test ws used for post-hoc nlysis of C nd N in bulk soil nd the three frctions in ech soil type, mngement system nd soil depth. Sttisticl nlyses were performed with softwre pckge SPSS 19.0 for windows. Differences were considered significnt t p < Results Surfce litter in lnd mngement systems Men C of litter for tillge systems ws 0.31 nd 0.22 Mg h -1 on sndy nd clyey soils respectively compred to 0.64 nd 0.60 Mg h -1 in respective forests (Figure 2.2). c c b b b Figure 2.2: Amount of litter persisting in fields before the cropping seson. CT = conventionl tillge; RP = ripping; DS = direct seeding nd NF = nturl forest. Different letters show significnt differences t p = Error brs ± 1 SE. There ws no significnt difference in surfce litter ccumultion mong tillge systems on sndy soils. The mount of surfce residue mong tillge 37

48 Chpter 2 systems in ech soil type ws 34% nd 46% lower thn respective NF on sndy nd clyey soils. On Clyey soils RP hd highest surfce litter ccumultion which ws hlf of the litter on forest floor. In ll fields, termites seemed to ply n importnt role s they were present nd ctive in ll plots nd they my hve significnt effect on the degrdtion of orgnic inputs nd promotion of the relese of nutrients to crops Soil bulk density, ph, C nd N in lnd mngement systems Despite some humn influence t the forested sites, (e.g. fire wood collection nd fire) the soils t the Nyrukund nd Hereford woodlnds were considered to represent nturl soil conditions for ech loction. There ws stronger reltionship between bulk density nd totl orgnic C on clyey (R 2 =0.987) thn sndy soils (R 2 = 0.016). Tillge neither ffected bulk density, soil ph (p>0.1) nor C:N rtios lthough higher bulk densities were more ssocited with CT nd lest with RP being higher on sndy soils (1.32 g cm -3 ) thn clyey soils (1.20 g cm -3 ) (Tble 2.2). Soil orgnic C stocks vried significntly (p<0.01) between tillge systems nd nturl forests being significntly higher on clyey soils (F = ; p< 0.01) thn on sndy soils in ll lnd mngement systems nd t ll depth levels (Figure 2.3). Men C stocks in the lnd use systems up to 30 cm depth rnged from 7.97 to Mg h -1 nd 30.6 to 43.9 Mg h -1 on sndy nd clyey soils respectively. CT, RP nd DS hd 36%, 32%, 32% nd 14%, 15% 14% lower C thn their respective forests on sndy nd clyey soils respectively. On Sndy soils C ws higher under DS (10.9 Mg h -1 ) thn RP (10.7 Mg h -1 ) nd CT (8.0 Mg h -1 ) t 0-30 cm wheres on clyey soils, C ws higher under RP (32.3 Mg h -1 ) thn CT (31.2 Mg h -1 ) nd DS (30.6 Mg h -1 ) though not significntly. Depth distribution showed significntly higher (F= 22.98; p<0.01) C stocks t 0-10 cm thn t cm in ll cropping systems (Tble 2.2). On sndy soils, t 0-10 cm depth C ws higher under DS (6.9 Mg h -1 ) thn RP (6.4 Mg h -1 ) nd CT (5.2 Mg h -1 ) whilst on clyey soils there ws more C 38

49 Effects of tillge on soil orgnic crbon nd nitrogen storge under CT (19.1 Mg h -1 ) thn RP nd DS lthough the differences were not significnt. Tble 2. Soil chrcteristics, C nd N stocks in three tillge systems nd nturl forests on Tble sndy 2.2: nd Soil clyey chrcteristics, soils (n=12). C nd N stocks in three tillge systems nd nturl forests on sndy nd clyey soils (n=12). Soil Mngement Totl orgnic C (Mg h -1 ) Totl orgnic N (Mg h -1 ) ph BD(g cm 3 ) 0-10 cm cm 0-30cm C:N Sndy CT 4.7(0.1) 1.37(0.02) 5.16(0.77) 2.81 (0.76) 7.97 RP 0.38(0.07) 0.22(0.07) (0.1) 1.32(0.03) 6.38(.76) 3.90(0.76)b (0.07) b 0.34(0.07) b 13 DS 4.7(0.1) 1.33(0.03) 6.91(1.76) 4.46(0.77)b (0.05) b 0.37(0.06) b 13 NF 5.1(0.1) 1.27(0.3) 14.96(1.09) b 14.29(1.09)c (0.10) c 1.09(0.10) c 13 Clyey CT 5.8(0.3) 1.22(0.06) 19.13(0.76) 12.04(0.77) RP DS NF 5.8(0.3) 1.21(0.05) 18.22(0.78) 14.05(0.76) b (0.07) 0.96(0.07) (0.07) b 1.20(0.07) b (0.3) 1.16(0.04) 18.03(0.76) 12.59(0.76) (0.06) b 0.99(0.07) (0.3) 1.21(0.2) 22.77(1.09) b 21.11(1.08)c (0.11) c 1.46(0.10) c 14 CT =conventionl tillge, RP = minimum tillge with ripper, DS= no tillge using direct seeder, BD = bulk density. Stndrd error of the men shown in prenthesis. Mens followed by different letters re significntly different t p=0.05. Tukey s HSD test. A comprison with the bseline C stocks (Tble 2.1) showed tht CF, RP nd DS incresed C stocks t 0-10 cm depth by 4%, 4%, 8% nd 8%, 6%, 2% on sndy nd clyey soils respectively (Tble 2.1 nd 2.2). The SOC stocks incresed under CT, RP nd DS by 0.10, 0.24, 0.36 Mg h -1 yr -1 nd 0.76, 0.54, 0.10 Mg h -1 yr -1 on sndy nd clyey soils respectively. Over the four yer period N stocks were reduced by 0.55, 0.40, 0.56 Mg h -1 nd 0.63, 0.65, 0.55 Mg h -1 on sndy nd clyey soils respectively. There were significnt differences in N content (p=0.015) between sndy (0.5±0.3 Mg 39

50 Chpter 2 h -1 ) nd clyey soils (1.2±0.3 Mg h -1 ) which did not hve corresponding effect on the C:N rtio. The men C:N rtios rnged from 9-20 nd on sndy nd clyey soils respectively. The C:N rtio nd totl orgnic N were not significntly different by depth (p=0.012). b b b b Figure 2.3: Soil orgnic crbon (SOC) nd totl orgnic nitrogen (TON) stocks in three tillge tretments (CT = conventionl tillge, RP = ripping nd DS = direct seeding) nd nturl forest (NF) in sndy nd clyey soils t 0 30 cm depth Crbon nd N in density frctions The light frction only ccounted for ±1% of the soil mss. However, the orgnic C nd N concentrtion of the density frctions were inversely relted to their msses. On sndy soils men C nd N concentrtion of flf ws 33±8% nd 1.5±0.5% respectively while the olf C nd N were 26±8% nd 1.2±0.3%. The men MHF C nd N were 0.7±0.3% nd 0.05±0.02% respectively. On clyey soils, men C nd N concentrtion of flf ws 40±6% nd 1.5±0.3% respectively while in the olf C nd N were t 41±4% nd 1.8±0.2%. The MHF C nd N were 2.8±1% nd 0.19±0.07% respectively. Despite the smll proportion of the two light frctions 40

51 Mngement flf + olf C flf C/ SOC flf N/TON olf C/SOC olf N/TON MHF/SOC MHF/TON Effects of tillge on soil orgnic crbon nd nitrogen storge to the soil mss, they contributed to 6±2% of the bulk SOC nd 1.7±1.3% of the totl N stocks (Tble 2.3). Tble 2.3. Distribution of soil orgnic mtter frctions nd their reltionship to SOC nd TON s ffected by different lnd mngement system t 0 30 cm depth. C:N Rtios (g C m -2 ) % flf olf MHF Sndy CT 24.1(5.3) Soil RP 37.8(5.3) b DS 35.7(5.3) NF 76.9(7.6) b HSD Clyey CT 97.3(5.6) b Soil RP 109.9(5.3) b b 26.9 b DS 89.9(5.3) b b 27.8 b NF 85.7(7.5) 2.7 b 1.5 c b 97.7 b HSD CT =conventionl tillge, RP = minimum tillge with ripper, DS= no tillge using direct seeder, NF= nturl forest, BD = bulk density. flf = free light frction, olf = occluded light frction, MHF = minerl ssocited hevy frction. Stndrd error of the men shown in prenthesis. Mens followed by different letters re significntly different in ech soil type t p=0.05. HSD = Honest Significnt Difference. The mount of orgnic C stored in density frctions nd their depth distribution differed between lnd use types (Figure 2.4). The flf C nd N were significntly lrger in clyey soils thn in sndy soils nd significntly lrger in the top lyer thn the lower lyer (P<0.01). Tillge systems on sndy soils hd smllest verge flf nd olf C stocks (25.3 ± 1.3 g m -2 nd 7.3 ± 1.2 g m -2 ) t 0 30 cm when 41

52 Chpter 2 compred with corresponding NF (58.4 ± 4 g m 2 nd 18.5 ±1.0 g m -2 ). The two light frctions were not significntly different between RP nd DS on sndy soils. Clyey soils, hd the opposite, hving ll flf C nd N in tillge systems being higher (80. ± 12 g C m -2 nd 2.7 ± 0.4 g N m -2 ) thn NF (57.4 ± 2.0 g C m -2 nd 2.4 ± 0.3 g N m -2 ). The flf N ws significntly lower under CT thn DS nd RP on sndy soils wheres on clyey soils flf N ws lowest in NF. c b c c b b b b b b b c b c b b b Figure 2.4: 2.4: Distribution of of crbon nd nd nitrogen in free in free light light frction frction (flf) (flf) nd nd occluded light frction (olf) occluded in three light tillge frction systems (olf) in (CT three conventionl tillge systems tillge, (CT conventionl RP- Ripping nd tillge, DS RP- Direct seeding) nd nturl Ripping forest(nf) nd DS in Direct sndy seeding) nd clyey nd soils nturl up forest(nf) to depth in of sndy 30 cm. nd Different clyey soils letters show significnt differences up to depth of the of men 30 cm. t Different p=0.05. letters show significnt differences of the men t p=

53 Effects of tillge on soil orgnic crbon nd nitrogen storge The mount of orgnic C stored in density frctions nd their depth distribution differed between lnd use types (Figure 2.5 nd 2.6). The flf C nd N were significntly lrger in clyey soils thn in sndy soils nd significntly lrger in the top lyer thn the lower lyer (P<0.01). Tillge systems on sndy soils hd smllest verge flf nd olf C stocks (25.3 ± 1.3 g m -2 nd 7.3 ± 1.2 g m -2 ) t 0 30 cm when compred with corresponding NF (58.4 ± 4 g m 2 nd 18.5 ±1.0 g m -2 ). The two light frctions were not significntly different between RP nd DS on sndy soils. Clyey soils, hd the opposite, hving ll flf C nd N in tillge systems being higher (80.9 ± 12 g C m -2 nd 2.7 ± 0.4 g N m -2 ) thn NF (57.4 ± 2.0 g C m -2 nd 2.4 ± 0.3 g N m -2 ). The flf N ws significntly lower under CT thn DS nd RP on sndy soils wheres on clyey soils flf N ws lowest in NF. b b c b b b Figure 2.5: Distribution of crbon nd nitrogen in minerl ssocited hevy frction (MHF) in three tillge systems (CT conventionl tillge, RP- Ripping nd DS Direct seeding) nd nturl forest(nf) in sndy nd clyey soils up to depth of 30 cm. Different letters show significnt differences of the men t p=0.05. The olf C nd N frctions were lowest under CT nd highest under DS on sndy soils. On clyey soils RP significntly incresed olf C nd N t 0-10 when 43

54 Chpter 2 compred with DS nd CT. At cm DS hd more C nd N thn RP nd CT lthough they were ll lower thn NF. The lrgest proportion of soil C nd N ws found in the MHF (C: 93±6%; N: 96±2%) nd (C: 94±4%; N: 97±1%) on sndy nd clyey soils respectively (Tble 2.3). Thus, MHF represented consistent trend reltive to lnd mngement system (Figure 2.6) nd ws significntly correlted with totl SOC (R 2 = 0.998; p <0.01; n=168; SE =0.29) lthough the rnge ws 87-98% of totl SOC nd TON. The MHF C nd N were higher in top thn lower soil depth in ll mngement systems. Interction of depth nd texture significntly ffected MHF nd olf C nd N contents. The C:N rtios for MHF were smller thn flf nd olf rnging from 9-21 compred to for flf nd for olf. The men C:N rtios decresed in the order flf (23±6) > olf (20±3) > HF (13±3) cross sites on sndy soils with similr trends on clyey soils of flf (26±4) > olf (23±2) > HF (13±3). The C:N rtios of olf decresed with soil depth for most sites while C:N rtios of flf remined constnt. 2.4 Discussion Surfce litter retention in griculturl nd forest systems Forests hd more C nd N in litter thn griculture systems (Figure 2.2) lthough their potentil is retrded by the removl of forest litter often used s soil mendments in griculturl fields (Kowero, 2003; Giller et l., 2006) ffecting soil orgnic mtter input. Regenertion of forest ecosystems cn be seriously ffected by fctors such s seed fll, seed bnk, nutrient vilbility nd microclimte (Holl, 1999). In ddition to soil texture, size nd composition of bove ground biomss, history of utilistion lso ffects the ccumultion of orgnic mtter (Sleutel et l., 2011). Thus, in res from which lrge prt of the surfce lyer of the soil is removed or disturbed, s in this study nd most severely in the Nyrukund (sndy) re, restortion of the SOC nd N levels cn be slow. Despite this, the forests remin importnt in C storge. Inputs through litter ccumultion in miombo woodlnds is lso be ffected by frequent fires. 44

55 Effects of tillge on soil orgnic crbon nd nitrogen storge Figure 2.6: Distribution of orgnic C nd N in density frctions (flf = free light frction, olf = occluded light frction nd MHF = minerl ssocited frction) in three tillge systems (CT conventionl tillge, RP ripping nd DS direct seeding) nd nturl forest (NF) t two depth intervls. 45

56 Chpter 2 Conservtion tillge systems (RP nd DS) should receive nnul orgnic mtter inputs of Mg h -1 which is normlly supplemented by thtch grss mteril which is esily degrded nd only persists till end of the growing seson fter which the crop residues tke over. The effects of residue retention my even be lrger thn other fctors such s soil texture or rinfll s the residue retention ws more pplicble in the initil yers when the plots were protected from free rnging livestock resulting in DS plots hving more C thn CT nd RP (Thierfelder & Wll, 2012). At the time of the study ll fields were no longer protected. Crop residues re sometimes collected to feed livestock in the dry seson or the livestock feed in situ resulting in very low orgnic inputs into the soil. This hs n effect on the mount of C system cn store nd my result in low C ccumultion under conservtion griculture (CA) systems. The ttinment of complete cover in CA systems my not be prcticl in Zimbbwen rurl systems under existing locl institutionl rrngements. In ddition, the thtch grss used to supplement residues is sometimes thretened by nnul wildfires strengthening the need for community fire mngement nd fire prevention policies. However, climtic condition, including the long dry spell from April to November, is mjor limiting fctor to the preservtion of orgnic mtter in ll tillge systems s reflected by the low mounts of surfce litter. During this period the sites re eqully disturbed by being subjected to trmpling by nimls, excessive dryness, termite degrdtion of orgnic residues nd sometimes wind erosion (mostly in August nd September) Soil bulk density, C nd N in different lnd mngement systems Bulk densities found in the study t 0-10 cm (Tble 2.2) re comprble to those found by King nd Cmpbell (1994) nd Wlker nd Desnker (2004) who compred miombo woodlnds nd rble lnds in the southern Africn region. Sndy soils tend to hve higher bulk densities by virtue of their higher snd 46

57 Effects of tillge on soil orgnic crbon nd nitrogen storge content which mkes them more vulnerble to trmpling by grzing nimls nd humns especilly during the dry seson. Soil orgnic C stocks differed gretly between griculture nd forested lnd with the two forests hving higher C nd N contents s compred to griculture lnds (Tble 2.2; Figure 2.3). Our results (46.11 Mg h -1 nd 53.9 Mg h -1 ) re comprble to C stocks found in other woodlnds of southern Afric rnging from Mg h -1 (Woomer, 1993 ; Wlker & Desnker, 2004; Zingore et l., 2005; Ryn et l., 2011). In this study, soil texture ws importnt in determining the mounts of C nd N storge in soils since the two sites receive equivlent mounts of rinfll. Clyey soils hve greter protection llowing slow decomposition nd depletion of orgnic mtter while on sndy soils orgnic mtter is more exposed nd vulnerble to rpid decomposition in the presence of fvourble environmentl conditions. The cly nd silt size prticles hve lrger surfce res which llow better stbilistion of orgnic mtter thn sndy soils (Powlson et l., 2013). Although the current stocks were not significntly different mong tillge prctices. A comprison of C stocks t 0-10 cm (Tbles 2.2 nd 2.3) on sndy soils shows n increse of 0.38, 0.93 nd 1.46 Mg h -1 over the four yer period for CT, RP nd DS respectively. Increse of C stocks in top lyers hs been observed elsewhere (Gulde et l., 2008; Anikwe, 2010; Dercon et l., 2010; Blnco-Moure et l., 2011). Brhim et l. (2009) lso found DS hving greter C stocks in 0-10 cm nd the cm lyers with higher C stocks under CT t cm. Contrry to this study, Anikwe (2010) nd other scholrs (Tn et l., 2007; Jgdmm & Ll, 2010 ) found significnt differences between C nd N stocks of CT nd no tillge (DS). Crop rottion using leguminous crops hs been done in ll tillge systems nd this prctice is known to enhnce microbil ctivity (Dinesh et l., 2004) nd my hve n impct on the insignificnce of C stored in the three tillge system lthough usully little residue ws left in the field to lst until the next cropping seson. The C stocks under CT in this study re higher thn reported by Thierfelder et l. (2012) who worked in the sme study re nd reported stock 47

58 Chpter 2 of 3.2 Mg C h -1 t 0-10 cm while t cm their stocks were 7.8 Mg h -1 compred to 7.97 Mg h -1 in this study. Their study ws however, bsed on fields initited in 2004 wheres in this study the results re the men of four frmers fields estblished in 2005/2006 seson. In their DS fields, C stocks t 0-10 cm were less thn hlf of those reported in this study while t they found 10 Mg C h -1 compred to 4.46 Mg C h -1 in this study. C stocks in clyey soils under CT, RP nd DS incresed t by 3.05, 2.14 nd 0.38 Mg h -1 respectively 0-10 cm when compred with the bseline dt (Tble 2.1 & 2.2). The CT mngement system hd the lowest C stocks t cm where RP ws highest resulting in the RP hving highest stocks t 0-30 cm lthough the difference between CT nd DS ws not significnt. All tillge systems hd significntly lower C nd N when compred to NF. In Zmbi, Stroosnijder nd Hoogmoed (2004) found C sequestrtion rtes of Mg h -1 yr -1 under conservtion tillge systems over nine yer period. Increses of totl orgnic C in the short term hve been reported fter three yers (McCrty et l., 1998) lthough the increses cn be insignificnt (Frnzluebbers & Arshd, 1996; Ling et l., 2007). Similrly, Thierfelder nd Wll (2012) lso nlysed C stock chnges t some of the fields in Hereford nd found increses of 19%, 21% nd 38% under CF, RP nd DS from 2004 to 2008 t depth of 0-20 cm. They showed increses of 6.9, 7.6 nd 10.2 Mg h -1 respectively (i.e. 1.7, 1.9, 2.6 Mg h -1 yr -1 respectively). They found highest increses under DS while CT nd RP were not significntly different. Differences with our results minly under DS systems, could be ttributed to the lck of continuous cover in the designted fields by the yer 2010 s the fences were broken. This shows the importnce of residue retention to gin greter positive impcts under RP nd DS in both sndy nd clyey soils. Despite sptil heterogeneity nd the differences in mounts of C nd N, CT nd RP systems my hve benefited from homogenistion of soil profile with the plough (CT) or ripper (RP) nd orgnic mtter ccumultion which however, resulted in decrese in the concentrtion grdient of C with soil depth when compred with NF on sndy soils. Clyey soils tend to benefit more from minimum tillge while sndy soils could be more sensitive to this disturbnce. 48

59 Effects of tillge on soil orgnic crbon nd nitrogen storge These results suggest tht RP could be n effective soil mngement technique for incresing soil C storge on clyey soils. Studies which hve shown mjor differences between CT nd DS hd continuous cover under the no till system (Brhim et l., 2009), which ws not the cse in this study. Our results gree with Poirier et l. (2009) who found no significnt differences between tillge systems lthough they considered the profile up to 60 cm. They found significnt differences in the top 20 cm which is similr to this study t cm. Studies on similr soils show decline of C nd N through CT (Chivenge et l., 2007) which is not evident in this study. In this study the ox-drwn plough my not be going s deep s 30 cm leving the soil below 30 cm depth reltively unffected. Contrry to most tillge studies, de Rouw et l. (2010) found significnt differences between DS nd CT with highest C (+590g C m -2 ) in CT nd loss of (- 133 g C m -2 ) under DS over five yer period. The loss of C ws minly ttributed to the slow process of biomss decomposition in no till systems (DS). They concluded tht tillge helps to incorporte residues tht my never be cptured for decomposition under no till. In ddition, the mouldbord plough helps to prepre weed-free seedbed nd fcilittes percoltion of wter into the soil profile (Riches et l., 1997). This could be the sme cse in our study re were vrible mounts of residue re not preserved in plce in CA systems fter hrvesting nd equl mounts of fertiliser re dded to ll tretments nnully. Other studies on sndy soils hve shown tht the incorportion of orgnic mtter into the minerl soil by cultivtion fcilittes interctions between orgnic residues nd inorgnic colloids thus physiclly stbilising the residues nd reducing decomposition rtes resulting in preservtion of soil C (VndenBygrt et l., 2002). The high C:N rtios, reflect less microbil ctivity nd lso shows the inherent N deficiency in these soils (Nymngr et l., 2000) supporting the need for supplementing with fertiliser nd/or mnure to increse crop production (Giller, 2002; Zingore et l., 2007) nd C sequestrtion. The observed increse in C stocks in ll tillge systems could be result of better nutrition nd improved 49

60 Chpter 2 mngement of the fields compred to the time prior to the strt of the experiments nd the ddition of fertiliser hs been shown to increse soil C stocks in continuously tilled fields fter nine yers (Mujuru et l., Submitted) Soil C nd N dynmics in density frctions We observed contrsting effects of tillge on the flf C nd N concentrtions in the sndy nd clyey soils (0-10 cm), (i.e. in the sndy soils incresing flf C present under respectively CF, DS, RP nd NF systems, while for the clyey soils we observed decresing flf C under respectively RP, CT, DS nd NF (Figure 2.4). The significnt difference between flf C nd N on sndy nd clyey soils shows the frction s vulnerbility to mngement ctivities on soils of different texture when compred with the mount of light frction in nturl forests (Figure 2.4). This shows the sensitivity of this frction to chnges in soil mngement (Shrifi et l., 2008; Yoo & Wnder, 2008; Sequeir et l., 2011; Xu et l., 2011) proving tht the light frction mybe most ffected by lnd mngement prctices (Tn et l., 2007) on both sndy nd clyey soils. For sndy soils, the less fvourble microbil living conditions my induce lrger turnover time of C in the unprotected flf when compred to clyey soils. Conventionl tillge my hve enhnced the incorportion of bove ground litter into the minerl soil nd therefore contributed to the initil phse of soil C stbilistion. This mixing effect turned out to be more effective in the well-structured clyey soils thn in the less-structured sndy soils. Tn et l. (2007) found significntly higher flf C under DS thn CT which is consistent with our results on sndy soils. The flf C ws lso reduced with depth in ll lnd use systems. Studies by Blesdent et l. (2000) suggested tht tillge ffected flf through incresing the rte of ggregte destruction which cuses decrese in orgno- minerl complexes within ggregtes ssocited with the flf. The flf C is considered to be more lbile thn olf C (Hssink, 1995). The significntly higher flf C nd N in tillge systems thn NF on clyey soils could be result of persistent nnul fires tht retrd ddition of flf to forest soils. 50

61 Effects of tillge on soil orgnic crbon nd nitrogen storge The olf ws higher in NF thn tillge systems. Among tillge systems the olf C ws higher in DS nd RP thn CT. However, the olf C nd N frctions were mrginlly smll in the sndy soil, showing tht SOM stbilistion by physicl protections is nerly bsent in cropping systems wheres on clyey soils, the physiclly protected olf C nd N ws considerble compred to flf. Fvourble moisture nd tempertures during the cropping seson promotes greter minerlistion of light frction orgnic mtter under RP nd DS resulting in less ccumultion of flf C nd N. Differences in C:N rtios between flf nd olf were probbly due to slightly dvnced form of decomposition in olf (Hssink, 1995). The flf C:N rtios were similr to those reported by Gregorich et l. (2006) which were between 17 nd 22. The N contents which were close to the detection limit minly in the light frctions my contribute to greter vribility of C:N rtios t the two depths in ll lnd mngement systems. In ddition, lrger C utilistion in the clyey top soil might be result of (micro-) biologicl ctivity minly during the cropping seson. The utilistion of fresh litter (substrte) in soil involves processes where, microbes respire C s CO 2 nd ssimilte C nd N resulting in decrese of the verge C:N rtio of the flf. In contrst to mounts of C nd N in light frctions, the MHF contributed most of the C n N with vlues of 86-97% nd 90-98% on sndy nd clyey soils respectively. Thus MHF represented consistent trend reltive to lnd use system (Figure 2.5) incresing under RP nd DS on clyey nd sndy soils respectively. In ech soil type this frction ws not ffected by tillge s shown by no significnt differences. The totl SOM retention cpcity of the clyey soils bsed on physicl protection (olf) nd dsorption onto minerl surfces (MHF) ws twice s much s on sndy soils. The MHF ccounted for the lrgest proportion of C nd N in ll tillge nd forest systems suggesting tht C loss fter conversion from forest to croplnd cused reduction in both light nd hevy frction of orgnic mtter (Figure 2.4 nd Figure 2.5) minly in the top 10 cm of the clyey soils nd the cm lyer 51

62 Chpter 2 of the sndy soil. Thus the higher the cly content, the lrger the difference between LF nd MHF. 2.5 Conclusion The present C nd N stocks were lower in tillge systems thn NF in the two soil types. More C nd N were stored t 0-10 cm thn cm depth. Although the totl C nd N stocks were not significntly different, the mount of C ddition to the soil differed significntly between CT nd DS on sndy soils nd ws highest under CT nd lowest under DS on clyey soils. Over four yer period, the three tillge systems under CT, RP nd DS hd positive impcts on SOC stocks t 0-10 cm with mgnitudes of 0.38, 0.93, 1.46 Mg h -1 nd 3.05, 2.14, 0.38 Mg h -1 on sndy nd clyey soils respectively. Our results do not support the hypothesis tht losses of C nd N from the soil re ssocited with conventionl frming nd less with no-till prctices, thus conflicting with other studies, which found decreses of both C nd N stocks in conventionlly cultivted lnds. The SOM frctions were dominted by MHF C nd N which ccounted for 86-93% nd 94-98% on sndy nd clyey soils respectively. In these semi-rid res, the protection of orgnic C nd N in MHF seems to be min cuse for soil enrichment by orgnic mtter. Results lso show tht clyey soils cn store more C nd N fter mild disruptions (RP) lthough difference with CT were not significnt. The MHF in the sndy soils indictes tht SOM dsorption onto minerl surfces is by fr the most effective stbilistion mechnism lthough, when compred to the clyey soils, this totl SOM stbilistion cpcity is rther limited. The C:N rtios for MHF were smller thn flf nd olf rnging from compred to for flf nd for olf. The seprtion of SOM into flf, olf nd MHF llowed us to ssess the cpcity of tillge systems to enhnce the physicl protection of orgnic C nd N within sndy nd clyey soils. Results from this study my lso suggest tht crop residues re importnt for successful RP nd DS in both sndy nd clyey soils nd the effects of residue my even be lrger thn other fctors. Therefore in these soils, under the previling climtic nd mngement conditions, the issue of tillge lone cnnot 52

63 Effects of tillge on soil orgnic crbon nd nitrogen storge help in C sequestrtion unless institutionl rrngements for residue retention re improved. The exploittion of the benefits of CA nd corresponding sustinbility of systems need support for surfce cover retention (de Mores Sá, et l. 2011) which cn lso be extended to conventionl tillge. 53

64 Chpter 2 54

65 Effects of fertilistion on orgnic C nd N storge CHAPTER 3 Effects of nitrogen fertiliser nd mnure ppliction on soil orgnic crbon nd nitrogen storge under continuous mize cropping on Arenosols nd Luvisols of Zimbbwe Submitted to Journl of Agriculture s Mujuru L, Rusinmhodzi L, Nymngr J nd Hoosbeek MR 55

66 Chpter 3 Effects of nitrogen fertiliser nd mnure ppliction on soil orgnic crbon nd nitrogen storge Abstrct The mintennce of soil orgnic crbon (SOC) pool in griculturl soils is importnt for long term crop productivity nd represents potentil sink for tmospheric crbon dioxide. Dt from nine yer continuous mize experiment were nlysed to ssess the effects of fertilistion on SOC nd totl orgnic nitrogen (TON) in clyey (Luvisols) nd sndy (Arenosol) soils of Murew district, Zimbbwe. Density frctiontion ws used to ssess the distribution of SOC nd TON in three soil orgnic mtter (SOM) frctions nd their sensitivity to fertilistion on two fertility grdients. The reltionship between light nd hevy frction orgnic C were nlysed to determine equilibrium levels. Tretments included: unfertilised control (control), nitrogen fertiliser (N Fert ) nd nitrogen fertiliser plus cttle mnure (N Fert + mnure). Results showed significntly higher SOC nd TON stocks under N Fert nd N Fert + mnure thn control t ll depths. In clyey soil, C nd N stocks in N Fert were significntly higher thn control nd significntly less thn N Fert + mnure. On sndy outfields, N Fert hd highest C nd N thn control nd N Fert + mnure t ll depths. On clyey soil, men totl orgnic C t 0-50 cm depth ws: control (17.98±1.46 Mg h -1 )<N Fert (26.73±1.58 Mg h -1 )< N Fert + mnure (31.99±1.42 Mg h -1 ) while on sndy soil it ws: control (5.98±1.42 Mg h -1 )< N Fert + mnure (10.71±1.41 Mg h -1 )<N Fert (11.76±1.34 Mg h -1 ). Nitrogen followed similr trends. Compred with control, N Fert nd N Fert + mnure enhnced flf C on homefields nd outfields by 19%, 24% nd 9%, 22% on clyey soil nd 17%, 26% nd 26%, 26% respectively on sndy soil. Homefields on clyey soil, under N Fert nd N Fert + mnure hd similr equilibrium levels, being 2.5 times more thn control. The equilibrium levels for MHF C under N Fert nd N Fert + mnure tretments were higher thn current stocks in both homefields nd outfields. Crbon stocks under control in outfields were similr to theoreticl equilibrium levels of 16.1 Mg h -1 wheres in homefields equilibrium levels were only 3 units higher thn current stocks. Results suggest tht the ppliction of mnure cn be low cost lterntive for enhncing soil qulity nd promoting soil C sequestrtion under conventionlly tilled continuous mize cropping in Zimbbwe. 56

67 Effects of fertilistion on orgnic C nd N storge Key words: mnure, fertiliser, sequestrtion, light frction, hevy frction, equilibrium level. 3.1 Introduction The importnce of griculturl soil for climte chnge mitigtion through soil C sequestrtion hs dominted current globl climte chnge debtes (Food nd Agriculture Orgnistion (FAO), 2010) nd importnt for sustinble soil productivity (Ll 2004). Soil orgnic crbon (SOC modifies soil physicl, chemicl nd biologicl properties, eventully ffecting nutrient provision, wter retention nd microbil nourishment (Jimenez et l., 2002). The mount nd stbility of SOC in griculturl soils determines their productivity nd contribution to sustining environmentl qulity (Ll, 2004). Agriculture prctices therefore, influence the mounts of C stocks in smll holder frming systems including potentil role in sequestering C to diminish GHG concentrtions nd globl wrming. Prctices tht hve been identified for enhncing SOC sequestrtion in griculturl soils include i) ppliction of mnure nd inorgnic fertiliser (Liu, 2004; Gulde et l., 2008; Gong et l., 2009), ii) conservtion tillge (Six et l., 2000; Chivenge et l., 2007b; Mrongwe et l., 2011), iii) crop rottion (Hvlin et l., 1990; Pustin et l., 1998), iv) improved fllowing, v) mulching (Mfongoy & Dzwowel, 1999) nd vi) intercropping. A combintion of mnure nd inorgnic fertiliser ws superior for improving SOM contents nd its frctions eventully enhncing C sequestrtion (Mnn et l., 2005; Rudrpp et l., 2006; Purkysth et l., 2008; Li et l., 2010) minly from incresed dry mtter production. Contrry to these positive results, Wu et l (2004) found decresing SOC fter ppliction of fertility mendments while others found no effect of fertility mendments on SOC storge (Šimon, 2008). Most rble fields in smllholder frming res of estern nd southern Afric re chrcterised by grdients of decresing soil fertility with incresing distnce from homesteds nd the fertility grdients re result of preferentil 57

68 Chpter 3 ppliction of mnure, household wste, nd inorgnic fertilisers on fields closest to homesteds (Tittonell et l., 2005; Zingore et l., 2007b). These resource lloction strtegies cuse significnt increses in the quntity of SOC fter ddition of more frmyrd mnure nd inorgnic fertilisers to fields close to homesteds thn fields wy from homested on both clyey nd sndy soils (Zingore et l., 2008; Dunjn et l., 2012). Incresed SOC nd TON were closely relted to increses in crop yields. Progressive studies were done in Murew district, Zimbbwe t 3 yers (Zingore et l., 2008) nd t 7 yers (Dunjn et l., 2012) nd confirmed decresing C nd N with distnce from homesteds. Similr results hve lso been reported elsewhere (Mpfumo & Giller, 2001; Tittonell et l., 2005; Zingore et l., 2008; Msvy et l., 2010). However, Hileslssie (2007) found contrdictory results with greter soil fertility in fields wy from homesteds while working in the est Africn highlnds of Ethiopi. The losses or gins of SOC nd TON over short nd medium term re difficult to detect due to sptil nd temporl vribility in bulk soil nd my not be sensitive indictor of short nd medium term chnges in soil qulity (Hssink, 1997). Lbile orgnic C frctions such s free light frction (flf) nd dissolved orgnic crbon cn be used s erly nd sensitive indictors of chnges in SOC (Hynes, 2000). Light frction orgnic C is importnt in reflecting short term turnover of nutrients (Hssink, 1997) with greter concentrtions of N thn the minerl ssocited hevy frction (Compton & Boone, 2000). Methods such s density frctiontion, size seprtes or combintion of the two divide SOM into three bsic levels of functionl, structurl complexity nd different turnover rtes (von Lützow et l., 2006). The light frction is esily decomposed, hs greter turnover rte nd is smller thn the minerl ssocited frction (MHF) nd cn be used to chrcterise impcts of different soil mendments. The importnce of the free light frction (flf), occluded light frction (olf) nd minerl ssocited frction (MHF) is widely recognised in studies outside Southern Afric (e.g. (Dll & Myer, 1986b; Jnzen et l., 1992; Liu et l., 2005; Swnston et l., 2005) but the reltionship between the three frctions is poorly understood in Africn soils. Yin et l. (2005) nd Yin nd Ci (2006) studied this reltionship by using linerized form of the Lngmuir eqution nd found 58

69 Effects of fertilistion on orgnic C nd N storge positive correltion between light frction orgnic mtter (LFOM) nd the rtio of LFOM nd hevy frction orgnic mtter (HFOM) (i.e. LFOM/HFOM rtio) under orgnic nd inorgnic fertiliser tretments in Chin. They clculted the potentil increse nd the mximum equilibrium vlue of HFOM of soils from three longterm experimentl fields in Chin. Their LFOM/HFOM rtios rnged between 0.01 nd 0.50 with higher vlues under orgnic fertiliser mendments. Knowledge of the equilibrium levels llows evlution of the potentil of different soil types nd griculturl prctices for more soil C sequestrtion (Yin & Ci, 2006). Mintining soil nd crop productivity is mjor chllenge especilly in rin fed rid nd semi - rid res chrcterised by long nnul dry spells. The objectives of this study were to determine how nitrogen fertiliser nd cttle mnure ppliction in continuously tilled mize cropping systems ffect: 1) totl SOC nd totl soil orgnic nitrogen (SON) reltive to distnce from homested nd soil depth; 2) C nd N storge in three density seprted SOM frctions nd their sensitivity to fertilistion nd 3) the reltionship between light nd hevy frction orgnic C. We hypothesised tht long term fertiliser nd mnure ppliction will increse SOC in whole soil nd eventully in stbilised C frctions; ccumultion of SOC nd TON in SOC frctions is influenced by the long-term ppliction of N fertiliser nd mnure in conventionlly tilled continuous mize cropping systems; the light frction would be responsive indictor of short nd long term chnges in SOC. We lso hypothesised tht there is stronger reltionship between LF nd MHF in clyey soils thn in sndy soils, becuse of the lrger dsorption cpcity of cly prticles for orgnic molecules. 3.2 Mterils nd methods The study site The soils were collected from Murew district of Zimbbwe which is locted ~80 km est of Hrre, ltitude 17 39' 13" S nd longitude 31 48' 30" E. The re receives unimodl rinfll of between mm nnully, with men mximum temperture of 26 C nd men minimum temperture of 14 C. The soils in the re re predominntly grnitic sndy soils (Hplic Arenosols) 59

70 Chpter 3 (FAO/IIASA/ISRIC/ISSCAS/JRC, 2012) strongly wethered hving, low levels of vilble nutrients nd low nutrient reserves. The sndy soils re interspced with pockets of dolerite intrusions tht give rise to smll ptches of more fertile cly soils (Chromic Luvisols) (Nympfene, 1991; FAO, 2006). Frming prctices re bsed on integrted crop/livestock systems with mize (Ze mys L.) s the dominnt stple crop. In ddition, other crops such s sun flower (Helinthus nnuus L.), groundnut (Archis hypoge L.), sweet pottoes (Ipomoe btts L.) nd ssorted vegetbles re lso grown. Cttle re the min livestock, communlly grzed in open ccess res nd in rble fields during the dry seson. They re corrlled close to homesteds t night. In most cses crop residues re used to feed cttle over the dry period nd orgnic fertiliser is used to fertilise crops, with smll proportion of frmers using residues s mulch in fields where conservtion griculture (CA) is prctised. Zingore et l. (2007b) showed tht vilble P nd SOC decresed significntly with distnce from homesteds, prticulrly where frmers hve cttle due to preferentil ppliction of mnure nd minerl fertilisers on fields closest to the homesteds. Cttle provide drught power for conventionl tillge nd for ferrying fertility inputs to the fields Chrcteristion of experimentl sites Soils were collected from experimentl positions fter nine yers of continuous sole mize (Z. mys L.) cultivtion, bsed on soil fertility grdients nd tretments described by Rusinmhodzi et l. (2013). Two frms in the medium welth ctegory were selected on fields with contrsting soil types: one on Arenosol (sndy soil) nd nother on red cly soil which is Luvisol (clyey soil). Ech field represented typicl homefields (<50 m from homested) nd outfields ( m from homested) s described by Zingore et l. (2007). Men SOC concentrtions given by Zingore et l. (2007) for homefields nd outfields on sndy soil were 5 nd 3 g C kg -1 wheres homefields nd outfields on clyey soils hd 14 nd 7 g C kg -1 respectively. The TON concentrtions in homefields nd outfields were 0.4, 0.3 g kg -1 nd 0.9, 0.5 g kg -1 on Arenosols (sndy soil) nd 60

71 Effects of fertilistion on orgnic C nd N storge Luvisols (clyey soil) respectively. The C:N rtios were 13, 10 nd 16, 14 respectively. Biomss yields generlly incresed with time in tretments under N fertiliser nd Mnure (Tble 3.1). Residue retention ws miniml nd less thn 0.5 Mg h -1 due to competing uses nd the free rnge grzing in the off seson period. Experimentl design in ech soil type ws split plot design within rndomised complete block. This pper reports the results of three tretments in ech soil type nd field position selected on bsis of pplicbility to smllholder frmers: 1) unfertilised control (control), 2) Nitrogen Fertiliser (N Fert ) = 100 kg N h -1 split pplied s mmonium nitrte in equl mounts t bout 3 nd 6 weeks fter plnt emergence nd 3) Cttle mnure ( 0.9% N) + mmonium nitrte (100 kg N h -1 ) (N Fert + mnure) where cttle mnure ws pplied t 5 Mg h -1 (equivlent of 10 kg P h -1 ), prior to ech cropping seson. Tble 3.1: Men mize grin yields for the period Soil type position Tretment % chnge in grin yield Increse/decrese grin yield (Mg h -1 ) Clyey Homefield Control Clyey Homefield N Fert Clyey Homefield N Fert+ mnure Clyey Outfield Control Clyey Outfield N Fert Clyey Outfield N Fert+ mnure Sndy Homefield Control Sndy Homefield N Fert Sndy Homefield N Fert+ mnure Sndy Outfield Control Sndy Outfield N Fert Sndy Outfield N Fert+ mnure Source: Rusinmhodzi et l in 61

72 Chpter Soil smpling nd nlysis Four representtive soil smples were collected from ech soil type nd field position (i.e. homefields nd outfields) t four incrementl depths up to 50 cm. Smples were composited for ech depth nd repliction, ir dried nd pssed through 2-mm sieve before nlysis of C nd totl N concentrtions by dry combustion in EA1108 CHN Elementl nlyser (Fisons Instruments). Additionl smples were tken from ll four depths using metl ring (volume 100 g m 3 ) for bulk density mesurement nd results were expressed on oven-dry-bsis. Bulk density ws expressed s the rtio of dry weight of soil core nd volume of metl core. Totl SOC stock for ech of the four depths (0 10, 10 20, nd cm) ws computed seprtely for ech tretment, field position nd depth using eqution [1]. SOC stock (Mg h -1 ) = SOC concentrtion (%) * BD * Depth (cm) [1] Where BD is bulk density (g/cm 3 ). The SOC nd TON in mended tretments ws used to estimte the sequestrtion rte for ech tretment reltive to the unfertilised control Soil density frctiontion Sodium polytungstte (SPT) (3N 2 WO 4.9WO 3.xH 2 O) ws used to seprte soil orgnic mtter by density into three frctions, (flf, olf nd MHF) following the method described by Roscoe et l.(2000). A brief description of the method is given in Mujuru et l., (2013). Briefly, ten (10g) of ir dried soil <2 mm were dispersed in sodium polytungstte solution (1.60 g cm-3), centrifuged t 4500 rpm, filtered nd dried t 40 C nd weighed. Mteril left in the superntnt ws considered to be flf (mostly prtilly decomposed plnt residues), wheres tht in the sediment ws mixture of olf nd MHF (more fully-decomposed residues nd minerl mteril). SPT solution with density 1.6 g cm -3 ws dded to the residul soil mteril nd n ultrsonic probe ws used to disperse the occluded light frction nd centrifuged for 23 minutes, decnted over Büchner funnel, oven dried t 40ºC. The remining precipitte, the MHF, needed to be rinsed 62

73 Effects of fertilistion on orgnic C nd N storge thoroughly, (i.e. t lest 5 times for clyey soils nd three times for sndy soils, or until conductivity of the filtrte ws <50µS cm -1 ) in order to remove ll SPT. Totl C nd N contents of flf, olf nd MHF were determined fter grinding by dry combustion using CN nlyser Soil C sequestrtion potentil The Lngmuir eqution ws used to evlute the dsorption of flf onto minerl surfces, where the dsorbed orgnic mtter, i.e. MHF, is regrded s sequestered OM (Yin et l. (2005). We ssumed tht light frction (LF) C is decomposed nd dsorbed onto minerl soil prticles s MHF C over time nd tht soil minerls cn rndomly dsorb LF C until the MHF hs reched C sturtion. Therefore, interction between LFC nd soil minerls ws regrded s dsorption nd desorption process which cn be described using the Lngmuir eqution. The following lineristion ws used to fit the dt (Yin & Ci, 2006; Bolster & Hornberger, 2007). LFC / MHFC = LFC / MHFC mx + 1 / (k MHFC) [2] Where MHFC mx is the mximum dsorption cpcity for orgnic C (equilibrium vlue for soil orgnic C in the MHF) nd k is the equilibrium constnt. LFC/MHFC versus LFC yields liner reltionship with slope 1 / (MHFC mx ) nd intercept 1 / (k MHFC). Eqution [2] ws used to determine the reltionship between light frction (flf +olf) C nd its dsorption onto minerl surfces. The goodness of fit ws checked through the coefficient of determintion R 2 nd the p vlue Sttisticl nlysis Dt on C nd N contents were tested for normlity nd homogeneity of vrince using Kolmogorov-Smirnoff nd Levene s test respectively. Effects of N fertiliser nd cttle mnure ppliction on SOC nd TON nd density seprted frctions within ech depth were nlysed using one wy ANOVA. Differences were considered significnt t p Tukey s HSD test ws used to compre the men 63

74 Chpter 3 differences between fertility tretments nd depth for bulk soil nd the three frctions (flf, olf nd MHF) in the two soil types. Liner regression ws used to evlute the reltionship between light frction nd minerl ssocited hevy frction crbon. All sttisticl nlysis ws performed with SPSS for windows version 19 nd GENSTAT Results Soil crbon nd nitrogen contents For ech soil type, bulk density in homefields ws not significntly different from bulk density in outfields rnging from 1.1 to 1.4 g cm -3. In ddition, bulk densities were not sttisticlly different by depth in ech field position. SOC nd TON stocks of the 0-50 cm depth increment were significntly (p=0.000) higher (26.3 Mg C h - 1 nd 1.6 Mg N h -1 respectively) on clyey soil thn sndy soils (12 Mg C h -1 nd 0.6 Mg N h -1 ) nd lso higher in homefields thn outfields (Figure 3.1). b b b b Figure 3.1: Soil C nd N contents (0 50 cm) of homefields nd outfields of clyey nd sndy soils. 64

75 Effects of fertilistion on orgnic C nd N storge The storge of C nd N ws significntly ffected by fertility mendment (Figure 3.2) when compred to the unfertilised control. Men SOC on clyey homefields ws significntly higher (p < 0.015) thn outfields nd followed the order: control (19.8±1.46 Mg h -1 ) < N Fert (28.8±1.58 Mg h -1 ) < N Fert + mnure (31.9±1.42 Mg h -1 ), while on sndy soil it ws control (8.2±1.42 Mg h -1 )< N Fert + mnure (14.1±1.41 Mg h -1 ) N Fert (13.8±1.34 Mg h -1 ). Outfields followed similr trends for control, N Fert nd N Fert + mnure hving 16.1, 24.6, 32.1 Mg C h -1 nd 3.4, 9.5, 7.7 Mg C h -1 on clyey nd sndy soils respectively. Therefore, ppliction of N Fert nd N Fert + mnure incresed C stocks on clyey soils by 1.0, 1.3 Mg C h -1 yr -1 nd 0.9, 1.8 Mg C h -1 yr -1 in homefields nd outfields respectively when compred to the control. On sndy soils, C stocks incresed by 0.65, 0.62 Mg h -1 yr -1 nd 0.68, 0.48 Mg h -1 yr -1 in homefields nd outfields respectively reltive to control. b b b b b b b b b b b Figure 3.2: Effects of nitrogen fertiliser (N Fert) nd nitrogen fertiliser plus cttle mnure (N Fert + mnure) ppliction on SOC nd TON stocks in homefields nd outfields of clyey nd sndy soils. Different letters show significnt differences in ech soil type nd field position t p = Error brs show ±1 stndrd error of the men. 65

76 Chpter 3 TON followed similr trends to those of SOC in ll tretments except N Fert + mnure in homefields. The nitrogen stocks on cly soils were lso higher in homefields (1.7 Mg N h -1 ) thn outfields (1.5 Mg N h -1 ) being significntly higher (p=0.000) under N Fert + mnure (2.1 Mg h -1 ) thn control (1.1 Mg h -1 ) whilst on sndy soil, nitrogen stocks were lso greter in homefields (0.73 Mg N h -1 ) thn outfields (0.37 Mg N h -1 ) nd significntly higher (p= 0.000) in fertility tretments thn control. At ll smpling depths, fertility mendments enhnced SOC nd TON stocks better thn the unfertilised control irrespective of field position (Figures 3.3 nd 3.4). Appliction of N fertiliser plus cttle mnure significntly incresed the SOC stocks in soil compred to ppliction of N Fert lone t ll depths on clyey soils. On sndy soil, ppliction of N Fert resulted in greter SOC thn N Fert + mnure nd control t ll depths except the cm depth on homefields nd cm depth on outfields (Figure 3.3). Clyey Homefield Clyey Outfield Sndy Homefield Sndy Outfield Figure 3.3. Depth distribution of SOC stocks under different fertility tretments on clyey nd sndy soils. N Fert = nitrogen fertiliser, N Fert + mnure = nitrogen fertiliser plus cttle mnure. 66

77 Effects of fertilistion on orgnic C nd N storge Appliction of N Fert nd N Fert + mnure resulted in significntly higher (p=0.012) ccumultion of SOC (33.3 nd 36.4 Mg h -1 respectively) thn the control (17.1 Mg C h -1 ) t 0-10 cm nd TON followed similr trends being higher under N Fert (2.1 Mg h -1 ) nd N Fert + mnure (2.3 Mg h -1 ) thn control (1.1 Mg N h -1 ) (Figure 3.4). At 0-10 cm, ppliction of N Fert + mnure incresed SOC by 15% while N Fert incresed SOC by 11% reltive to control tretment. Although the C nd N decresed with incresing depth in ll except N Fert in sndy homefields, there were no significnt differences between the cm depths of N Fert nd control in homefields. Clyey Homefield Clyey Outfields Sndy Homefield Sndy Outfields Figure 3.4 Depth distribution of TON stocks under different fertility tretments on clyey nd sndy soils. N Fert = nitrogen fertiliser, N Fert + mnure = nitrogen fertiliser plus cttle mnure. In clyey soils, TON ws significntly higher under N Fert + mnure thn control in both homefields nd outfields except cm depth in outfields where N Fert nd N Fert + mnure were similr. On sndy soils, significnt differences between three tretments were only t cm in homefields nd cm in outfields. 67

78 Chpter 3 The C:N rtios were higher in clyey (1:17) thn sndy soils (1:15) lthough within ech soil type there were no significnt differences. The C:N rtios of homefields under control, N Fert nd N Fert + mnure were 17, 17, 15 nd 14, 18, 17 on clyey nd sndy soils respectively wheres in outfields the C: N rtios were 15,16,18 nd 11,16,15 for clyey nd sndy soils respectively Crbon nd nitrogen in density frctions Similr to totl SOC nd TON stocks, density frctions lso showed significnt difference between soil types p<0.001), field position (p= 0.007), fertility tretments (p<0.001) soil depth (p<0.01), soil type x tretment (p<0.001) nd soil type x field position x tretment (p<0.001). The C nd N stocks in the three density frctions were generlly in the order olf< flf< MHF (Tble 3.2). The ddition of nitrogen fertiliser nd cttle mnure hd vrible influence on the three frctions resulting in significnt increses of flf C nd MHF C compred to control tretments. On clyey soils, the proportions of SOC in flf, olf nd MHF were similr under control nd N Fert + mnure (3%, 2%, 95%) while the proportions under N Fert were 3%, 1% nd 96% respectively. On sndy soils, the proportions of flf, olf nd MHF were different between control (4%, 8%, 88%), N Fert (4%,1%, 95%) nd N Fert + mnure (5%, 4%, 91%) respectively. The flf C nd N were 0.78 Mg C h -1 nd 0.04 Mg N h -1 on clyey soils wheres on sndy soils they were 0.41 Mg C h -1 nd 0.02 Mg N h -1. The olf followed similr trends being 0.32 Mg C h -1 ; 0.02 Mg N h -1 nd 0.12 Mg C h -1 ; 0.01 Mg N h -1 on clyey nd sndy soils respectively (Figure 3.5). The olf N ws sttisticlly similr in the two soil types. Field position showed greter flf C in homefields thn outfields but with no significnt effect on olf N. N Fert nd N Fert + mnure tretments enhnced flf C in homefields nd outfields by 19%, 24% nd 9%, 22% respectively, when compred with the control. In sndy soils, N Fert nd N Fert + mnure tretments incresed C storge by 17%, 26% nd 26%, 26%, when compred with control on homefields nd outfields respectively. Thus, on sndy outfields, N Fert nd N Fert + mnure hd similr impcts on flf C storge. N Fert hd more olf C on clyey homefields. On 68

79 Effects of fertilistion on orgnic C nd N storge sndy soils, N Fert nd N Fert + mnure hd less olf C thn control in homefields wheres in outfields, N Fert enhnced olf C stocks by 15% nd N Fert + mnure ws 8% lower thn control. Tble 3.2: Totl SOC, TON (Mg h -1 ) nd C:N rtios for density frctions s ffected by fertiliser nd mnure pplictions on clyey nd sndy soil under continuous mize cropping. Soils Position SOC TON C:N Rtio Tretment flf olf MHF flf olf MHF flf olf MHF Clyey Home Control (1) 20(2) 16(2) Field Fertiliser (1) 16(2) 17(2) Mnure (1) 19(2) 15(2) Outfield Control (1) 19(2) 15(1) Fertiliser (1) 19(2) 16(2) Mnure (1) 20(2) 18(2) Men Sndy Home Control (1) 19(2) 13(2) Field Fertiliser (1) 18(2) 17(2) Mnure (1) 20(2) 16(2) Outfield Control (1) 20(2) 14(2) Fertiliser (1) 17(2) 16(2) Mnure (1) 20(2) 15(2) Men MSE <0.01 < Different letters show significnt difference per soil type nd field position t α = flf = free light frction, olf = occluded light frction, MHF = minerl ssocited hevy frction. Fertiliser = N Fert 100 kg h -1,Mnure = 100 kg N h Mg h -1 cttle mnure 0.9 N. The MHF dominted the SOM ccounting for greter thn 98% of the totl soil mss nd 95-97% nd 84-95% of SOC on clyey nd sndy soils respectively. The MHF N ccounted for 95-96% of TON in the two soil types. Minerl ssocited hevy frction C concentrtion rnged from 12 to 27 g kg -1 on clyey soils nd from 4 to 12 g kg -1 on sndy soils. The resulting C stocks in MHF were lso significntly different between clyey homefields nd outfields (25.7 nd 22.7 Mg h -1 ) nd sndy homefields nd outfields (11.3 nd 6.6 Mg C h -1 ) (Figure 3.6). There were significnt differences in MHF C stocks were significntly different between tretments; control, N Fert nd N Fert + mnure hving 17.3, 25.6, 30.8 Mg C h -1 nd 5.6, 11.2, 10.1 Mg C h -1 on clyey nd sndy soils respectively. On clyey soils, the N Fert + mnure nd N Fert ppliction incresed MHF C by 21%, 16% nd 6%, 16% in homefields nd outfields respectively 69

80 olf C (t/h) olf N (t/h) flf C (t/h) flf N (t/h) Chpter 3 wheres on sndy soils increses under N Fert + mnure nd N Fert tretments were 16%, 18%, nd 19%, 27%, in homefields nd outfields respectively. MHF N ws 1.11, 1,56, 1.86 Mg N h -1 nd 0.42, 0.69, 0.64 Mg N h -1 for control, N Fert nd N Fert + mnure tretments on clyey nd sndy soils respectively. b b b b b b b b b b b b 1 b b b b b b b b 2 Figure 3.5: Soil C nd N stocks in free light frction (flf) nd occluded light frction (olf) under different fertility tretments t four depths lyers on clyey nd sndy soils seprted into homefields nd outfields. N Fert = nitrogen fertiliser, N Fert + mnure = nitrogen fertiliser plus cttle mnure. Different letters show significnt differences in ech soil type nd field position t p = Verticl distribution of C nd N in density frctions showed tht control tretments on clyey soils hd the lowest flf C nd N t ll depth levels with the olf N being significntly higher in control (p=0.013) thn N Fert + mnure t ll depths except cm depth on sndy soil. The MHF C nd N decresed with incresing soil depth hving higher mgnitudes on clyey thn sndy soils nd ws strongly relted to bulk soil orgnic C (R 2 = 70

81 Effects of fertilistion on orgnic C nd N storge 0.996, p<0.01 nd R 2 = 0.998, p<0.01) for sndy nd clyey soils respectively. MHF N hd similr reltionships (Figure 3.6). Figure 3.6: Depth distribution of minerl ssocited hevy frction (MHF) C nd N in homefields nd outfields of clyey nd sndy soils s ffected by fertility tretments. N Fert = nitrogen fertiliser, N Fert + mnure = nitrogen fertiliser + cttle mnure. The C:N rtios for MHF were smller thn flf nd olf rnging from compred to for olf nd for flf (Tble 3.2). The MHF C:N rtios were significntly higher in clyey thn sndy soils (p=0.007). The flf (p=0.001) nd olf (p=0.024) C:N rtios were significnt higher in control thn N Fert + mnure tretments Soil C sequestrtion potentil There ws liner reltionship between light frction C nd the rtio of LF C/MHF C with ll fertility tretments on clyey soils being highly significnt (p<0.01) i.e. control (R 2 = nd 0.903), N Fert (R 2 = nd 0.780) nd N Fert + mnure (R 2 = nd 0.615) in homefields nd outfields respectively) (Tble 3.3). 71

82 Chpter 3 Tble 3.3: Regression equtions for fertility tretments in homefields nd outfields on clyey soils nd ssocited theoreticl mximum dsorption cpcity of minerl ssocited hevy frction (MHF). Field Position Tretment Regression eqution df R 2 SE Theorticl mximum dsorption cpcity (Mg h -1 ) Homefield Control LFC/MHFC = LFC * N Fert LFC/MHFC = LFC * N Fert + mnure LFC/MHFC = LFC * Outfield Control LFC/MHFC = LFC * N Fert LFC/MHFC = LFC * N Fert + mnure LFC/MHFC = LFC * LFC = light frction crbon, N Fert = nitrogen fertiliser, N Fert + mnure = nitrogen fertiliser +, N Fert = nitrogen fertiliser, N Fert + mnure = nitrogen fertiliser + cttle mnure. *Significnt t p= 0.05, SE = Men Stndrd Error. On sndy soils, the three tretments hd wek R 2 vlues (R 2 < 0.30) nd were excluded from further nlysis using regression. Thus, LFC dt filed to fit n pproprite liner or non-liner regression model s ws shown by the low R 2 vlues despite sndy soils hving significnt gins in C nd N under N Fert nd N Fert + mnure tretment when compred with the control. The regression equtions were used to estimte the equilibrium level nd results showed tht the control tretments hd lowest equilibrium vlues (Tble 3.3; Figure 3.7). The slopes of the regression equtions were steepest under control (5.8% nd 6.2%) on homefields nd outfields respectively. The equilibrium levels for MHF C in homefields under control (17.15 Mg h -1 ), N Fert (44.05 Mg h -1 ) nd N Fert + mnure (42.19 Mg h -1 ) tretments were higher thn current stocks in Figure 3.1. In outfields only the control tretment hd current stocks lower thn the equilibrium vlue of Mg h

83 Effects of fertilistion on orgnic C nd N storge Control N Fert Contro l N Fert N Fert + mnure N Fert + mnure () Figure 3.7: The reltionship between light frction C nd minerl ssocited hevy frction (MHF) C in () homefields nd (b) outfields under control, nitrogen fertiliser (N Fert) nd nitrogen fertiliser plus mnure (N Fert + mnure) tretments on clyey soils. Unfertilised control =, nitrogen fertiliser (NF)=, nitrogen fertiliser plus cttle mnure (N Fert + mnure) = 3.4 Discussion (b) Crbon nd nitrogen in bulk soil The higher C nd N stocks in clyey soils (Figure 3.1) confirm the importnce of texture in SOC dynmics. Cly soils hve greter cpcity to protect orgnic mtter from decomposition s the finer texture my result in presence of occluded nd dsorbed stble humus. Mngement ctivities however, could lso be n importnt fctor influencing the cpcity of soil to store SOC. In this study, C stocks in N Fert + mnure were not significntly ffected by distnce from homested on clyey homefields (Figure 3.2). Homefields hd higher orgnic C nd N thn outfields supporting the findings of Zingore et l. (2007) nd shows tht the fertility grdients hve been mintined over the nine yer period. Nitrogen followed similr trends except for N Fert tretment on homefields. Benefits of inorgnic nd orgnic fertiliser mendments re supported by Mtsumoto et l. (2008) nd Gregorich nd Jnzen 73

84 Chpter 3 (1996). Nitrogen fertiliser is known to enhnce growth of crop roots nd shoots nd promotes C inputs into the soil lthough it cn lso negtively ffect locl micro-environment thus controlling decomposition thereby cusing no effect on C ccumultion (Cmpbell et l., 1997). Bremer et l. (1994) nd Eghbll nd Ginting (2003) found the ppliction of mnure nd compost to hve significnt C sequestrtion effects while chemicl fertilisers hd no effect. Appliction of N fertiliser lone hd less effect on SOC nd TON storge thn combintion of N fertiliser nd cttle mnure on clyey soil wheres on sndy soils similr observtion ws mde only t 0 10 cm depth (Figures 3.3 nd 3.4). Thus, increse of SOC contents under N Fert nd the N Fert + mnure in both clyey nd sndy soils reltive to the control tretments could be ttributed to 9 yers of continuous ddition of C vi mnure, root biomss nd residues ssocited with higher crop yields (Liu et l., 2005; Bnger et l., 2009; de Rouw et l.; Ding et l., 2012) in both homefields nd outfields. Return of crop residues to fields is thretened by competing uses s some frmers collect nd store the crop residues for livestock feed over the dry seson. If left in the field, existing policies llow free rnge livestock grzing over the dry period, thretening the persistence of residues in the fields. Therefore, if the crop residues re grzed in-situ, input of orgnic mtter is most likely result of vrying mounts of root biomss, remnnts of crop residues nd the orgnic mnure. These directly dd C substrte nd nutrients to the soil (since mnure contins significnt mounts of C s mjor constituent). Therefore, vritions in C nd N stocks re likely result of different C inputs from biomss leftovers (Purkysth et l., 2007). Rusinmhodzi et l., (2011) reported incresed crop yields under fertility tretments nd decresed crop yields in the control tretments over the nine yer period. The 0.9% difference in N content between N Fert nd N Fert + mnure tretments resulted in no significnt gin in mize grin yields in homefields. The lck of significnt differences between N Fert nd N Fert + Mnure on sndy soil could be result of enhnced decomposition under N Fert + mnure nd low C inputs under N Fert. The orgnic component of mnure cn esily be minerlised on sndy soils unlike on clyey soils where it cn be complexed with cly minerl surfces. On clyey soils, N Fert + mnure tretment hd more SOC 74

85 Effects of fertilistion on orgnic C nd N storge nd TON thn N Fert lone thus greeing with Wu et l. (2004) who worked on Chinese soils nd found orgnic dditions more effective in C ccumultion thn inorgnic fertiliser lone. Stroosnijder nd Hoogmoed (2004), while working in Burkin Fso, found n increse of 1.8 g C kg -1 soil fter 10 yers of nnul ppliction of 10 Mg h -1 of mnure nd they concluded tht combintion of either hnd hoeing or conventionl tillge with mnure cn increse SOC nd crop yields. Differences in C nd N ccumultion under ech tretment could lso be influenced by initil C nd N stocks. The mgnitude of differences mong tretments ws greter t 0-10 cm nd cm thn lower depths (Figures 3.3 nd 3.4). Similrly, Brr et l. (2013) reported greter build-up of SOC nd TON with fertilistion. In support of the results on clyey soils, their studies lso found combintion of mnure nd inorgnic fertiliser hving significnt cumultive benefits of both incresed C stocks nd crop yield (Berzsenyi et l., 2000; Ho et l., 2002) thn inorgnic fertiliser lone t 0 15 cm (Liu et l., 2005). In this study, beneficil effects of orgnic nd inorgnic mendments were possibly not fully mximised in surfce lyers on sndy soil s result of possibility of leching of nutrients to lower lyers resulting in significntly higher ccumultion of C nd N t cm depths under the N Fert tretment (Figures 3.3 nd 3.4). This is becuse sndy soils re low in cly nd silt nd hve reduced cpcity to protect SOC. Sndy soils re lso prone to leching losses of both C nd N resulting in ccumultion t lower depths fcilitted by fster infiltrtion rtes. In clyey soils, mcro nd micro ggregtion protects SOC from degrdtion through binding with cly nd silt size prticles (Six et l 2002). Addition of mnure nd other orgnic fertilisers improves nutrient efficiency nd enhnces biomss yields (Nymngr et l., 2003). Inorgnic fertilisers my lso cuse ccelerted decomposition of SOM on clyey soils when compred with orgnic inputs resulting in less C ccumultion. Soil conditions in the study re re prejudiced by reduced cover nd continuous surfce disturbnce during the dry period. In most cses, the unfenced smllholder frms re subjected to depletion of bove ground crop residues consumed by free rnging livestock during the dry seson. Some frmers, however, store the crop 75

86 Chpter 3 residues for personl livestock feed over the dry period or they use the residues s bedding mteril in livestock pens. In most prts of southern Afric, smllholder frmers use conventionl tillge with smll ptches of lnd dedicted to conservtion tillge prctices despite the poor soil fertility (Nympfene, 1991). In most cses, lrge mounts of chemicl nd/or orgnic fertiliser re required to improve crop productivity. Given these conditions, N Fert nd N Fert + mnure tretments hd limited impct on C:N rtios within ech soil types s shown by the smll vritions in C:N rtios Crbon nd nitrogen in density frctions The lrgest mounts of light frction were found under N Fert + mnure tretment reflecting the presence of inherent lbile mterils. The proportion of flf nd olf in the soils ws 1 2% of initil soil mss which is similr to the proportions obtined by Swnston et l. (2005) lthough it ws less thn tht obtined from elsewhere. For exmple, Yin et l. (2005) worked on three soil types in Chin nd found the light frction ccounting for 1 27% of bulk soil orgnic mtter nd Tn et l. (2007) found rnge between 5% nd 12% while working on tillge experiments in the USA. The ddition of mnure plus nitrogen fertiliser nd ddition of nitrogen fertiliser lone showed greter prtitioning of flf C in homefields thn outfields. Fertility tretments hd less effect on olf C nd N (Tble 3.2). This could be n indiction of the importnce of lnd use history with flf reflecting chnges cused by lnd mngement e.g. effect of fertility grdients which decrese with incresing distnce from homested (Zingore et l., 2007). In this cse, the nlysis of SOC frctions were used to show the influence of lnd use ctivities on orgnic mtter qulity becuse the frctions hve different stbilities nd turnover rtes nd re extrcted from different positions in the soil mtrix (Golchin et l., 1994b). The flf is the most dynmic nd sensitive to frction known to be esily influenced by mngement prctices (Jnzen et l., 1992) nd cn be ffected by qulity nd quntity of mterils dded to the soil (i.e. inorgnic or orgnic inputs). The mount of flf in ech tretment might lso be 76

87 Effects of fertilistion on orgnic C nd N storge ttributed to the quntities of crop root biomss which is proportionl quntity of better yields (Bnger et l., 2010). Similr to this study, Bremer et l.(1994) found more light frction under inorgnic fertiliser plots thn the other tretments, while Mirsky et l. (2008) found the flf unffected by fertility tretments. The olf, which consists of processed orgnic mteril nd prtilly degrded plnt mterils ssocited with minerl prticles nd ggregtes (Golchin et l., 1994b) ws however, less in mnure thn control. N Fert + mnure hd greter beneficil effects on SOC in flf nd to greter extent MHF wheres it did not enhnce the olf indicting tht ggregtion ws not ffected. Across the four smpling depths, significnt differences were observed mostly between N Fert + mnure nd control tretments showing the importnce of the orgnic component of mnure in contributing to lbile frction. The ddition of mnure nd inorgnic fertiliser ws observed to ffect distribution of flf in surfce lyers decresing with incresing soil depth, depending on tillge prctice (Rudrpp et l., 2006; Brr et l., 2013). The N Fert nd N Fert + mnure tretments significntly incresed C nd N in flf in the top 20 cm s compred to the lower nd cm depths. This could be ttributed to the fct tht orgnic mendments induce rpid nd conspicuous ltertions to the function nd structure of soil microbil communities thus ffecting soil orgnic mtter frctions. The form nd ppliction rte of mnure nd ssocited C content hve been reported to influence the comprtive rection of soil microorgnisms (Pul & Beuchmp, 1996). In support of these results, Bnger et l. (2010) found higher LFC in surfce thn the lower lyers under inorgnic fertiliser nd orgnic fertiliser ppliction. Smllholder frms ssessed in this study hd less or no residue input strtegies leving root biomss s the min source of LF orgnic mtter. The remnnt residue left fter grzing by livestock nd root biomss hd different impcts flf deposition on clyey nd sndy soils with the sndy soils ccumulting more flf thn clyey soils. The flf C my be used s n indictor of chnge in C ccumultion following ppliction of mnure nd inorgnic fertiliser on both clyey nd sndy soils. This more vilble lbile C pool (i.e. flf) increses microbil ctivity, subsequently incresing ggregtion, nd SOC stbilistion through occlusion minly in clyey soils. In ddition, incresed microbil turnover lso enhnces 77

88 Chpter 3 production of dissolved orgnic crbon (DOC) which in turn increses stbilistion of SOC by dsorption. Furthermore, the qulity of the incoming bove nd below ground plnt residues nd orgnic mendments determine the qulity nd degrdbility of orgnic mtter by soil microorgnisms (Gregorich & Ellert, 1993). Although the LF ws considerbly smller thn the hevy frction (Figures 3.5 nd 3.6), it plys n importnt role in soil biologicl processes s n energy source for the soil microorgnisms (Gong et l., 2009). The MHF ws the most bundnt density frction on both clyey nd sndy soils nd is considered more stble C pool with long turnover time (>100 yers) (von Lützow et l., 2008) nd high mounts of protected C. The proportion of MHF C to totl SOC ws 96% nd 92% for clyey nd sndy soils while N ws 96 97%. The results suggest tht on clyey soils, ddition of N Fert + mnure ws possibly more effective in stbilising C nd N in the MHF (Figure 3.6), when compred to N Fert lone. The interction between SOC nd minerl surfces cn result in incresed protection of SOC from microbil degrdtion, nd cly prticles cn encpsulte prticles or ptches of the SOM. Furthermore, the presence of cly prticles in soil provides greter surfce re onto which orgnic mteril cn be dsorbed (Bldock & Skjemstd, 2000).The results suggest tht some of the mnure dded nnully to the soil becomes stbilised in the soil to form the MHF C nd N. On the other hnd, N Fert cn stimulte microbil ctivity nd enhnce C turnover resulting in incresed dsorption of decomposition products onto minerl surfces minly on clyey soil. The increse in reclcitrnt C fter ddition of combintion of N Fert + mnure is normlly ttributed to higher lignin content of the mnure when compred with nitrogen fertiliser lone nd some of the lignin is ssumed to enter directly into the slow pool (Prton et l., 1987). However, study by Hofmnn et l. (2009) showed tht lignin my not be n importnt fctor in SOM decomposition. Therefore, the significnt increse of C nd N in MHF nd ssocited frctions fter mnure ddition suggests the importnce of mnure in stbilistion of SOC nd N griculturl soils. The potentil my not hve been fully exploited since the soils studied re subjected to conventionl tillge known to cuse incresed minerlistion of lbile LFC without significnt effect on MHF. 78

89 Effects of fertilistion on orgnic C nd N storge Lrger C:N rtios of the light frction (Tble 3.2) cn be ttributed to decresed net N minerlistion nd N immobilistion to meet metbolic demnds for microbil decomposers (Swnston et l., 2004). In nother study Mujuru et l. (2013) nlysed C:N rtios in density frctions from similr soils under different tillge systems nd found higher C:N rtios in flf (17 32) thn those in this study Soil C sequestrtion potentil Sndy soils showed no sttisticlly significnt liner dependence of the LFC/MHFC rtio nd LFC lthough there were positive liner reltionships between SOC or TON nd ssocited MHF. Yin et l. (2005) lso found tht Chinese fluvo-quic nd reddish pddy soils hd no liner reltionship between LF nd the LF/MHF rtio. The reltionship of LFC nd LFC/MHF C rtio on clyey soils (Tble 3.3) showed tht ppliction of both N Fert + mnure nd N Fert lone on homefields enbled greter LFC nd MHFC over time nd cn be mximised when the mngement prctices remin unltered. Potentil for more C storge cn be relised when C stocks re below equilibrium. In outfields, only the control hd reched equilibrium wheres the rest hd potentil to store more C. The differences in slopes cused corresponding effects on the equilibrium levels in ech field position nd tretment. The slope ws gretest under control showing greter mplitude in the loss of LF over the nine yer period nd the intercept ws lso highest under control showing tht the mount of C minerlistion my be greter thn the other tretments nd field positions. The lowest intercept ws in inorgnic fertiliser of outfields. Similr regression ws used to determine equilibrium levels of orgnic mtter in hevy frction (Yin et l., 2005; Yin & Ci, 2006) under orgnic nd inorgnic fertiliser ppliction in Chinese soils. Their results showed tht C stocks under orgnic fertilisers were below equilibrium levels while inorgnic fertilisers were bove equilibrium levels. Results showed tht in homefields, the equilibrium vlues under N Fert + mnure nd N Fert were similr but 2.5 times more thn in the control. This could be ttributed to better mngement in homefields thn outfields nd control tretments. In outfields however, the equilibrium vlue of control tretments ws 79

90 Chpter 3 lowest whilst in current stocks in homefields under control tretments were 3 Mg h -1 lower thn equilibrium levels. Outfields hve inherent infertility (Zingore et l., 2007) nd the nitrogen fertiliser inputs could be utilised by crops leving the soil with insufficient orgnic inputs. Yin nd Ci (2006) found mtching equilibrium levels under orgnic nd inorgnic fertilisers while working on soils from long term experiment in pddy soils of Chin wheres in fluvo-quic soils, orgnic fertilisers hd twice s much hevy frction s in inorgnic fertilisers. The long term blnce between ddition nd loss of orgnic mtter under control could result in blnce between the crop uptke nd inputs from residues nd crop roots. In this cse, lnd use history nd mngement become importnt in ttinment of equilibrium levels of soil. The dsorption of orgnic mtter onto minerl surfces mostly depends on the mount of orgnic mtter input nd its qulity (Rsmussen et l., 1980). Control tretments nd outfields reflect long term blnce between C inputs nd outputs nd show tht the soils hve pproched new lower stedy stte. When MHF reches equilibrium levels, it cn no longer increse wheres the LF cn increse with more inputs striving to ttin new equilibrium level (Yin & Ci, 2006). The minerl ssocited hevy frction C cn increse over time in soils tht re below equilibrium level provided the sme lnd use mngement prctice is mintined or improved. Results suggest potentil C nd N sink on clyey soils s indicted by the differences between ctul nd estimted equilibrium levels. Although cttle mnure hs greter potentil to enhnce soil fertility nd soil C storge, smllholder frmers do not hve lrge herd of livestock to supply dequte mnure to their fields. Only few cn fford to pply mnure nnully to some of their fields while the mjority use inorgnic fertilisers (which re lso beyond the rech of mny) (Rusinmhodzi et l., 2013). This suggest need for promotion of cheper lterntives nd ssistnce for inorgnic fertiliser inputs for smll holder frmers to enhnce soil C sequestrtion. Alterntives such s use of compost, legume crops, reduced tillge nd improved fllows should be some of the extension options for promoting soil C sequestrtion. Currently, residue retention is chllenge for smll holder frmers to mintin residues in situ becuse of unfvourble policies which llow free rnge grzing during the dry 80

91 Effects of fertilistion on orgnic C nd N storge seson (6-8 months). There is therefore need for strtegic mngement of crop residues so tht frmers cn mke decisions of whether to keep their residues or to use them for other lterntive uses. The use of bundnt grss biomss to improve quntity of SOM in gro ecosystems cn be more beneficil nd reduces emissions from wildfires. 3.5 Conclusions The results hve shown tht C nd N stocks cn be enhnced in home fields nd outfields on clyey nd sndy soils using N Fert nd combintion of cttle mnure + nitrogen fertiliser respectively. The higher mounts of C nd N in N Fert nd N Fert + mnure tretments thn control shows the importnce of ppliction of soil fertility mendments for the mintennce or improvement of C nd N stocks in griculturl systems. Crbon nd nitrogen in 0-10 cm depth incresed considerbly under N Fert nd N Fert + mnure tretments compred to the control nd supported the hypothesis tht SOC nd TON decresed with incresing soil depth. Despite being under conventionl tillge, fertility mendments mitigted the decrese of totl SOC nd TON in both homefields nd outfields but t different mgnitudes. Nitrogen fertiliser lone or in combintion with cttle mnure consequently incresed the ccumultion of C nd N in flf, olf, MHF fter nine yers of continuous cropping except tht mnure tretment hd limited cpcity to increse olf C nd N. The distribution of C nd N in the three frctions ws greter in homefields thn outfields with greter stbilistion of SOC under N Fert + mnure tretment on clyey soils wheres N Fert hd gretest impcts on sndy soils. On clyey soil, estimtion of the equilibrium level enbled us to ssess the reltionship between SOC frctions nd determine the potentil of fertilistion to sequester dditionl C in clyey soils. Assessment of the mximum SOC protective cpcity suggested potentil for dditionl SOC storge in the clyey homefields nd outfields under N Fert nd N Fert + mnure tretments. In outfields, the combintion of mnure nd nitrogen fertiliser showed highest potentil to sequester more C in MHF. Sustining SOC levels in these soils cn be chllenged 81

92 Chpter 3 by the low crop yields which give rise to low biomss residues coupled with long nnul dry nd fllow seson. 82

93 Soil orgnic crbon nd nitrogen storge in Pinus ptul chronosequence CHAPTER 4 Soil crbon nd nitrogen sequestrtion over n ge sequence of Pinus ptul plnttions in Zimbbwen Estern Highlnds Published s : L. Mujuru, T. Gotor, E.J. Velthorst, J. Nymngr nd M.R. Hoosbeek Forest Ecology nd Mngement. 313:

94 Chpter 4 Soil crbon nd nitrogen sequestrtion over n ge sequence of Pinus ptul plnttions in Zimbbwen estern highlnds ABSTRACT Forests ply mjor role in regulting the rte of increse of globl tmospheric crbon dioxide (CO 2 ) concentrtions creting need to investigte the bility of exotic plnttions to sequester tmospheric CO 2. This study exmined pine plnttions locted in the estern highlnds of Zimbbwe reltive to crbon (C) nd nitrogen (N) storge long n ge series. Smples of stnd chrcteristics, forest floor (L, F nd H lyers) nd 0 10, nd cm soil depths were rndomly tken from replicted stnds in Pinus ptul Schiede & Deppe stnd ges of 1, 10, 20, 25, nd 30 yers plus two nturl forests. Sodium polytungstte (density 1.6 g cm -3 ) ws used to isolte orgnic mtter into free light frction (flf), occluded light frction (olf) nd minerl ssocited hevy frction (MHF). In both nturl nd plnted forests, bove ground tree biomss ws the mjor ecosystem C pool followed by forest floor s humus (H) lyer. In ddition, the minerl soil pool hd 45%, 31% nd 24% of SOC stored t the 0 10, nd cm soil depths respectively. Stnd ge cused significnt differences in totl orgnic C nd N stocks. Crbon nd N declined initilly soon fter estblishment but recovered rpidly t 10 yers, fter which it declined following silviculturl opertions (thinning nd pruning) nd recovered gin by 25 yers. Soil C nd N stocks were highest in moist forest (18.3 kg of C m -2 nd 0.66 kg of N m -2 ) nd lowest in the miombo (8.5 kg of C m -2 nd 0.22 kg of N m -2 ). Averge soil C mong the Pinus stnds ws 11.4 kg m -2 of C m -2, being highest t 10 yers (13.7 kg of C m -2 ) nd lowest t 1 yer (9.9 kg of C m -2 ). Some inputs of chrcol through bioturbtion over the 25 yer period contributed to stbilistion of soil orgnic crbon (SOC) nd its depth distribution compred to the one yer old stnds. Nitrogen ws highest t 10 yers (0.85 kg of N m -2 ) nd lest t 30 yers (0.22 kg 84

95 Soil orgnic crbon nd nitrogen storge in Pinus ptul chronosequence of N m -2 ). Crbon nd N in density frctions showed the 20 yer old stnd hving similr proportions of flf nd olf while the rest hd significntly higher flf thn olf. The contribution of flf C, olf C nd MHF C to SOC ws 8 13%, 1 7% nd 90 91% respectively. Crbon nd N in ll frctions decresed with depth. The minerl ssocited C ws significntly ffected by stnd ge whilst the flf nd olf were not. Conversion of depleted miombo woodlnds to pine plnttions yield better C gins in the short nd long run whilst moist forests (MF) provide both crbon nd biodiversity. Our results highlight the importnce of considering forestry ge bsed C pools in estimting C sink potentil over rottion nd the possibility of considering conservtion of existing nturl forests s prt of future REDD + projects. Key words: crbon, Soil orgnic mtter frctions, Forest floor, C:N rtios, Pinus spp., Plnttion forestry. 4.1 Introduction Chnges in soil orgnic mtter (SOM) cn result in significnt contributions to emissions or uptke of greenhouse gses from forests nd other lnd use systems. Forests govern C trnsfers directly through photosynthesis nd respirtion nd indirectly by influencing the structure nd size of plnt-lef development (Elisson, 2007; Vn Minnen, 2008). They represent n importnt C pool (Brown, 2002) tht fvour sequestrtion of C due to their incresed woody biomss, extensive roots, nd bundnt litter (Shrrow & Ismil, 2004). The extensive rooting system of forest species influence soil microbil biomss thus control the cycle of C between the tmosphere nd the soil (Brown, 2002). In generl, tropicl forests contin less C in soils thn their biomss C, storing bout 60% C boveground nd 40% belowground (Dixon et l., 1994). However, especilly in these forests, roots go deeper nd thus, root turnover my dd to C sequestrtion in deeper horizons due to slow C turnover (Jobbgy & Jckson, 2000). 85

96 Chpter 4 The ccumultion of soil C nd N vries within different soil horizons nd depths. Some forest sub-soils hold bout 45% of totl SOC bound to the cly prticles to form microggregtes. This complexion of SOC in the forest sub soils is essentil for long-term stbilistion (von Lützow et l., 2006). The extent of this stbilistion is determined by orgno-minerl interctions, micropores, type nd nture of cly surfces, nd C loction within the microggregtes. Next to climte nd soil type, the sequestrtion of C depends on forest species nd mngement (Ll, 2003; Lmlom & Svidge, 2003) hving compromise between ecosystem C storge cpcity nd timber extrction. Longterm differences in SOC storge mong three tree species hve been studied by Seely et l. (2002) who concluded tht ll tree species re importnt C pools lthough they hve different C storge cpcities. Vesterdl et l. (2002) compred soils under Norwy spruce (Pice bies L) nd ok (Quercus robur L.) nd showed them to sequester 0.9 kg m 2 of C nd 0.2 kg m 2 of C respectively fter 29 yers nd the SOC being mostly concentrted in the upper soil horizons. In Hwii, Kye et l. (2000) reported storge of C by 17-yer old Euclyptus nd Albizi lebbeck trees nd reported tht Albizi hd 2 kg m -2 more soil C nd 0.23 kg m -2 more soil N. The gretest potentil for bove ground biomss C storge in coniferous plnttions (e.g. pines) is found in tree biomss (Peichl & Arin, 2006) with dditionl mounts from forest floor nd minerl soil C (Tylor et l., 2007; Noh et l., 2010). Net rte of C uptke is gretest when forests re young, nd slows with time. Old forests continue to sequester C t decresed rte with decresed rte of respirtion (Mrris, 2008). When forests re cut, C is returned quickly to the tmosphere if the woody tissue is burned or converted to products tht re short-lived (Ecologicl Society of Americ, 2000). Depending on hrvesting prctices, most of soil C remins in the soil nd become prt of the C stock of growing forest or subsequent cycle in plnttion system. In ddition to type of tree species, stnd ge is lso criticl in determining the mount of C in n ecosystem influencing the qulity nd quntity of C inputs relesed into n ecosystem (Mtos et l., 2010; Penne et l., 2010). Some studies hve shown tht conversion of ntive forests to conifers cn cuse up to 15% losses of SOC depending on period following conversion while others estimted up to 20% SOC reductions over periods below 40 yers (Guo & Gifford, 2002). 86

97 Soil orgnic crbon nd nitrogen storge in Pinus ptul chronosequence The generl impcts of plnttion forests hve been outlined by region (Nilsson & Schopfhuser, 1995) nd the IPCC (2003) suggested tht the rel C stock estimtes might be much lower thn indicted s some of the C hs not been ccounted for. Some studies hve indicted reltive increses in surfce soil C stocks in plnttions (Schwertmnn et l., 1986) while other studies found limited cpcity for soil C ccumultion (Richter et l., 1999; Lio et l., 2012). In this study we will not only look t quntities of C but lso t its stbility. Next to quntity, type nd degree of stbiliztion is lso importnt for the ssessment of C sequestrtion. Most studies on soil C nd N dynmics over stnd ge re minly from other regions either from experimentl sttions reflecting site specific conditions e.g. (Covington, 1981; Rit et l., 2011) or model estimtions on locl or regionl scle e.g. (Peltoniemi et l., 2004). Forest systems of Zimbbwe include rinforest, indigenous woodlnds, plnttions nd bushlnd/grsslnds covering 0.1, 65.9, 0.4% nd 1.5% of the lnd re respectively. Plnttion forests consist of Pinus spp.(68%), Euclyptus (20%) Acci mernsii (11%) nd Poplr spp.(1%) (Forestry Commission Zimbbwe 1996). The reltively extensive woodlnd cover mkes Zimbbwen forests potentil C sink, but the sink is thretened by griculturl expnsion nd demnd for wood. The chrcteristion of C in the bove ground biomss of forests is well dvnced, but the below ground C dynmics is poorly understood cusing need for correlting the below ground biomss to the bove ground biomss to predict C storge in forest soils (Brown, 2002). Determintion of the flux of globl C cycle needs substntil reserch which cn link ptterns nd long term effects of C nd N ccumultion in the soil reltive to forest ge. The role of forests in the globl C cycle hs therefore initited gret interest in exploring the cpcity of forest ecosystems to increse C uptke by mens of fforesttion nd sustinble forest mngement through inititives such s reduced emissions form forest degrdtion nd deforesttion (REDD+). There re few studies quntifying the potentil for soil C ccumultion nd stbilistion under nturl nd exotic plnttions in Zimbbwe nd this cretes need for studies on the soil C sink potentil of forest plnttions. Relible knowledge of the C nd N dynmics in 87

98 Chpter 4 forest soils is therefore fundmentl to understnding sustinble forest mngement prctices nd their role in climte chnge mitigtion In this study we mesured forest floor nd soil C nd N pools induced by plnttion forestry t different stges during rottion cycle. Our im ws to describe the distribution, ccumultion nd stbility of forest floor nd soil C nd N pools nd their temporl shifts over time. We hypothesise tht 1) forest floor nd soil C nd N pools under plnttion forestry re lower thn nturl forest, 2) more C nd N is stored in the forest floor nd soil pools with incresing stnd ge within cycle nd 3) soil C stbilistion increses with stnd ge. 4.2 Study site nd methodology The study ws crried out within the Nyng Pine Division of Wttle Compny P/L in Estern Zimbbwe t Mutrzi Estte situted t S E , lying t the extreme South of the Nyng District (Figure 4.1). The ltitude rnges between m nd m bove se level. The terrin is chrcterised by reltively moderte slopes, nd forms prt of the Estern escrpment of the Nyng mountin rnge. It is drined minly by the perennil Mutrzi river. The totl plnttion re is 3,806, 990 h of which 2,548 h ws re-plnted s t September 2010 (WATCO., 2010). Mutrzi Estte flls into Nturl Region (NR) I of the Zimbbwe gro ecologicl clssifiction system (WATCO., 2010) with nnul rinfll estimted t round 1500 mm nd the sesonlity follows the sme generl pttern s the rest of the country (bulk of rinfll being confined to the months of November to Mrch). Smll mounts of winter precipittion in the form of mist, fog nd rinfll do occur on res of high elevtion. Averge mximum temperture is 28 C with minimum of 0 C. Lowest tempertures occur between My nd August while the highest re from October to Februry. Reltive humidity vries between men 58% in September to men of 86% in Jnury/ Februry. The previling wind is esterly blowing dominntly during the months of November to My. The soils re orthoferrllitic within the Kolinitic order (Zimbbwen clssifiction) which corresponds to Rhodic ferrlsols in FAO clssifiction (FAO, 2006). The soils re chrcterised by good depth, permebility nd structurl 88

99 Soil orgnic crbon nd nitrogen storge in Pinus ptul chronosequence stbility exhibiting high degree of resistnce to erosion. They hve extremely poor chemicl chrcteristics, with prticulrly high levels of cidity nd low wethering rtes (WATCO., 2010). A wide vriety of brodlef, lrge tree species occur in nturl forests including Mcrng mellifer, Ilex mitis, Schreber lt, Rpne melnophloeos, Ole cpensis nd Scheffler umbellifer. The understorey of these forests is usully dominted by extensive bnks of ferns comprising minly of Asplenium nd Cythe spp. Widespred stnds of Psychotri zombmontn lso occur. Forest fringes re dense dominted by species including Hypericum Figure 4.1: Mp of Zimbbwe showing the loction of Nyng pine s Mutrzi forest. revolutum, Pteridium, Rubus nd Smilx nceps. These forests re usully protected from invsions by commercil pine species nd from fires by wide fire gurds (20 50 m) nd inside fire trces. Individul comprtment records showed 89

100 Chpter 4 tht nturl forests were clered for estblishment of pine plnttions in ll stnds considered for the study (WATCO., 2010). Mutrzi estte is mde up of 14 blocks with totl of 206 comprtments of pine trees t different stnd ges in rottion. Totl lnd under Pinus species is h covering bout 70% of totl lnd re. The verge ge of the whole estte s t September 2010 ws 10.9 yers. The Pinus species plnted in the estte include P. elliottii (Slsh Pine), P. ted (Loblolly Pine), P. ptul (Ptul Pine), nd P. tecunumnii (Tecun Umn Pine). P. ptul, Schiede & Deppe is the most dominnt comprising more thn 95% of the plnted re. The comprtments re estblished with stems per h (s.p.h.), thinning twice t 4 yers to 650 s.p.h. nd t 12 yers to 400 s.p.h. before cler felling t 25 yers. Clening or weeding is crried out twice in the first yer, nd one slsh weeding in the second yer (WATCO., 2010). Before the estblishment of pines, ntive forests were clered nd burnt before mrking nd pitting for initil plnting Experimentl design nd dt collection Plots were selected to represent pine ge clsses of 1, 10, 20, 25, nd 30 yers. Also, moist brod lef forest nd miombo woodlnd were included in the study to represent soils prior to clering for pine plnttion. The miombo woodlnd is frequently ccessed by neighbouring communities wheres the moist forest is less ccessible. Among the pine stnds, two stnds were selected from different mngement blocks nd in ech stnd, three smpling plots (0.04 h ech) were rndomly selected. All pine stnds were in their first rottion except the 1 yer old which is now entering second rottion. At ech smpling plot, geo-loction nd ltitude were recorded using Grmin GPS device. Slope ws recorded using clinometer. Aspect, undergrowth species nd ground cover were observed nd noted. At ech smpling point tree mesurements, forest floor nd soil smples were collected s described below. At ech plot centre, forest floor ws smpled from inside metl ring of 30 cm dimeter. The forest floor ws strtified into three lyers: (1) litter lyer (L) consisting of fresh nd recently fllen, non-decomposed mteril. The mteril is identifible by the nked eye s plnt residues. This lyer usully contins less 90

101 Soil orgnic crbon nd nitrogen storge in Pinus ptul chronosequence thn 10% fine orgnic mtter, (2) frgmented lyer (F) orgnic mteril is frgmented nd prtly decomposed with plnt residues being mcroscopiclly recognizble nd (3) the humus lyer (H) consists of decomposed orgnic mtter, originting from litter fll from decdes go nd root turnover. The mterils re decomposed nd their origins re no longer distinct from ech other with more thn 70% being fine orgnic mtter (Currie, 1999; Schulp et l., 2008; Keith et l., 2010). Next, pits were dug to depth of 60 cm. At ech depth increment, i.e. 0 10, nd cm, composited g bulk smple ws tken by smpling the four sides of the pit. In ddition, bulk density (BD) smples were tken using 100 cm 3 metl ring smpler t ech depth increment centre. All soil smples were put into lbelled ir tight plstic bgs nd stored in cool dry plce. A totl of 108 bulk soil smples nd 108 bulk density smples were collected (5 pine stnd ges 2 stnds ech 3 pits 3 depths plus 2 nturl forests x 3 pits ech x 3 depths). At ech smpling point, 1 m 2 1 m 2 re ws clered to trp litter to ssess nnul litter fll in ech ge clss. Dimeter t brest height (DBH, t 1.3 m) nd tree height were recorded for every tree within rdius of m. Tree height ws mesured for every third tree using Suunto hypsometer. Stnd stem volume (V) for pine stnds ws clculted from stnd bsl re (BA=π D 2 /4) nd men tree height (H) using the stndrd biometric eqution: V= BA x H x f [1] The eqution includes stndrd stem form fctor ( f) of 0.4 (Cnnell, 1984). Bsic wood density ws obtined from Muneri nd Blodis (1998) nd biomss expnsion fctor of 1.3 (FAO, 1997) were used to convert stem wood volume to biomss. For the nturl forests, generlised llometric equtions intended for ll species types in brod forest types nd ecologicl zones were used to determine the forest C stocks using equtions 2 (Brown et l., 1989) nd eqution 3 (Mlimbwi et l., 1994) nd n verge of the two ws used DBH DBH 2 [2] Exp ln (DBH) [3] 91

102 Chpter 4 The equtions use DBH to explin vrition in boveground forest C stocks. Bsed on this we ssumed tht we will be ble to generte relible estimtes of C stocks bsed on trees with DBH 5 cm in the nturl forests without using species-specific llometric equtions. The men of two equtions ws tken s the biomss of ech forest. Crbon stocks were clculted using fctor of 0.5 to obtin C stocks (s 50% of biomss is C) Lbortory nlysis The field moisture content ws determined grvimetriclly by drying ech bulk density smple in n oven t 105 C for 48 hours. Smples were weighed before nd fter drying nd percentge field moisture nd the BD grms (g) of dry soil/100 cm 3 (volume of ring) were clculted. Soil ph ws mesured with ph meter (Orion 701A) in 1 M KCl solution suspension for ech stnd ge nd soil depth. The bulk soil smples (BS) were pssed through 2.00 mm Retsch sieve fter which > 2.00 mm prticles were discrded. These prticles included roots, lrge chrcol nd rock mteril in most cses. A smple of 10 g of the dry soil sieved to 2.00 mm (BS) ws put into moisture free hrd grphite continer with metl bll inside. The continer ws put on Retsch mill nd spn t 85 rpm for 5 minutes. The resultnt ground soil ws seled in glss continer. A mg subsmple of the ground soil ws weighed into trred 5 x 5 mm Aluminium foil, seled nd nlysed for totl C nd N by dry combustion in EA1108 CHN Elementl nlyser (Fisons Instruments). The totl C in forest floor nd SOM were used to obtin totl orgnic crbon (TOC, kg m 2 of C) for ech ge clss thus quntifying the reltive contribution of ech forest or plnttion ge. Forest floor C stocks were clculted by multiplying C concentrtion with smple mss nd dividing this by the re of the smple. Soil orgnic C stocks were clculted by multiplying C concentrtion with bulk density nd thickness of the soil lyer with correction for stone content following equtions 4 nd 5: Crbon stock = d BD SOC CF st [4] 92

103 Soil orgnic crbon nd nitrogen storge in Pinus ptul chronosequence where : Crbon stock (kg/m 2 ), d = depth of horizon (m), BD = bulk density (kg/m 3 ) of the soil lyer. SOC = SOC concentrtion expressed s weight bsed percentge nd CF st = correction fctor for stone nd grvel content: CF st = 1 (%stone +%grvel) /100 [5] Soil orgnic mtter frctions nd chrcteristion Soil Orgnic Mtter frctions were obtined from bulk soil following the method by Golchin et l, (1994b) but s described by Roscoe et l. (2000) with three sttes of physicl protection for soil orgnic C: free light (flf) (non-protected nd extrctble without soniction), occluded (olf) (extrctble by soniction) nd protected (MHF) (remined in the residue fter soniction). Sodium Polytungstte (SPT) solution with density of 1.6 g cm -3 ws used to seprte the frctions t rpm in Mistrl 6000 centrifuge. Ultrsonic energy t 90%, 30 W output for 5 minutes from Vibrcell (Sonic Mterils) ws pplied to the smple fter removing the first frction, to seprte the occluded frction from the minerlised frction. The free nd occluded frctions were extrcted by vcuum filtrtion, filtering through Whtmn 0.5µm glss fibre filter, using vcuum filtering unit, decnted into trred beker, wshed with distilled wter to remove excess SPT nd dried t 40 C. The minerlised frction ws obtined by totlling the dry weights of the first two frctions nd subtrcting from 10.0 g which ws the originl weight of the soil smple. The obtined frctions were prepred nd nlysed for totl C nd N by dry combustion Sttisticl nd Dt Anlysis Dt ws nlysed fter testing for normlity (Kolmogorov Smirnov test) nd homogeneity of vrince (Levene s test). One wy nlyses of vrince (ANOVA) in SPSS v.18 (SPSS Inc., Chicgo, Illinois, USA) ws used to ssess the effects of ge nd depth on the forest floor (C nd N) soil ph, bulk density, whole soil C nd N contents, soil orgnic C nd N storge in density frctions nd the ssocited C:N rtios. A seprte nlysis ws done to ssess differences between older pine ge 93

104 Chpter 4 clsses (25 nd 30 yers) nd nturl forests. Tukey s HSD tests were used to test significnt effects t p Results Forest floor C nd N distribution The L, F nd H lyers of the forest floor were distinct in ll nturl nd plnttion forests except the 1 yer old plnttion stnds which hd no L lyer but hd bundnt ground cover dominted by pioneer species. Men thickness of the three forest floor lyers (L, F, H) for 1, 10, 20, 25 nd 30 yers were 0.0, 3.0, 3.5 cm; 1.3, 2.5, 3.7cm; 1.3, 1.9, 2.9 cm; 2.2,3.0,1.8 cm nd 1.7, 5.2, 3, 2 cm respectively. In the MW ech lyer ws 0.5 cm wheres in the MF thickness of L, F nd H lyers ws 0.5, 1.0 nd 1.0 cm respectively (Figure 4.2). Among the pine stnds, totl forest floor C ws lowest in the 1 yer old nd highest in the 30 yer old. All pine stnds hd significntly higher totl forest floor C nd N thn nturl forests except the 1 yer old which hd totl N stocks sttisticlly similr to MF (Figure 4.3). The mount of C nd N in the forest floor ws highest t 30 yers (5.4 kg of C m -2 nd 0.22 kg of N m -2 ). Miombo woodlnds hd significntly low (p < 0.01) C nd N stocks in ll three forest floor lyers with 1.1 kg m -2 of C nd 0.05 kg m -2 of N while the moist forest hd 2.2 kg of C m -2 nd 0.12 kg N m -2. Despite the bsence of the L lyer in the one yer old stnd, there were no significnt differences in cumultive forest floor C nd N with the 10 nd 20 yer old stnds. Among the Pinus stnds the C in the L lyer of forest floor incresed under respectively the 10, 20 nd 25 yer old stnds nd t 30 yers it ws lower thn t 25 yers. Nitrogen followed similr trends with decrese fter 25 yers. The increse between 10 nd 25 yers old stnds ws supported by incresing men nnul litter fll of 0.304, nd kg m -2 dry mss for 10, 20 nd 25 yers respectively (Tble 4.1). The men nnul litter fll of the 30 yer stnds ws slightly higher with kg m -2. The nnul C dditions to the L lyer were therefore, 0.15, 0.37, 0.47 nd 0.49 g C m -2 yr -1 for the 10, nd 30 yer old 94

105 Soil orgnic crbon nd nitrogen storge in Pinus ptul chronosequence stnds with significnt differences (p < 0.001) between successive yers before ge 25. Figure 4.2: Depth of forest floor litter (cm) s function of stnd ge nd forest type. Crbon nd N content of the F lyer decresed significntly (p=0.024) from 1 yer to 10 yers nd the incresed significntly in subsequent yers from 10 to 20 yers (p= 0.008) nd from 20 to 30 yers (p = 0.022) of stnd ge. The L nd F lyer C nd N were significntly higher (p < 0.01) in pine stnd ges of 20, 25 nd 30 yers thn nturl forest. The 25 nd 30 yer old stnds hd significntly higher (p<0.01) C in the F lyer thn the nturl forests. The H lyer, C nd N contents were significntly higher (p = 0.010) under the 10 yer old stnds (2.4 kg m -2 of C nd 0.11 kg m -2 of N) thn the 1, 20 nd 25 yer old stnds pine stnds. The C in H lyer of the 10 yer old nd MF ws significntly higher (p<0.01) thn in the F lyer. The C:N rtios of the plnttion 95

106 Chpter 4 stnds nd the MF showed similr trends with C:N rtio decresing from L to F to H wheres, in MW the H lyer C:N rtio ws higher thn the F lyer (Tble 4.2). b b b b b cd b d c bc d d bc c b c d b b () b b b b b b d bc bc b c b cd bc c bc d (b) Figure 3: Figure 4.3: The prtitioning of ) (b) orgnic crbon nd b) orgnic nitrogen in forest floor of plnttions (1, 10, 25, 30 yers), miombo woodlnds (MW) nd moist forests (MF). Different letters show significnt differences in ech stnd nd forest floor lyers t p

107 Soil orgnic crbon nd nitrogen storge in Pinus ptul chronosequence Tble 4.1: Men stnd chrcteristics of Pinus ptul stnds of different ges nd nturl forests. Stnd Chrcteristics 1 yer 10 yers 20 yers 25 yers 30 yers Miombo woodlnd Elevtion (m) Site Index n n Moist forest Stocking (SPH) 1100 (0) 650 (0) 395 (2) 397(2) 398(1) 308 (15) 712 (21) Men dbh (cm) nd 23.1(0.2) 32.1(0.6) 36.7(0.5) 38.1(0.1) 9.2(2.5) 29.6(5.1) Men Ht (m) 0.74(0.1) 18.3(0.2) 22.6(1.0) 29.4(0.5) 29.9(0.9) 6.3 (2.2) 14.9(4.2) BA (m 2 h -1 ) 0.45(0.30) 27.2(0.5) 32.4(1.2) 42.1(1.1) 45.4(0.4) 8.2(3.3) 70.93(5.1) Volume (m 3 h -1 ) 0.13(0.07) 199.3(4.9) 292.8(18.3) 494.1(19.2) 543.3(19.1) nd nd litter fll (kg m -2 yr -1 ) nd nd nd Biomss C (Mgh -1 ) 0.02(0.01) 23.0(0.6) b 33.8(2.1) b 57.1(2.2) c 62.8(2.2) c 10.7(3.0) 103.1(11.6) d nd = not determined, SPH SPH =Stems =Stems per hectre, per hectre, BA = BA bsl = bsl re, Ht re, = height, Ht = height, dbh = dimeter t brest dbh = height dimeter (1.3 m t bove brest ground) height (1.3 m bove ground) Tble 4.2: C:N rtios of three forest floor litter lyers in pine stnd ges nd nturl forests (MW nd MF) (stnd men ± S.D). Stnd MW MF Litter lyer L - 26(2) b 29(4) bc 26(1) b 31(2) c 23(2) 26(1) b F 31(3) 23(1) bc 25(2) c 26(2) c 24(2) c 21(4) b 20(1) b H 26(4) 22(3) b 20(3) b 21(1) b 23(3) b 23(1) b 16(1) c Overll men 28(4) 26(4) b 24(5) b 24(3) b 26(4) b 20(4) c 20(3) c Mens followed by different superscripts in row represent significnt difference t p< Tukey s HSD Forest stnd nd soil chrcteristics Generlly moist forests hd significntly higher moisture content thn the rest followed by the 1 yer old pine stnd (Tble 4.1). Bulk density in MF ws significntly higher thn the 25 yer old stnd but significntly lower thn ll stnds except the 10 yer old. Among the pines, the 25 yer old stnd hd significntly lower bulk density thn ll. Depth hd no significnt effect on soil ph (Tble 4.3) nd therefore only men ph is recorded (Tble 4.1) nd it rnged from 97

108 Chpter to 5.1 with the MF hving significntly higher (p < 0.01) ph thn ll stnds. The 25 yer old stnds hd significntly lower ph thn MW, 1, 10, 20 nd 30 yer old stnds nd MF. Tble 4. 3: Results of the ANOVA on effects of forest stnd ge nd soil depth on soil ph, soil orgnic crbon nd nitrogen in bulk soil nd density frction. Chrcteristic Age Depth Age * Depth R 2 F vlue P vlue F vlue P vlue F vlue P vlue Soil ph < < SOC (kg m -2 ) < < < TON (kg m -2 ) 7.22 < < < flf C (Mg h -1 ) 7.37 < < flf N (Mg h -1 ) 9.39 < < olf C (Mg h -1 ) 6.04 < olf N (Mg h -1 ) MHF C (Mg h -1 ) < < < MHF N (Mg h -1 ) < < C:N whole soil < < C:N flf C:N olf C:N MHF < SOC= soil orgnic crbon, crbon, TON =totl TON orgnic =totl nitrogen, orgnic flf= nitrogen, free light frction, flf= olf=occluded free light frction, light frction, MHF=minerl ssocited hevy frction olf=occluded light frction, MHF=minerl ssocited hevy frction Men dimeter t brest height (dbh), height, bsl re nd stnd volume incresed with incresed stnd ge hving higher rte of increse from 1 to 10 nd 20 yers but incresed t decresing rte from 25 yers to 30 yers (Tble 4.1). Biomss C ws 0.02, 23.0, 33.8, 57.1 nd 62.6 Mg C h -1 for the 1, 10, 20, 25 nd 30 yer old stnds respectively. In MW nd MF the biomss C stocks were 11 nd 103 Mg h -1 respectively. The cumultive totl C nd N stocks of 0 60 cm depth were lrgest under moist forest (18.3 kg C m -2 ) nd lowest in MW (8.5 kg C m -2 ) (Tble 4.4). Among the plnttion stnds, highest stocks were found under 10-yer old stnds (13.7 kg C m -2 ) nd lowest in the one yer old stnds (9.9 kg C m -2 ). Among the pines there ws n increse in C nd N from 1 to 10 yers followed by decrese t 20 yers fter which there ws n increse t decresing rte (see Tble 4.4). The concentrtion of C nd N ws significntly different (p < 0.01) (Tble 4.3) between the three soil lyers with highest C percentges in the 0 10 cm depth except for 98

109 Soil orgnic crbon nd nitrogen storge in Pinus ptul chronosequence the MW. At the 0 10 cm depth increment, soil C nd N concentrtion were lowest under MW nd highest under MF (Figure 4.4). Soil C concentrtion decresed for ech stnd ge nd nturl forest with incresing soil depth from men of 36.9 (± 0.5) g C kg -1 t 0 10 cm to bout 19.4 g kg -1 (± 0.5) t cm depth lthough there were devitions in the one yer old stnds. Nitrogen followed similr trends from 2.1 g C kg -1 (± 0.1) t 0 10 cm to 1.3 g C kg -1 (±0.1) t cm. Tble 4.4: depth distribution of bulk SOC nd TON in Pinus ptul stnds of different ges nd nturl forests. Soils 1 yer 10 yers 20 yers 25 yers 30 yers Miombo Moist forest Chrcteristics woodlnd Moisture (%) 12.5(0.3) 9.1(0.1) b 9.7(0.1) b 11.1(0.1) b 9.2(0.0) b 9.2(0.4) b 18.0(0.1) c BD (g cm -3 ) 1.68 (0.02) 1.54 (0.01) b 1.69 (0.01) 1.43 (0.02) b 1.64(0.01) 1.93 (0.04) c 1.45 (0.01) b Men ph 4.5(0.6) 4.7(0.1) b 4.6(0.0) b 4.2(0.1) b 4.5(0.1) 4.6 (0.1) b 5.1(0.1) c Bulk soil C (kg m -2 ) 0-10 cm 4.0(1.9) 7.2(1.9) 4.3(1.8) 6.6(1.9) 4.6(1.9) 2.2(0.3) 8.4(0.3) cm 3.5(1.9) 4.0(1.6) 3.2(0.18) 2.5(1.9) 3.8(1.9) 3.4(0.2) 5.8(0.2) cm 2.4(1.9) 2.5(0.19) 2.6(0.19) 3.0(1.9) 2.8(1.9) 2.9(0.3) 4.1(0.3) Totl ( 0-60 cm) 9.9(2.0) 13.7(1.2) b 10.1(0.1) b 12.1(1.1) b 11.2(2.1) b 8.5(0.2) 18.3(1.2) c Bulk soil N (kg m -2 ) 0-10 cm 0.20(0.02) 0.36( ( ( ( (0.03) 0.53(0.02) cm 0.30( ( ( ( ( (0.03) 0.33(0.03) cm 0.18( ( ( ( ( (0.03) 0.20(0.03) Totl (0-60 cm) 0.66(0.01) b 0.85(0.01) b 0.66(0.01) 0.67(0.01) 0.22(0.01) b 0.22(0.02) 0.25(0.02)) b C:N Rtio 15(2) 16(1) 15(2) 19(3) 17(2) 13(3) b 24(4) c Different superscripts show show significnt significnt difference difference t p < t Tukey s p < HSD test. Tukey s Figures HSD in test. Figures prenthesis in prenthesis show stndrd show error stndrd of the men error of the men. The C:N rtios for whole soil incresed with stnd ge from 10 yers to 25 yers nd decresed gin t 30 yers. Depth distribution of minerl soil C:N rtios decresed with incresing depth in ll pine stnds with incresing ge to mximum t 25 yers fter which it decresed t 30 yers s result of decresed C nd incresed N content (dt not shown). Nturl forests (MW nd MF) showed no trend with depth hving highest C:N rtios in MF t cm. 99

110 Chpter 4 d b c b b b b () c d b b b b c b b b c b b b b b b b b (b) () 4.4: Soil orgnic () Crbon nd (b) Nitrogen concentrtion up to depth of 60 cm c in pine stnds (1, 10, 20, 25 nd 30 yers) nd nturl forests (MW nd MF). Different letters show significnt differences for ech depth t p Error brs b b b b show stndrd error of the men. b b b b b b c 100 Figure (b)

111 Soil orgnic crbon nd nitrogen storge in Pinus ptul chronosequence Crbon nd nitrogen in density frctions The distribution of flf C nd N ws significntly higher thn olf in ll stnds. Among the pines the flf nd olf were highest t 10 yers nd 20 yers respectively (Figure 4.5 nd b). Crbon nd N contents of soil frctions were in the order MHF > flf > olf in ll forest types nd ges t the three depths levels. The flf C contributed between 8% nd 13% to totl orgnic C whilst the olf C contributed the lest (1 7%) nd MHF C the most (90 91%) to totl SOC. The mount of flf nd olf C nd N decresed with incresing soil depth. Nitrogen followed similr trends. The C:N rtios of SOM frctions showed difference between the light frctions nd the minerl ssocited frction. There ws generl decrese in C:N rtios in ech ge clss nd nturl forest in the order; flf>olf>mhf (Figure 4.6). The 25 yer old stnds hd highest C:N rtios while the 30 yer old showed; MHF <flf< olf. Significnt differences in olf nd MHF C:N rtios were mostly between nturl forests nd older pine stnds. The mount of flf nd olf C nd N decresed with incresing soil depth. The 10 yer old flf C ws significntly higher thn the 20 nd 25 yer old nd the MW. Free light frction N ws significntly lower in the 20 yer old stnd thn ll except MW. Verticlly there ws no significnt difference in flf C lthough there ws decrese with incresing depth in ll stnds. The flf N t 0 10 cm ws significntly higher thn the lower lyers (P<0.01) (Figure 4.7). The olf C in the 25 yer old stnds ws significntly lower thn ll stnds. Nturl forests (MW nd MF) hd significntly higher C thn the 25 yer old. Verticlly, the olf C t cm ws significntly lower thn the 0 10 cm nd cm depths. The olf N t ws significntly lower thn the 0 10 cm depth (p = 0.042). Among the pine stnds MHF C ws significntly different between successive yers being significntly lower thn MF (p< 0.02) except the 10 yer old stnds. The 25 yer old stnd ws not significntly different from the 20 nd the 30 yer old stnds. The MHF C content of MW nd MF ws significntly different 101

112 Chpter 4 () (b) Figure 4.5:Distribution of soil orgnic () Crbon nd (b) nitrogen in free light frction (flf) nd occluded light frction( olf) in pine stnds nd nturl forests. ) Figure 4.6: C:N rtios of three density frctions (free light frction (flf), occluded light frction (olf) nd minerl ssocited hevy frction (MHF)) in pine stnds nd nturl forests. 102

113 Soil frction Crbon (Mg C h -1 ) Soil frction nitrogen (Mg N h -1 ) Soil orgnic crbon nd nitrogen storge in Pinus ptul chronosequence from the pine stnds being significntly lower in MW nd higher in MF. Significnt differences by depth were shown between ll depths with the 0 10 cm lyer being significntly higher thn the cm nd cm depths (p < 0.02). The C nd N in MHF decresed with incresing depth except MW. Figure 4.7: Depth distribution of crbon nd nitrogen in density frctions in pine stnds of 1, 10, 20, 25 nd 30 yers nd nturl forests (MW nd MF). flf = free light frction; olf = occluded light frction nd MHF = minerl ssocited hevy frction. Correltions between forest floor C nd C in density frctions showed stronger reltionship (67%) between forest floor nd MHF followed by flf (60%). There ws lso positive correltion between flf nd MHF C (64%). The olf hd wek reltionships with the other two frctions nd with forest floor frctions. 103

114 Chpter Discussion Forest floor C nd N The thickness of forest floor lyers in the 30 yer old stnd ws significntly higher thn ll stnds (Figure 4.2). The thickness of the forest floor lyers did not hve corresponding effects on the mounts of C in pines. The MW hd lowest forest floor thickness nd soil C nd N stocks. The importnce of forest type nd mngement in determining C nd N stocks in forest ecosystems ws demonstrted by the differences in C nd N in L, F nd H forest floor lyers in pine stnds nd nturl forests. These differences in C nd N stocks my in turn hve n impct on mechnisms of nutrient cycling (Kim et l., 2010). In this ecosystem, fire ws used s prt of mngement tool to prepre the 1-yer old sites nd this hd n effect on the mounts of litter thus impcting forest floor pools s demonstrted by the lck of the L lyer in the 1- yer old stnds (Figure 4. 3). Czimczik et.l. (2003) lso ttributed low forest floor litter dry msses to effects of fire while working in Scots pine forests. At yer one, there ws no L lyer but the F nd H lyers were similr nd higher thn the 10 nd 20 yer olds. By the ge of 10, there ws n increse in H lyer nd decrese in F with dditions onto the L lyer. There is possibility tht some of the F mteril ws trnsformed into H while some of the H mteril might hve been incorported into minerl soil by the ge of 10 yers nd beyond. The period shortly fter estblishing new rottion by plnting seedlings, shows higher decomposition thn ccumultion of orgnic mteril on forest floor. As the young trees grow older, higher mount of biomss is ccumulted leding to higher litter-fll. By the ge of 10, there ws more H lyer C nd N from the decomposition of ccumulted orgnic mtter tht survived the fires during lnd preprtion coupled with the ccumultion from decying pioneer species. The net C input in the initil yers is not only from litter fll, but lso from residue decomposition fter conversion nd lso decy of pioneer species including grss species which dominte the forest floor before cnopy closure. The reltionship between ge nd C conforms to the Covington s curve only for the L lyer where the lyer strts to develop with time up to mximum level. Covington (1981) 104

115 Soil orgnic crbon nd nitrogen storge in Pinus ptul chronosequence lso found generl decrese in forest floor orgnic mtter in the first 15 yers fter hrvesting of Northern hrdwoods. In rottion, litter-fll becomes importnt for nutrient cycling in the forest-soil, being the lrgest inflow of C nd nutrients to the forest floor (Strr et l., 2005). After yer 10 there ws reduction in H with increses in F nd L lyers. At 20 yers L, F nd H were not significntly different while t 25 more L hd ccumulted on the forest floor nd prt of F nd H incorported into the soil. There ws reduction of H lyer nd n increse in F nd L lyers. More minerlistion nd incresed brekdown of L to F constituents continued up to the ge of 30 yers with reduced quntities of fresh litter. This cycle substntites the importnce of the current 25 yer rottion ge of pines where thickness of the forest floor lyers nd subsequently the C they contin re ssocited with n increse in stnd ge in pine tree forests (Dmes et l., 1998; Bens et l., 2006; Mtos et l., 2010; Penne et l., 2010; Shresth & Chen, 2010). Pinus plnttions re known to culminte in volume nd biomss production t the ge of 25 yers which is much erlier thn nturl forests (Augustin et l., 2007). Although totl C in forest floor incresed between the 10 nd 30 yer old stnds, our results suggests tht forest floor C incresed t decresing rte beyond the ge of 25 yers. The results gree with the findings of Li et l. (2011) who found no further increse beyond the ge of 35 yers while working in Koren pine stnds. There ws decrese in C:N rtios in ll except MW from the L to F to H lyers (Tble 4.2), suggesting n increse in humifiction with depth. Although C:N rtios in forest floor nd soil generlly remin stble (Yng & Luo, 2011), there were significnt differences between overll C:N rtios of the one yer old stnd nd the 20 nd 25 yer old stnds (Tble 4.2). The C:N rtios of forest floor F nd H lyers were lower under MF thn the other stnds indicting better qulity of F nd H mterils. Fine root decomposition cn lso dd C to these lyers (Hoosbeek et l., 2011). The influence of fine roots nd chrcol on C:N rtios is lso reported by Golchin et l.(1994b). Burning cuses short term increses in soil vilble N which results in stimultion of growth of pioneer species in the one yer old 105

116 Chpter 4 stnds. In ddition, fire reduces qulity of the substrte resulting in lrger C:N rtio in one yer old stnds Forest stnd nd soil chrcteristics Among the pines, the 1 yer old stnds hd more moisture despite hving less shde. The thick ccumultion of F nd H forest floor mteril fter the process of burning for lnd preprtion provides ground cover thus reducing soil nd wter loss (vn Bodegom et l., 2008). In ddition, one yer old trees tke up less wter thn trees of 10 or more yers of ge. Higher soil moisture content nd temperture during the erly yers of the rottion ccelerted litter decomposition nd incresed the trnsformtion of F into H litter which ws mnifested in the 10 yer old stnds. This my lso hve cused n increse of dissolved orgnic crbon (DOC) production nd leching of the DOC into minerl soil, thus subsequently dding more C into the soil. There were significnt differences in ph mong pine ges which followed no trend nd between MF nd ll the other stnds (Tble 4.1). The ph ws significntly lower in pine stnds thn MF thus supporting the ide of pines cidifying soils (Prfitt & Ross, 2011; Kundhlnde et l., 2012). Lio et l. (2012) however, found no significnt differences in ph of nturl nd plnted forests while working on other coniferous species. The bulk density in plnttion stnds ws lower thn MW but significntly higher thn MF being significntly different between the one yer old nd the 25 yer old stnd (Tble 4.1). There ws no trend in bulk density with ge lthough Lio et l. (2012) found significnt bulk density increses in plnttion stnds with ge. In this study, storge of C remined high in MF despite the low bulk density nd ws lso lowest in MW despite hving the highest bulk density. We therefore ttribute C storge in the different stnds with differences in C concentrtion (Figure 4.4). Totl boveground tree C incresed from 1 to 25 yers fter which it incresed t decresing rte, demonstrting rpid increse from the 1 yer to the 10 yer old stnd, nd from the 10 to the 25yer old stnd (Tble 4.1). Crbon storge estimted for the MW in this study (10.7 Mg h -1 ) is higher thn biomss C 106

117 Soil orgnic crbon nd nitrogen storge in Pinus ptul chronosequence stocks of costl miombos in Tnzni (Mlimbwi et l., 1994) but lower thn miombo woodlnds in Mozmbique (21.2 Mg h -1 ) nd Tnzni (23.3 Mg h -1 ) (Shirim et l., 2011). The estimted storge of C in MF (103 Mg h -1 ) is lower thn estimtes from tropicl rinforests of Afric (202 Mg C h -1 ) (Lewis et l., 2009). The species diverse, nturl moist forest which is ssumed to be n old nd more stble ecosystem contined more biomss nd soil C nd N compred to the homogenous 25 nd 30 yer old pine forests nd the MW (Tbles 4.1 nd 4.4). Studies in Mozmbique showed 7.6 kg C m -2 in MW soils (Ryn et l., 2011), figure which is lower thn the C in the MW in this study re nd within the rnge of other MW in Southern Afric ( kg m -2 ). The conversion from moist forest to plnttion, cn however result in depleted C nd N stocks (Hudiburg et l., 2009; Gonzlez-Benecke et l., 2010; Wendling et l., 2010; Lio et l., 2012) whilst C benefits my be relised when MW is converted to pine plnttions beyond 25 yers. Similr results of differences between brod leved forests nd pine plnttions were reported by Jndl et l., (2007) nd Wendling et l., (2010), who showed the former ecosystems to contin more SOC thn shllow rooted pine plnttions. In ddition, pine plnttions hve poorly developed rooting systems which mke them less efficient t trpping nutrients when compred to nturl moist forests (vn Bodegom et l., 2008). Nevertheless, Brown et l. (1986) stted tht plnttions cn sequester more C with time s they develop nd grow into older ge clsses nd supports the results of this study. A rottion hs set of progrmmed silviculturl opertions such s thinning (4 yers nd 14 yers) nd pruning (3 yers nd 12 yers) nd weeding which cn continully dd C inputs into forest floor nd soil C pools nd this could be the reson why the one yer old stnd ws not severely depleted. Although the forest ws under pines for 25 yers, the SOC stocks were not restored to the levels of MF though prt of the C is stored s root biomss. In ddition, the rottion system contins chrcol produced during burning for lnd preprtion cusing redistribution of C nd N t depths of cm t 10 yers nd even deeper fter severl yers. There were no significnt differences between MW nd the older ge clsses of Pinus stnds of 20, 25 nd 30 yers t depth of cm. The lower biomss C in MW t 0 10 cm nd cm could be ttributed to frequent 107

118 Chpter 4 disturbnce s neighbouring communities utilise the woodlnd for timber nd non- timber forest products nd this results in higher degrdtion pressure ccompnied with lower C storge thn their potentil (Brown, 1997). A diverse undergrowth cn produce litter exposed to decomposers following estblishment of new rottion nd hence the 10 yer old stnd hd the highest C nd N concentrtions t time when ll pioneer species re gone which could be ttributed to incorportion of F nd H mteril into minerl soil. This high ccumultion of C could be ssocited with litter input from the nontree vegettion (Blck et l., 2009). In this wy the process of succession nd decomposition of pioneer species enhnced more fine roots nd litter in the upper soil thereby incresing C concentrtions in the erly yers of estblishment (Hoosbeek et l., 2011). There ws lso n increses in C inputs into upper soil lyers despite the burning for lnd preprtion where fine roots re burnt nd chrcol is dded into the soil. Blck et l. (2009) studied Pice sitchensis, Bong. Crr. nd found highest sequestrtion rtes t 10 yers, which subsequently declined fter cnopy closure in older nd thinned stnds. There could lso be other fctors ffecting C nd N dynmics including mngement ctivities such s pruning, thinning nd other tending opertions. In this study, the C nd N were lowest soon fter estblishment but recovered rpidly by the ge of 10 yers, fter which it declined possibly s result of silviculturl opertions such s thinning nd pruning fter which ws recovery gin by the ge of 25 yers. This shows tht plnttions cn be used efficiently to crete C sinks becuse of the rpid growth rtes soon fter estblishment nd dditions tht come s the trees develop nd grow into older ge clsses. There ws smll decrese in soil C:N rtios before 10 yers of stnd development which is similr to the findings of Georgidis (2011) who ws working in temperte forests. This ws supported by Olsson (1996) who showed decresed C:N rtios during the initil 8 to 15 yers of stnd development. The highest C:N rtio ws in the 25 yer old stnds showing limittion in the rte of decomposition which my be ffected by the low ph vlues. 108

119 Soil orgnic crbon nd nitrogen storge in Pinus ptul chronosequence Crbon nd nitrogen in soil orgnic mtter frctions There were no outstnding differences in mss distribution of the MHF mong stnd ges nd forest types. Differences were distinct in flf nd olf showing highest proportion of flf in moist forest whilst olf ws not outstnding but only lowest t 25 yers (Figure 4.7). The pttern of C nd N storge in SOM frctions ws consistent with bulk soil C. Crbon nd N in different forests showed greter quntities of flf C in moist forest due to incorportion of bove ground litter into the minerl soil by bioturbtion. Under pine stnds, bioturbtion is negligible leving the bove ground litter on top of the minerl soil. Crbon nd N in flf under pines is mostly the result of root turnover. The flf C showed decrese with incresing depth (Figure 4.7) in ll pine stnd ges except the 1 yer old nd nturl forests which is in line with the observed root distribution. Tropicl soils, such s the ones in the study re, re reported to contin high flf C in the top 30 cm of the soil (Trumbore et l., 1996) decresing with depth due to less inputs (litter nd fine roots become fewer with increse in depth). However, this trend ws not evident in the one-yer old stnd, MW nd MF. This devition could be ttributed to the influence of fire before lnd preprtion nd dditions through root decomposition in the soil lyers fter subsequent hrvesting in pine stnds nd root decy fter lnd clering. A combintion of the effects of preprtory fire, mngement (hrvest removl nd pruning) nd decomposition could result in ddition of more free light orgnic mtter to deeper soil lyers without ny pttern. Totl C nd N in soil nd in occluded nd minerl ssocited frctions generlly decresed with increse in depth in ll forest types nd ges which is similr to other studies (Tn et l., 2007; Usug et l., 2010; Jiménez et l., 2011). Age of forest plys n importnt role in determining SOC quntities s shown by significnt differences in SOC frctions. The MHF is importnt for contining reclcitrnt C nd thus contributes more to the long term stbilised SOC pool (Tn et l., 2007). Incresing stble C with successive rottions were noted by Zhng et l. (2009). 109

120 Chpter 4 The C:N rtios mong the density frctions were different between the light frctions nd the minerl ssocited frctions. A wider vrition of C:N rtios mong density frctions were lso shown by Golchin et l. (1994). The observed decrese of C:N rtios with incresing SOC stbilistion, except for MW where nthropogenic effects my be dominnt, is due to incresed humifiction of SOM nd the ccumultion of lrge N-rich orgnic molecules originting from microbil biomss (Chn et l., 2008). Differences between C:N rtios of flf, olf nd MHF re probbly due to differences in decomposition sttes where olf is slightly more decomposed whilst MHF is more dvnced (Hssink, 1995). 4.5 Conclusion We investigted the C storge potentil of Pinus ptul ge sequence (1, 10, 20, 25 nd 30 yers) nd two nturl forests. Cumultive C nd N storge in stem biomss, forest floor nd soil incresed with stnd ge in pine stnds. Although we my not hve perfect reference forest, ssumption of stble equilibrium condition nd C storge potentil cn be ssessed using existing forest frgments (MF nd MW). In this regrd the conversion from moist forest to plnttion forest, results in depletion of C nd N stocks but conversion of miombo woodlnd to pine plnttion cn be beneficil in the long run. Forest floor C nd N peked t 30 yers nd this my be relted to dditions from fine root biomss nd litter fll. Stem biomss incresed from 1 to 10 yers nd from 10 to 25 yers nd incresed t decresing rte therefter. As stem biomss nd forest floor C incresed t 30 yers, SOC decresed. Pine plnttions store significntly more C nd N in the forest floor thn nturl forests. Crbon in the forest floor decresed from L to F to H while in minerl soil it decresed with incresing soil depth. Tree biomss incresed with incresing ge in this pine ge sequence nd corresponded with incresing forest floor C. Soil orgnic C nd N concentrtion however decresed for ech stnd ge nd nturl forest with incresing soil depth from men of 36.9 g of C kg -1 t 0 10 cm to bout 19.4 g of C kg -1 t cm depth lthough there were devitions in the MW. Nitrogen followed similr trends from 2.1 g of N kg -1 t 0 10 cm to 1.3 g of N kg -1 t cm. 110

121 Soil orgnic crbon nd nitrogen storge in Pinus ptul chronosequence In pine plnttions soil C storge is mximised t 10 yers nd declines fter thinning nd grdully increses gin towrds the rottion ge of 25 yers. Thickness of litter lyer did not hve corresponding effect on the mounts of C stored in forest soils lthough there ws positive correltion between litter lyer nd MHF nd flf. Totl C nd N in bulk soil nd density frctions generlly decresed with increse in soil depth for ll forest types nd ges. The contribution of flf C, olf C nd MHF C to SOC ws 8 13%, 1 7% nd 90 91% respectively. The C:N rtios of SOM frctions decresed s: flf > olf > MHF. Plnttions cn therefore be n efficient mens of creting C sinks owing to the rpid development rtes soon fter estblishment nd dditions tht come s the trees develop nd grow into older ge clsses. Thus, the period of stnd mturing between the ge of 20 nd 25 my be considered most importnt for C sequestrtion due to n increse in both the bove nd below ground C nd N storge. If plnttion forestry is to benefit from globl rrngements such s REDD+, mitigtion should im t reducing disturbnces such s fire nd other forms of C emissions. The focus should be on fforesttion nd enrichment plnting to increse nd mintin the re of forest lnd coupled with proper monitoring nd silviculturl prctices tht increse C sequestrtion. Mintennce of high conservtion vlue moist forests hs gretest benefits of both C nd biodiversity conservtion nd these should be conserved nd if possible be considered s prt of future REDD+ projects. Additionl studies re needed on biomss C prtitioning. 111

122 Chpter 4 112

123 Modelling SOC in griculturl nd forested systems CHAPTER 5 Modelling soil crbon from griculture nd forest res of Zimbbwe Chpter to be submitted s Mujuru L. Murev A nd Hoosbeek MR (2014) 113

124 Chpter 5 Modelling soil crbon from griculture nd forest res of Zimbbwe ABSTRACT Assessment of the potentil for soil orgnic crbon (C) sequestrtion bsed on lnd mngement, soil type nd climte conditions is importnt for selecting griculturl prctices tht cn be used to mitigte greenhouse gs emissions. The Rothmsted crbon model (RothC) ws used to predict soil orgnic crbon (SOC) chnges in response to lnd mngement prctices nd climte chnge. Agriculturl nd nturl forest soils on Luvisols nd Arenosols under tillge nd fertility tretments were ssessed. Density seprted soil orgnic mtter frctiontions were compred with conceptul pools of the RothC model. Modelled SOC stocks were comprble to mesured stocks of There ws good correltion between the density frctions nd modelled vlues of Humified Orgnic Mtter (HUM) + Inert Orgnic Mtter (IOM) nd Resistnt Plnt Mteril (RPM) with poor correltion for the decomposed plnt mteril (DPM). Microbil biomss ws prt of RPM + DPM. Results showed strong positive reltionships between mesured MHF nd HUM + IOM (R 2 = 0.98). All tretments showed rpid increse in the initil yers with slow increse therefter except for the control which showed decline in C stocks with time. The results suggest greter increse of SOC stock in clyey soils nd nturl forests thn cropping systems on sndy soils. Sndy soils hve less cpcity to store more C unless there re supplementry orgnic inputs nd/or integrtion of prctices such s groforestry. Results hve shown tht linking RothC model with mesured soil dt, cn be 114

125 Modelling SOC in griculturl nd forested systems useful for estimting the potentil C sequestrtion resulting from lnd mngement prctices in Zimbbwen forest nd cropping systems. Key words: RothC, soil crbon pools, modelling, climte chnge, tillge, fertilistion 5.1 Introduction Sequestrtion of C in griculturl nd forest soils is seen s wy of decresing tmospheric crbon dioxide concentrtions nd mitigting climte chnge. The cpcity of soil to store C depends on soil type, lnd mngement prctice nd climtic conditions. Humn ctivities continue to increse greenhouse gs emissions nd sttistics hve shown tht tmospheric concentrtions of crbon dioxide (CO 2 ) nd other het-trpping gses incresed from 1.7 ppm between 1993 nd 2003 to 2.1 ppm over the decde. By Mrch 2013 the concentrtion of tmospheric C hd risen to ppm (Ntionl Climte Dt Center (NOAA), 2013). Lnd use chnge hs hd significnt impct on globl C stocks with cultivtion reported to cuse significnt depletion of orgnic mtter nd relesing crbon dioxide (CO 2 ) into the tmosphere (IPCC., 2000). Some of the mjor cuses of CO 2 relese from the erth to the tmosphere re deforesttion nd degrdtion which re driven mostly by griculturl expnsion nd shifting cultivtion (Willims et l., 2008), production of chrcol nd fuel wood (Chidumyo, 1991), legl nd illegl timber logging (Sunseri, 2009), construction nd wild fires. Agriculturl ctivities tht relese C from the soil into the tmosphere include tillge nd other forms of soil disturbnces tht fcilitte gseous exchnge between the soil nd the tmosphere. Soil disturbnces lso enble the incorportion of plnt mterils into the soil (Pretty et l., 2002). Although griculturl ctivities hve been identified s mjor sources of CO 2 emissions, it is lso possible to hve griculturl ctivities tht re dpted to reverse these negtive effects nd promote soil C sequestrtion in ddition to 115

126 Chpter 5 other benefits of food security nd ecosystem sustinbility (Food nd Agriculture Orgnistion (FAO), 2010). Soil C sequestrtion cn offer vluble offset for greenhouse gs emissions in griculture, forestry nd other lnd uses (AFLOU) (IPCC, 2006) nd benefit from existing C mrkets. Ll (1999) stted tht developing countries hve the gretest potentil for soil C sequestrtion since most of the soils re highly degrded nd therefore below C sturtion. Agriculturl prctices tht reduce emissions nd promote C storge cn be designed to chieve sustinble lnd use but the need for simple, rpid monitoring methods cnnot be overemphsised. A number of scientific evidence gps re linked to the ccurcy of C ccounting, scribed to lck of dt nd uncertinties relted to C storge nd C flux models (Stringer et l., 2011). Soil crbon models hve been developed for prediction nd provision of informtion on the rte of soil C sequestrtion or loss. Severl models hve been designed nd reviewed by Smith et l. (1997). Among these re RothC, DNDC, CENTURY nd DAISY ll bsed on conceptul crbon pools with different turnover rtes. The RothC model is mong the models which hve been identified by FAO (2004) s widely pplicble esy to use model. The model hs been pplied to estimte SOC chnges in response to lnd use or climte chnge in rble nd non- rble soils in mny prts of the world (Jenkinson et l., 1987; Colemn & Jenkinson, 1996; Kong & Colemn, 2008b; Yokozw et l., 2010) lthough with limited pplicbility in Southern Afric. Chnges in the rtes of soil C sequestrtion due to chnging environmentl or mngement fctors, cn tke severl yers to become pprent (Pretty et l., 2006). Therefore, future impct of griculture ctivities nd ssocited lnd use chnge cn only be predicted by the use of observtions in combintion with models which cn provide mens of evluting the chnging prctices in the future. Currently, there is limited informtion on the potentil for future C sequestrtion in sndy nd clyey soils of Zimbbwe especilly on smllholder frmers fields despite the importnce of SOC in soil biologicl, physicl nd chemicl processes. Equilibrium levels re importnt in determining the potentil of soil to store more C nd severl mthemticl models re used. In this study we compre the output of the Lngmuir eqution with the equilibrium levels estimted using RothC model. 116

127 Modelling SOC in griculturl nd forested systems There is lck of dt for monitoring long-term SOC in Zimbbwen soils mking it impertive to use the mesured soil dt s vlidtion dtset for testing the RothC model outputs for the time period of nd to investigte long term effects of rising tempertures on SOC storge. The objectives of the study were to: (1) ssess the effects of lnd mngement prctices on future soil C storge (2) test the reltionship between density seprted SOC pools nd modelled pools required in RothC nd compre estimtes of soil C chnges between mesured nd simulted C. (3) ssess the sensitivity of Roth C model to temperture rise nd (4) compre equilibrium levels from RothC with equilibrium levels estimted using the Lngmuir eqution. 5.2 Mterils nd Methods Study sites Modelling of tillge impcts ws done on soils representing clyey nd sndy soils from Hereford in Bindur district (17 42 S; E), Nyrukund in Shmv district (17 00 S; E) nd Murew district ltitude (17 39' 13" S nd longitude 31 48' 30" E). A description of the sites is given by Mujuru et l. (2013) nd Mujuru et l. (Submitted). Briefly, Hereford soils re red clys vrying from silty cly lom to cly, with chrcteristics corresponding to Rhodic Ferrlsols (Nympfene, 1991; FAO/IIASA/ISRIC/ISSCAS/JRC, 2012) nd flling into the ctegory of low ctivity clys (Btjes, 2010). Sndy soils, re derived from corse grnite covering lmost 70% of Zimbbwe (Thompson & Purves, 1981) nd re clssified s the Kolinitic order, Fersillitic group under the Zimbbwen soil clssifiction, which corresponds to Ferric Luvisols (Thompson & Purves, 1981; Scoones, 2001; FAO, 2006) but using IPCC defult clsses derived from the hrmonised world soils dtbse (Btjes, 2010) they cn be clssified them s Arenosols, (>70% snd nd <8% cly) nd re brodly referred to s sndy soils. Murew soils re grnitic snds (Hplic Arenosols) (FAO/IIASA/ISRIC/ISSCAS/JRC, 2012) which re strongly wethered hving low levels of vilble nutrients nd low nutrient reserves. These re interspced with pockets of dolerite intrusions 117

128 Chpter 5 tht give rise to smll ptches of reltively fertile clys (Chromic Luvisols) (Nympfene, 1991; FAO, 2006). Bindur represented clyey soils wheres Shmv represented sndy soils for tillge ssessments wheres Murew represented both clyey nd sndy soils under fertility tretments. Seven lnd mngement prctices (A-G) were selected with considertion to the current frming systems nd exmined using density seprted SOC pools for soils t 0-10 nd cm depths: [A] - conventionl tillge (CF) with mize legume rottion consists of n ox drwn plough to depth of cm once before plnting, residues were removed nd the remining biomss incorported into the soil during ploughing in the next seson, [B] - Ripping (RP) - minimum tillge with n ox drwn ripper to depth of cm with mize- legume rottion, crop residues were to be retined in the field fter hrvesting, ground cover of Mg h -1 ws required in RP, [C] - Direct seeding (DS) - no tillge using n ox drwn direct seeder with synchronised seeding nd fertiliser ppliction with mize-legume rottion, residues were lso retined or supplemented to chieve the Mg h -1 ground cover nd cropping in ech tillge system ws mize (Ze mys. L) /cowpes (Vign unguicult L. Wlp) rottion t Nyrukund or soy ben (Glycine mx L. Merr) t Hereford, ech tretment received nnul bsl fertiliser of 165 kg h -1 compound D (i.e. 11kg h -1 N, 10 kg h -1 P, 10 kg h -1 K), which ws followed by 69 kg h -1 N pplied s mmonium nitrte in splits t 4 nd 7 weeks fter germintion (Thierfelder et l., 2012; Thierfelder & Wll, 2012), [D] - Nturl forest (NF), [E] - Conventionl tillge with continuous mize cropping nd no fertility mendments (control), [F] - Conventionl tillge under continuous mize cropping with nnul ddition of nitrogen fertiliser (N Fert), [G] - Conventionl tillge with continuous mize cropping with combintion of nitrogen fertiliser nd cttle mnure (N Fert + mnure) where mmonium nitrte supplies 100 kg N h -1 nd cttle mnure pplied t 5 Mg h -1 supplied n equivlent 10 kg P h -1 nd 0.9 % N ech cropping seson. Ech scenrio ws run for sndy nd for clyey soil under current nd chnging 118

129 Modelling SOC in griculturl nd forested systems temperture conditions. The SOC stock predicted by RothC model were compred with the SOC dt for 2010 (Mujuru et l 2013; Mujuru et l submitted) Modelling of crbon sequestrtion potentil Dt for SOC for whole soil nd density frctions under tillge nd fertility tretments were obtined from Mujuru et l. (2013) nd Mujuru et l. (Submitted) respectively (Tble 5.1). The RothC model used in this study is bsed on monthly time step clcultions tht simulte SOC turnover over periods rnging from few yers to few centuries (Jenkinson & Ryner, 1977; Jenkinson et l., 1990). The model inputs were () climtic dt (monthly rinfll (mm), monthly evpotrnspirtion (mm), verge monthly men ir temperture ( C)), Climte dt were obtined from world climte dtbse collection of meteorologicl dt (2013). The ETo clcultor (Res, 2009) ws used to estimte potentil evpotrnspirtion bsed on temperture nd rinfll of ech specific loction (b) Soil dt (cly%, initil soil orgnic crbon (SOC) stock (Mg C h -1 ), depth of the soil lyer (30 cm), inert orgnic mtter (IOM) (Tble 5.1) pproximted using eqution [1] proposed by Flloon et l, (1998) becuse the rdiocrbon content ws not known nd becuse we did not do ny chemicl frctiontion to seprte this chemiclly resistnt pool. In this study, the IOM ws ssumed to be prt of the minerl ssocited hevy frction (MHF). IOM = 0.049TOC [1] Where: TOC is Totl orgnic crbon, Mg C h -1 IOM is Inert orgnic mtter, Mg C h -1 nd (c) lnd use nd lnd mngement dt (soil cover, monthly input of plnt residues (Mg h -1 ), monthly input of frmyrd mnure (FYM) (Mg C h -1 ), residue qulity fctor (decomposble plnt mteril (DPM)/resistnt plnt mteril (RPM) rtio) (Colemn & Jenkinson, 1999). Soil cover ws bsed on whether the soil is bre or vegetted in prticulr month nd is indicted s either covered or fllow (Colemn & Jenkinson, 1999). SOC is split into four ctive pools DPM, RPM, microbil biomss (BIO) nd humified orgnic mtter (HUM) which decompose by first-order process, ech with its own chrcteristic rte, nd n mount of inert orgnic mtter (IOM) 119

130 Chpter 5 resistnt to decomposition (Figure 5.1). The plnt mterils in RothC re subdivided into DPM nd RPM, wheres plnt debris in soils is found s free light Tble 5.1: Inert orgnic mtter (IOM), cly% nd SOC stocks in whole soil nd density frctions in griculturl lnds nd nturl forest systems (Men (SD)) to depth of 30 cm. Soil type Lnd mngement SOC LFC MHF IOM Cly content (Mg h -1 ) (%) Clyey CF 31.17(4.57) 2.18(0.36) 28.99(4.38) Clyey RP 32.27(3.04) 2.39(0.37) 29.88(3.05) Clyey DS 30.62(4.23) 2.02(0.27) 28.60(4.13) Clyey NF 43.88(5.70) 2.37(0.20) 41.51(5.17) Clyey Control 17.48(1.66) 0.67(0.10) 16.81(4.58) Clyey N Fert 24.74(1.80) 0.95(0.66) 23.79(7.08) Clyey N Fert + mnure 31.12(1.61) 1.18(0.09) 29.94(7.13) Sndy CF 7.97(1.69) 0.49(0.12) 7.48(1.62) Sndy RP 10.28(1.93) 0.76(0.15) 9.52(1.37) Sndy DS 11.37(2.08) 0.75(0.25) 10.62(1.99) Sndy NF 29.25(2.57) 1.58(0.20) 27.67(2.55) Sndy Control 5.92(1.15) 0.38(0.10) 5.54(1.54) Sndy N Fert 11.66(1.60) 0.56(0.08) 11.10(1.04) Sndy N Fert + mnure 10.72(1.61) 0.65(0.07) 10.07(1.52) C CF = conventionl frming, RP = Ripping, DS = direct seeding, N Fert = Nitrogen fertiliser, N Fert + mnure = nitrogen fertiliser + cttle mnure. frction (flf), occluded light frction (olf) nd forms of dissolved orgnic crbon (DOC). The three mesured frctions re prticulte orgnic mtter found outside of ggregtes (flf), POM found within ggregtes (olf), nd minerl-ssocited frction (MHF). In the model, plnt C inputs re ssumed to exclusively enter the DPM nd RPM in proportions, determined by the source of the plnt mterils (Figure 5.1). The process lgorithms in RothC re ffected by the three climtic 120

131 Modelling SOC in griculturl nd forested systems fctors (soil moisture, temperture nd plnt cover) with ech pool decomposing by first order kinetics hving chrcteristic decomposition rtes. The model pportions plnt litter input between DPM nd RPM depending upon the vegettion type. For most griculturl crops nd improved grsslnd, DPM/RPM rtio of 1.44 ws used, i.e. 59% DPM nd 41% RPM wheres DPM/RPM rtio of 0.25 ws used for deciduous or tropicl woodlnd i.e. 20% s DPM nd 80% RPM. In the model, the proportion tht goes to CO 2, BIO nd HUM is determined by the mount of cly in the soil (Colemn & Jenkinson, 1999). Although RPM estimted using the RothC model correlted well with mesured LFC in sndy soils nd nturl forests, LFC in the croplnd clyey soils neither correlted with RPM nor DPM in croplnds. Becuse of this discrepncy the model defult decomposition rte constnts for ctive comprtments were initilly used (i.e. RPM (0.3), DPM (10.0) BIO (0.66) nd HUM (0.02)) (Colemn & Jenkinson, 1999). Figure 5.1: Structure of the Rothmsted Crbon Model (Colemn nd Jenkinson, 1999). The mesured SOC content ws used for running the RothC model to equilibrium under constnt environmentl conditions. The constnt climtic conditions were tken s the verge of the climtic dt from For ech lnd mngement prctice, RothC ws initilly run to equilibrium (

132 Chpter 5 yers), itertively fitting crbon inputs to mtch the initil SOC stock nd the corresponding distribution in comprtments (DPM, RPM, BIO,HUM) with different decomposition rtes. Dt of C nd rdiocrbon ges in ll the comprtments received in equilibrium mode (initil soil stte, initil rdiocrbon ges) were used to run the model in short term mode ( from ). The ssumption implied in this inverse simultion procedure, ws tht RothC could simulte the dynmic chnges in SOC under the specified conditions. Model estimted pools of RPM + DPM + BIO were compred with LF C (flf +olf) whilst HUM + IOM were compred with MHF for ech lnd mngement system. The RPM, DPM, BIO, LF represent mounts of C input Comprison of equilibrium SOC levels from RothC nd Lngmuir eqution SOC equilibrium levels were estimted using the Lngmuir eqution nd compred with equilibrium levels estimted by RothC model. The Lngmuir eqution cn be used to evlute the dsorption of light frction C (LF C) onto minerl surfces, nd becoming minerl ssocited hevy frction C (MHF C) nd is regrded s sequestered C (Yin et l. (2005). We ssumed tht over time, the LF C decomposes nd in prt becomes dsorbed onto minerl soil prticles s the MHF C. In ddition, soil minerls cn rndomly dsorb LF C until the MHF hs reched C sturtion. Therefore, interction between LF C nd soil minerls follows dsorption nd desorption processes tht cn be described using the Lngmuir eqution. Eqution 2 shows the lineriztion used to fit the dt following Yin nd Ci (2005) nd Bolter nd Hornberger (2007). LF C / MHF C = LF C / MHF C mx + 1 / (k MHF C) [2] Where MHFC mx is the mximum dsorption cpcity for orgnic C (equilibrium vlue for soil orgnic C in the MHF) nd k is the equilibrium constnt. LFC/MHFC versus LFC yields liner reltionship with slope 1/(MHFC mx ) nd intercept 1 / (k MHFC). The equilibrium level estimted using Lngmuir eqution ws compred with the equilibrium level obtined from RothC model. 122

133 Modelling SOC in griculturl nd forested systems Dt nlysis Reltionships between men vlues (± SD) for modelled nd mesured SOC nd equilibrium C levels in ech lnd mngement system were nlysed using liner regression. The goodness of fit ws tested using correltion coefficient (r) nd root men squre error (RMSE) in ddition to stndrd devition. RMSE shows the percentge term for the totl difference between predicted nd observed vlues. The bis vlue ws clculted s Yi Xi where Yi = mesured SOC or equilibrium level nd Xi = modelled SOC or equilibrium level. A t-test ws conducted to compre mesured with modelled C. Significnt differences were tested t p Results nd Discussion Reltionship between mesured nd modelled SOC stocks The RothC model is designed to simulte soil orgnic C turnover using user estimtes of the C inputs mking the evlution of the soil C turnover components esier. If input dt is not vilble, inverse simultion techniques re used to determine inputs needed to mtch the observed SOC in prticulr yer. Running the RothC model in reverse mode estimted the inputs required to ttin the SOC stocks in 2010 for the seven lnd mngement prctices in clyey nd sndy soils. The simulted mounts of the SOC in the RothC ws initilly used to clculte the nnul plnt inputs to soils using the mode of known totl SOC content. Soil orgnic mtter concentrtion t ny time is the blnce of the C ddition to the soil pool nd the crbon lost from it through decomposition nd other loss mechnisms. One my rgue tht the lck of simultion of plnt production, hence C input into soil could be disdvntge if not done crefully. Inverse simultion techniques llow the determintion of input needs to mtch the observed SOC. With the defult setting of the decomposition rte constnt for resistnt plnt mteril, there ws good greement between the simulted SOC nd mesured SOC (2010) in ll tretments (Tble 5.2). However, the lrgest discrepncies were found in sndy soils. On the other hnd, the model ppered 123

134 Chpter 5 to under-estimte SOC in ll tillge systems minly on clyey soils. In sndy soils, SOC in CF, DS nd N Fert + mnure were overestimted. Crbon Stocks on clyey soils with continuous conventionl tillge, under control, N Fert nd N Fert + mnure hd higher C thn in sndy soils lthough they hd similr inputs. Differences re most likely result of cly content nd the root biomss input corresponding to higher crop yields. Highest mesured SOC ws found in nturl forests whilst lowest ws in control tretments of both tillge nd fertility tretments. Mesured SOC on clyey soil ws NF > RP > CF > N Fert + mnure > DS > N Fert > Control wheres the modelled SOC ws: NF > N Fert + mnure > DS > RP > CF > N Fert > Control. In sndy soils the trend for mesured SOC ws NF > N Fert > N Fert + mnure > RP > control > CF whilst the modelled ws NF > N Fert + mnure > N Fert > DS > RP > CF > control. The higher SOC in NF reflects the chnges ssocited with conversion of forests to croplnds. Tble 5.2: Soil orgnic crbon (SOC) mesured nd predicted using site specific soil input vlues for the 0-30 cm depth in seven lnd mngement prctices on clyey nd sndy soils. Difference % Mesured Modelled Soil type Tretment Mg h -1 Difference Clyey CF 31.17(0.77) Clyey RP 32.27(076) Clyey DS 30.62(0.77) Clyey NF 43.88(1.08) Clyey Control 17.48(1.58) Clyey N Fert 24.74(1.46) Clyey N Fert + mnure 31.12(1.42) Sndy CF 7.97(0.74) Sndy RP 10.28(0.77) Sndy DS 11.37(1.27) Sndy NF 29.25(1.09) Sndy Control 8.92(1.42) Sndy N Fert 11.66(1.41) Sndy N Fert + mnure 10.72(1.34) CT = conventionl tillge, RP = ripping, DS = direct seeding, NF = nturl forest, N Fert = Nitrogen fertiliser, N Fert + mnure = nitrogen fertiliser + cttle mnure. 124

135 Modelled SOC (Mg h -1 ) Modelling SOC in griculturl nd forested systems Under the current climte scenrio, there is potentil for dditionl crbon storge in Compred with mesured SOC, RothC underestimted ll SOC vlues in ll nturl forests nd tillge tretments on clyey soils wheres on sndy soils the model overestimted the three fertility tretments nd DS (Tble 5.2). Clyey soils hd lower devitions ( %) thn sndy soils ( %). The regression model combining both clyey nd sndy soils performed better thn seprted nlysis y = x hving significnt positive correltion between modelled nd mesured vlues on both sndy nd clyey soils (p<0.001, R 2 =0.978; SE = 1.62) Figure 5.2. Higher estimtes of C thn mesured could be cused by misrepresenttion of cly content coupled with soil depth which ws higher thn the model s 23 cm. In ddition, RothC estimtes lower thn mesured vlues could be reflection of potentil insensitivity of the model to tillge. Although the model ws developed in Rothmsted, UK, bsed on the frming systems where soil ws mnged by ploughing nd stubble incorportion soon fter crop hrvest, the simulted SOC mtched the observed SOC, suggesting tht lrge frction of the suitbility of the model to predict chnges in SOC in semi-rid soils. Clyey Sndy Figure 5.2: The reltionship between observed C stocks nd modelled C stocks from RothC crbon model. 125

136 Chpter Reltionship between mesured SOC pools nd conceptul RothC SOC pools Density frctiontions were compred with model outputs of RPM+DPM nd HUM+ IOM. Estimted resistnt plnt mteril crbon pool (RPM) in the RothC model nd mesured light frction crbon (LFC) content were compred. The conceptul pools estimted using the RothC model for ech site re shown in Tble 5.3. Tble 5.3: Conceptul SOC pools in the RothC model estimted for ech site Soil type Lnd mngement prctice RPM DPM BIO HUM Totl Clyey CF Clyey RP Clyey DS Clyey NF Clyey Control Clyey N Fert Clyey N Fert +Mnure Sndy CF Sndy RP Sndy DS Sndy NF Sndy Control Sndy N Fert Sndy N Fert +Mnure CT CT = conventionl tillge, RP = ripping, DS = direct seeding, NF = nturl forest, N Fert = Nitrogen fertiliser, N Fert + mnure = nitrogen fertiliser plus cttle mnure. Although there ws good reltionship between light frction (flf +olf) C nd RPM (R 2 = 0.674, p < 0.001, SD= 0.185, SS = 1.25) (Figure 5.3) nd between 126

137 Modelling SOC in griculturl nd forested systems MHF C nd HUM (R 2 = 0.797, p<0.01, SD=3.74, SS= ), they did not mtch perfectly for substitution in the model but MHF C nd HUM + IOM hd perfect mtch (R 2 = 0.98, p< 0.01) (Figure 5.3b). Clyey soil sndy soil () (b) Figure 5.3: Reltionship between () mesured LF C nd model RPM nd (b) MHF C vs modelled HUM + IOM Long term prediction of C storge in different systems The verge vlues of SOC stock estimted for 40 yer modelling period bsed on 2010 mesured C vlues, showed greter increses in C stock in clyey thn sndy soils. Additionl storge cpcity vried with lnd mngement nd rnged between -29% nd 36% ( to Mg h -1 ). This shows tht prctices such s conventionl frming without fertility mendments cuse loss of C from the soil (29%) (Figures 5.4). In cropping systems the conservtion tillge prctices reched more thn 70% of their potentil to store C. However, the overll ssessment shows tht cropping systems re only ble to sequester s much C s Nturl forests with ddition of lrge quntities of orgnic inputs s reflected in RP nd DS. Other studies confirm tht the cropping systems re chrcterised by lower SOC thn nturl forests on similr soils (Guo & Gifford, 2002). The lower C in control nd CF systems lso support the findings of Scholes nd Hll (1996) who showed tht 127

138 Chpter 5 () (b) Figure 5.4: Predicted SOC in seven lnd mngement prctices under current climte on () clyey nd (b) sndy soils. CF = conventionl frming with mize legume rottion, RP = Minimum tillge with mize legume rottion, DS = no tillge with mize legume rottion, NF = Nturl forest, Control = Conventionl tillge under continuous mize cropping (no fertility mendments), N Fert = conventionl tillge under continuous mize cropping (nitrogen fertiliser 100 kg h -1 ) nd N Fert + mnure = Conventionl tillge under continuous mize cropping (nitrogen fertiliser + cttle mnure). pproximtely 50% of SOC is lost in the first 20 yers following conversion of tropicl woodlnd, grsslnd or svnnh. Such losses cn be evident even fter 5 128

139 Modelling SOC in griculturl nd forested systems yers (Ll, 1999). Although tillge is considered mjor cuse of SOC loss in griculturl systems (Pretty et l., 2002), the model did not predict lrge difference between tilled nd untilled sites on clyey soils. Insted, soil fertility mendments, minly combintion of N fertiliser nd mnure, cn be more beneficil for SOC sequestrtion. The modelling of C to 2050 showed tht most sites hve theoreticl cpcity to store more C rnging from 0.06 t Mg h -1 over the 40 yer period. The resultnt nnul rtes of C sequestrtion re in the rnge of Mg h -1 nd Mg h -1 yr -1 in sndy nd clyey soils respectively depending on prctice. The rte of C sequestrtion in sndy soils is less thn the rnge for dry lnds ( Mg h -1 yr -1 proposed for conservtion tillge in dry lnds by Ll (1999) but clyey soils showed higher vlues thn Ll et l (1999). Though the increse is smll, the model showed higher rte of increse in the erly yers followed by n increse t decresing rte in lter yers. This confirms the fct tht SOC storge cn only be possible up to limit beyond which no sequestrtion occurs becuse SOC will only continue to increse up to equilibrium. Clyey soils hve greter potentil thn sndy soils whose increse is mrginl in ll tretments except NF. Lter, minly fter 2030, decrese of the rte of C increse is noticed in soils under N Fert nd N Fert + mnure on clyey soils. The rte of SOC stock increse in the initil yers of the modelling period cn be ffected by chnging input fctors or incomplete initilistion. Incresed cly content supports higher SOC storge potentil lthough it cn be ffected by soil depth. The im of ny lnd mngement system should be to increse plnt productivity while mintining the soil helth. Results suggest tht sndy soils re lmost sturted nd hve limited cpcity to sequester more C. Productivity nd C sequestrtion cn only be chieved through soil fertility mendments i.e. eliminting the constrints to plnt growth in order to chieve resonble chnges in future C stocks. Thierfelder et l., (2010), Thierfelder nd Wll (2012) nd Rusinmhodzi et l., (2011) showed tht verge crop yields for smllholder frming systems yield bout 2.5 Mg h -1 grin yield nd this yields bout 7 Mg h -1 of totl bove ground biomss. Thus bout 3.36 Mg C h -1 is produced nnully s biomss C ssuming plnt C content of 48%. 129

140 Chpter 5 The RothC model ws lso run under pst nd future chnging climte given temperture rise of 1.5 C rise nd bseline of A 1.5 C rise in temperture shows different responses in clyey nd sndy soils with the clyey soils hving greter C ccumultion wheres in sndy soils, modelled C stocks re below current stocks. Clyey soils benefit from incresed tempertures wheres sndy soils tend to decline. Under the chnging climte scenrio the potentil for dditionl crbon storge is limited in ll lnd mngement prctices on sndy soils. Clyey soils showed similr trends for NF but greter potentil for DS thn the two fertility tretments (N Fert nd N Fert + mnure) wheres on sndy soils, fertility tretments hd higher ccumultion thn DS nd RP but lower thn NF (Figure 5.5). An increse in temperture cn result in incresed C stocks. Although the N fertiliser sites ccumulted more C thn no till sites (DS) other studies found inorgnic fertilisers detrimentl to C storge e.g. Frge et l. (2007) prt from the costs ssocited with its cquisition. Incresed tempertures cuse incresed plnt production resulting in higher biomss nd C. However, soil respirtion lso increse with incresing temperture but the blnce between these two processes will mke the difference. In this ssessment, modelling with RothC suggests tht incresed biomss production my be lrger thn incresed decomposition resulting in incresed C stocks. In ll cses, modelling of SOC in clyey nd sndy soils of Zimbbwe showed tht new stedy stte will be reched if the current prctices re mintined, nd so subsequent declines in SOC become reltively smll with time. The modelling hs shown tht under current climtic conditions ll systems except the nturl forest on clyey soils hve reched stedy stte wheres 1.5 C rise in temperture cuses some of the systems on clyey soils to sequester more C. The results lso show tht when holding ll the other fctors constnt, the model is sufficiently sensitive to rise in globl tempertures with sndy soils reching n equilibrium much erlier thn clyey soils. 130

141 Modelling SOC in griculturl nd forested systems () (b) Figure 5.5: Fitted nd projected SOC stocks on () clyey nd (b) sndy soil up to yer 2050 under temperture rise of 1.5 C. CF = conventionl frming with mize legume rottion, RP = Minimum tillge with mize legume rottion, DS = no tillge with mize legume rottion, NF = Nturl forest, Control = Conventionl tillge under continuous mize cropping (no fertility mendments), N Fert = conventionl tillge under continuous mize cropping (nitrogen fertiliser 100 kg h -1 ) nd N Fert + mnure = Conventionl tillge under continuous mize cropping (nitrogen fertiliser + 5 Mg h -1 yr -1 cttle mnure). 131

142 Chpter Comprison of equilibrium level estimtion by Lngmuir eqution vs RothC model The Lngmuir eqution could not estimte equilibrium levels for the three fertility systems (control, N fertiliser nd N fertiliser + mnure) on sndy soils due to poor model fit (Tble 5.4) hving R 2 vlues below Informtion on equilibrium levels is importnt in determining the mount of dditionl C soil cn dd. This is normlly chieved using equtions such s the Lngmuir eqution whose limittions hve been pointed out (Bolster & Hornberger, 2007). Tble 5.4: Equilibrium C levels estimted by Lngmuir eqution for lnd mngement prctices on clyey nd sndy soils. Equilibrium C Soil type Prctice (Mg h -1 ) *R 2 *SE *P vlue Clyey CF < Clyey RP < Clyey DS < Clyey NF Clyey Control <0.010 Clyey N Fert <0.010 Clyey mnure Sndy CF Ns Sndy RP <0.001 Sndy DS Sndy NF Sndy Control ND <0.001 Sndy N Fert ND <0.001 Sndy mnure ND <0.001 CT CT = conventionl tillge, RP = ripping, DS = direct seeding, N Fert = Nitrogen fertiliser, Mnure = nitrogen fertiliser + cttle mnure, Ns = not significnt, ND= not determined, SE = Stndrd error.* from regression of the Lngmuir eqution A comprison of equilibrium levels estimted using the RothC model nd Lngmuir eqution shows positive correltion between the two methods on cly soils (R 2 = 0,87, P<0.01, SE = 4.86) (Figure 5.6). In clyey soils, mesured SOC in CF, DS, N Fert nd N Fert + mnure were below equilibrium levels wheres RP, control 132

143 Modelling SOC in griculturl nd forested systems nd nturl forest reched equilibrium. In sndy soils ll systems were below equilibrium levels estimted by RothC s the Lngmuir could not be pplied to sndy soils due to poor model fit. It is importnt to note tht uncertinties of up to 20% cn be found due to insufficient informtion bout soil nd climte (Poussrt et l., 2004). Therefore, since the dt used in this study ws obtined from short term experiments (4-9 yers), Sources of uncertinties between the mesured nd modelled dt cn be recognised nd bsed on the input dt. For exmple, the vlues of initil crbon stock. Another uncertinty is tht the dt of SOC stock were modelled using 30 cm soil depth whilst the model is bsed on 23 cm depth. The SOC stock is clculted from bulk density nd depth. Figure 5.6: Reltionship between equilibrium levels estimted by RothC model nd the Lngmuir eqution. 5.4 Conclusion RothC model is one of the most widely used models for the estimtion nd prediction of SOC stock on griculturl nd forest lnd due, bsed on successful pst evlutions nd the generlly good vilbility of required input dt. The RothC model is simple nd uses redily vilble input dt to estimte SOC stock 133

144 Chpter 5 on ny mngement systems in non-wterlogged soils. The modelling pproch represented n importnt nd promising method for the estimting C stock chnges nd provided some level of confidence for future soil C scenrios for cropping nd forest systems on sndy nd clyey soils of Zimbbwe. On the bsis of our results, it cn be concluded tht RothC 26.3 model is ble to estimte SOC stock chnges on Zimbbwen sndy nd clyey soils lthough it cnnot distinguish effects of soils disturbnce (such tillge intensity). Prctices tht hd more orgnic inputs such s the conservtion tillge prctices nd the N Fert + mnure showed greter potentil for future C storge. There ws lso good reltionship between equilibrium levels estimted by RothC model nd those estimted using the Lngmuir eqution. The model lso showed tht under current climtic conditions ll systems on sndy soils were pproching stedy stte. Therefore, the RothC model equilibrium output cn be used for ssessing the cpcity of soil to store dditionl C bsed on mesured vlues. The model s conceptul pools of RPM nd HUM showed good correltion with mesured LFC nd mesured MHF in both sndy nd clyey soils. Holding other fctors constnt, 1.5 C rise in temperture cuses some of the systems on clyey soils to sequester more C thn the current. A higher temperture hs positive effect on the stock of soil orgnic C in nturl forests nd in soils with N fertiliser nd mnure inputs. The model is therefore sufficiently sensitive to rise in globl tempertures with sndy soils reching n equilibrium much erlier thn clyey soils. The modelling pproch represents one of the most promising methods for the estimtion of SOC stock chnges nd llowed us to evlute the chnges in SOC in the pst nd future periods on the bsis of mesured dt. 134

145 Modelling SOC in griculturl nd forested systems 135

146 Chpter 5 136

147 Chpter 6 CHAPTER 6 SYNTHESIS 6.1 Introduction Agriculturl nd forestry systems of sub-shrn Afric (SSA), re more vulnerble to the chnging climtic conditions cused by incresed concentrtions of greenhouses in the tmosphere (Food nd Agriculture Orgnistion (FAO), 2010). Projections showed severl Sub-Shr Africn countries becoming incresingly more prone to severe droughts (Rojs et l., 2011). The models lso suggested mjor effects on griculturl production due to disturbnces in rinfll ptterns nd wter vilbility regime, thus, incresing Afric s vulnerbilities to food deficits. Such chllenges cn be overcome by choosing lnd mngement prctices tht increse productivity while mintining environmentl integrity. Agriculturl systems ccount for lrge shre of totl lnd use in Zimbbwe, mking them prime trget in ny strtegy imed t slowing, hlting, or reversing the emission of crbon into the tmosphere. Generlly, there is no compenstion for ecosystem services including benefits of environmentl goods such s C sequestrtion, fctor likely contributing to the historiclly observed reduction in some ecosystem services mnifested in form of lnd degrdtion in griculturl lnd nd deforesttion in nturl forests. Soil orgnic crbon is constituent of soil orgnic mtter importnt for mintining soil fertility, soil moisture, soil structure nd energy for soil biot. The dynmic processes tht influence SOM qulity nd quntity re complex, operting through time t different loctions nd situtions (Bldock & Skjemstd, 1999) resulting in SOM being both source of C relese (e.g. lnd degrdtion) 137

148 Synthesis nd sink for C sequestrtion. Mnging lnd for C sequestrtion is sustinble strtegy tht cn be implemented in both griculturl nd forest systems for productivity. The C storge potentil of soil is function of existing orgnic C levels nd biomss input reltive to other fctors (e.g. the prctice, climte, soil minerl composition, soil biot, nd position in the lndscpe) fcilitting the continuous exchnge of C between the erth nd the tmosphere. In this wy the, rte of sequestrtion or relese of C is strongly influenced by these interctions with isoltion of the influence of ech individul fctor often complicted (Ll, 2007). In met nlysis, Guo nd Gifford (2002) showed the importnce of lnd use on C storge by reveling incresed soil C stocks following lnd use chnge from crop to psture while the reversed lnd use chnge led to decresed C stocks. This supports the ide tht the fesibility of incresing soil C storge depends on the mngement prctices, type of soil nd fertilistion (This thesis). However, C (ir) is common property, where ny one individul, community, orgnistion or ntion tkes only smll shre of the cost of the C they dd to the tmosphere. In this regrd, the privte costs tht motivte decisions to sequester C my fll short of socil costs, resulting in too much crbon dioxide ccumulting in the tmosphere nd ffecting ll. Despite this fct, lnd mngement prctices tht minimise C losses by creting positive ecosystem C budget nd enhncing C storge need to be identified nd promoted. Such identifiction improves our understnding of the extent to which ech lnd mngement prctice ffects greenhouse gs reduction efforts nd mitigtion of humn induced increses in tmospheric crbon dioxide. Alterntive livelihood development strtegies hve been suggested s mens of enbling rurl people to shift from subsistence livelihoods through projects, such s fisheries nd beekeeping. Such projects help to decrese the mount of deforesttion nd forest degrdtion in line with REDD+ objectives by shifting locl economies wy from ctivities tht dmge forests, such s clering more lnd for cropping, unsustinble chrcol production nd firewood vending. Inititives with such focus hve met with vrying success e.g. in Keny Wild life Works hve estblished clothing fctory nd in Tnzni they hve strted beekeeping projects s prt of REDD+. 138

149 Chpter 6 The lnd mngement options should focus on incresing net primry production (NPP) nd reducing C loss from soils (e.g. combting lnd degrdtion nd deforesttion). In the quest to promote soil C sequestrtion to mitigte climte chnge, it is importnt to relise tht the C sequestrtion process is bsed on the fct tht: the process of soil C storge is reversible; soils hve finite cpcity to store C nd is linked to fluxes of other greenhouse gses, such s nitrous oxide (N 2 O) nd methne (Powlson et l. (2011). Chnges in SOC ssocited with lnd use nd mngement occur slowly nd cn be explined by the wy C nd N re llocted in different SOM frctions. Density frctiontion cn seprtes SOM tht is residing outside ggregtes (flf) from tht inside ggregtes (olf) nd orgnic mtter bound to minerls (MHF). The pools/frctions hve different turnover nd they cn show impcts of lnd mngement prctices on SOM (this thesis) which re normlly not evident in whole soil. Thus, SOM frctions serve s good indictors of both short term nd future chnges in totl C nd N stocks nd provide evidence of stbility of ech frction. However, the storge of SOC is suggested to be influenced by selective preservtion of reclcitrnt compounds, physicl protection ginst decomposition nd interctions with minerl surfces (von Lützow et l., 2008). 6.2 Potentil of tillge mngement prctices for SOC sequestrtion Agriculture ctivities hve been one of the key drivers of deforesttion in most of the tropicl countries nd griculture is likely to benefit from groforestry prctices tht hve potentil to increse soil fertility t miniml costs. The use of groforestry cn be prt of the fforesttion/reforesttion or it cn be specil option by itself to contribute to emissions reductions through dditionl sequestrtion nd/or voided emissions. When lnd is mnged using combintion of griculturl nd forestry pproches, there re benefits of sequestrtion of dditionl C in trees nd/or soil thus, reducing C emissions when compred to business-s-usul griculturl prctices. Agroforestry simultneously increses plnt cover more thn contined in nturl woodlnds. Plnting of trees in gro ecosystems cn be in the form of protection of existing trees in griculturl lnd, cretion of gro-forestry prks, soil fertility enhncement nd soil erosion 139

150 Synthesis control, other on frm tree plnting ctivities, fruit tree plnting or livestock mngement. Trees cn either be ntive or exotic species. In this regrd, plnting trees reduces the need to open up more nturl forests for crop production nd thus, help to strengthen smll holder forest mngement inititives while mitigting climte chnge since trees not only hve the potentil to sequester nd store dditionl C in bove ground biomss, but even more so significntly increse SOC. Nturl woodlnds on sndy soil hd lower C nd N stocks thn woodlnds on clyey soils (Figure 2.3). Although there re no estimtes of totl GHG emissions from deforesttion nd degrdtion of Africn woodlnds (Bond et l., 2010) the socioeconomic reltions between the stte, privte sector, nd locl people, coupled with the persistent poverty hve hstened the rte of deforesttion. Poverty, hunger nd incresing demnd for griculturl lnd hve chiefly driven locl communities to over exploit forest resources for their livelihoods. Greter loss of C into the tmosphere is ttributed to the conversion of ntive lnd to griculture nd much of the newly cropped lnd is unsuitble for griculture nd degrdes quickly, thereby forcing the frmer to convert even more lnd to griculture (Wlker & Desnker, 2004). There is however reltionship between SOC content nd soil texture s shown by the greter difference between ntive forests nd croplnds on sndy soils thn clyey soils (Tble 2.1). Such differences re likely result of hrvesting of mjor bove ground components in croplnds without substntil returns to the soils. There re competing uses for residues (Livestock feed nd fuel). Aboveground biomss only enters the soil lbile crbon pool vi roots, root exudtes nd litter input leding to incresed C nd N storge t deeper lyers of the soil. It is estimted tht bout 1.14 Pg C my be nnully emitted into the tmosphere through erosion induced processes (Jnzen, 2006). In ddition, ccelerted erosion nd other degrdtion processes cuse significnt loses of topsoil (Ll, 2012) lthough the impcts of erosion on C losses depends on the fte of the C fter the soil is deposited. Smllholder frmers hve trditionlly used the hnd hoe for tilling nd weeding lnd. The hoe only ploughed the upper surfce leving hrd pn t lower levels. A development from the hoe led to the use of the mouldbord plough going deeper nd breking the hrd pn thus llowing rin wter 140

151 Chpter 6 permetion. The plough exposed soils to greter bioturbtion. Tillge is known to ffect distribution of orgnic C in the soil profile, mixing soil mterils in the plough lyer nd destroying soil structure. Ploughing excerbtes depletion of SOC depending on climtic, edphic nd mngement fctors. To dte, severl tillge prctices re used to improve soil moisture nd fertility in croplnds. Cultivtion nd tillge however, ffect the distribution of SOC depending on the quntity nd qulity of SOM. Depletion of SOC in rble or other highly disturbed systems, cn lso occur due to lower plnt litter input (Figure 2.2), fster SOM brekdown (s it is more ccessible in soil ggregte disruption during ploughing) nd incresed displcement of C-rich surfce soil through erosion. The SOC cn be preserved using mize/legume rottions with mgnitudes vrying with soil texture nd tillge intensity (Chpter 2) nd cn lso be incresed using groforestry technologies. Furthermore, the inherent infertility of Zimbbwen soils (Nympfene, 1991) coupled with regulr tillge opertions result in lower C levels in croplnds thn ntive forests. Depletion of the SOC pool in tillge systems demonstrtes potentil impcts of conversion of nturl forests to croplnds lthough other intercting fctors such s vrible temperture regimes, low biomss C inputs, higher decomposition rtes nd ltertions in soil moisture determine the pce. Although the tillge studies (Chpter 2), were short term observtions, the nlysis took into considertion both the bulk/whole soil nlysis nd density frctions. Density frctions, minly the light frction re good indictors of the impcts of short term chnges in lnd mngement (Mrín-Spiott et l., 2008). Despite the short period of time in this ssessment, results suggest tht there were short term increses in whole soil C nd N stocks over four yer period with mgnitudes vrying with soil type (Chpter 2). In this study, SOC nd TON were greter under minimum nd no tillge ctivities t 0-10 cm depth in both sndy nd clyey soils when compred with initil stocks. Contrry to this, clyey soils showed conventionl tillge being comprble to ripping (minimum tillge) nd direct seeding (no till) (Tble 2.2). Sndy soils re however more sensitive to tillge nd tend to benefit more from no tillge prctices nd this supports other scholrs who lso found C storge benefits under no tillge (DS). Although the methodologies used to nlyse the initil C stocks could be different, 141

152 Synthesis comprison of conservtion tillge prctices with conventionl tillge showed similr trends to those obtined by compring with the bseline dt. In this study the mgnitude of SOC nd TON depletion ws minimised by ripping nd direct seeding (Chpter 2) when compred with conventionl tillge on sndy soils (Figure 2.3). Results suggests tht tillge nd other forms of soil disturbnce cn relese C from the soil enhncing gseous exchnge between the soil nd the tmosphere but cn lso fcilitte the incorportion of plnt mterils into the soil (Pretty et l., 2002). Better mngement, more C inputs nd the root biomss incorportion into the soil during ploughing contributed to n increse in SOC under conventionl tillge. In tillge systems, plnt roots re the mjor source of orgnic mtter input to the soil wheres in forest systems the mjor component is bove ground biomss retined fter litter fll in ddition to below ground prts. Soils with low C stocks hve less cpcity for SOC ccumultion s shown by sndy soils hving less cpcity to stbilise more C nd N thn clyey soils (This thesis) lthough the extent of C nd N storge is modified by lnd mngement prctice nd other environmentl fctors. Results show the likelihood of strong C decomposition regime fcilitting incresed sequestrtion of lbile C during the cropping seson. Although sndy soils showed low greenhouse gs mitigtion potentil (shown by low potentil of C storge) when compred with clyey soils, results suggest tht the no tillge system (DS) is the best mngement for C storge on sndy soil whilst the conventionl tillge (CT) system is the worst. Sndy soils re hence more sensitive to disturbnce. Currently tillge prctices re confined to frmer mnged experimentl plots with potentil for up scling on smll portions of lnd. Inputs nd equipment re supplied by the project where frmers shre the direct seeder nd the ripper. The experimentl plots hve provided some informtion on technicl performnce of ech tillge prctice. The relevnce of the prctices is however hmpered by two mjor chllenges: 1) Policy tht does not llow residue retention becuse livestock grze freely in the dry seson. Residues either hve to be removed or left in the field for livestock to grze.2). Even if left in the field, residues re consumed by termites nd other smll orgnisms during the long dry period lsting five to six months. Given this scenrio, ll conservtion tillge 142

153 Chpter 6 prctices get supplementry residues (grss) t the beginning of ech cropping seson. In sndy soils, the prctice of conservtion tillge increses soil C nd N in the top soil lyers when compred to conventionl tillge. Other fctors such s climtic conditions nd possible incorportion of residues under conventionl tillge did not led to more soil C loss when compred with no tillge (DS) t 0 10 cm. In this cse conservtion tillge prctices did not show expected results of greter C nd N possibly due to competing needs for crop the residues (minly s niml feed). In clyey soils, SOC nd TON stocks in conservtion tillge prctices (RP nd DS) were not significntly different from conventionl tillge but were lower thn in nturl forests. Clyey soils seem to be more ffected by vrious stress fctors including long dry spell nd trmpling by both smll nd lrge livestock cusing possible mixing of soils in the upper soil lyers nd reduction of residues left in situ. Idelly, no till nd reduced tillge plots should hve continuous surfce cover nd this is usully violted becuse of institutionl nd socio-economic fctors. Frmers re not ble to fence off their fields over the dry period subjecting ll fields to open ccess grzing. Only smll portions on experimentl plots nd home grdens re protected. Fencing does not mke the fields inccessible to smll livestock e.g. gots. To compound the mtter, the erected fences re usully vndlised s is the cse in the study re. Although conventionl tillge prctices re often known to hve greter rte of SOC loss when compred to conservtion tillge, there ws reltive SOC gin under conventionl tillge t 0 10 cm when compred to reduced tillge (RP) nd no tillge (DS) prctices on clyey soils wheres on sndy soils no tillge (DS) hd better C storge thn RP nd CT. Some studies hve shown tht conservtion tillge increses C storge in the 0 10 cm depth while conventionl tillge decreses C stocks t the sme depth level. The lck of significntly different C nd N gins under conservtion tillge systems could be due to the limited residue cover which mkes the soils even more vulnerble to gents such s wind erosion thn ploughed soils, where the trnsient roughness creted by tillge my reduce wind nd wter erosion (Blnco et l., 2009). Thus, the indequte mounts of crop residues coupled with climtic conditions (with long dry spell) 143

154 Synthesis could hve direct influence on soil wter conservtion, soil erosion control nd nourishment of microbil communities in ech system. Despite hving lower C nd N stocks thn NF, tillge systems incresed C stocks t 0-10 cm. Interestingly, the rte of C increse in CT on sndy soil ws similr to the rte of C increse under DS on clyey soils lthough the ctul vlues were higher in clyey soils thn sndy soils. Increse in C nd N stocks in cropping systems hs been ttributed to better nutrition nd mngement intensity compred to the time prior to the strt of the experiment. For exmple, fertiliser ppliction is limited in most smllholder frms s inorgnic fertiliser is usully beyond the rech of most frmers. As result, most fields re highly depleted creting potentil for C sequestrtion. Sndermn et l. (2010) reported between 0.2 nd 0.3 Mg C h -1 yr -1 increses in C stocks under improved lnd mngement of croplnds (including enhnced rottion nd no-tillge) thn under conventionl mngement cross rnge of Austrlin soils. They found gretest lrgest gins within the first 5 to 10 yers with the rte of chnge decresing to lmost zero fter 40 yers nd they ttributed bsolute declines to continuous responses of soils to the initil disturbnce of the ntive soils. Although Ll et l (1997) suggested tht conservtion tillge my provide even less opportunity to increse SOC in the tropics, results suggested tht conservtion tillge cn led to high SOC ner the surfce compred to conventionl tillge only in sndy soils. The hypothesis tht soils under no tillge mngement ccumulte more SOC in surfce soil lyers thn conventionl tillge (Jgdmm & Ll, 2010) cn only be pplicble to sndy soil in this study since on cly soils CF ccumulted more C in surfce soil thn RP nd DS. However, t lower depths, RP nd DS hd more C nd N (Tble 2.2) which is contrry to most studies which ssocite CF with distribution of soil orgnic C nd N in the profile. VndenBygrt et l. (2002) found tht t 0-30 cm, DS ccumulted more C nd N thn CF nd RP. This ws however contrry to clyey soils where the highest mounts t 0 30 cm were under RP nd the lowest C stock were under DS. There ws demonstrtion of differences in C storge s ffected by texture, minly the cly content. Cly content is known to exert mjor control on mounts of orgnic C soil cn store (Schimel et l., 1994). Biotic fctors influence the storge of C nd N in the soil lthough they were not prt of this study. 144

155 Chpter 6 There is need for continuous mintennce of environmentl benefits resulting from incresed SOC content. Fundmentl soil biologicl processes, which govern soil nutrient cycles re bsed on key processes of soil C storge nd cn be chieved by either reduced decomposition or improved mngement, either by fertility mendments, or by doption of reduced tillge. Exposure of orgnic mteril to microbil degrdtion results in less flf. The SOC pool receive fresh orgnic mtter through litter input whereby soil microbil ctivity drives the process of conversion of litter into stble humus nd is lso relted to bioturbtion (e. g. termite nd erthworm ctivities), which ffect both ertion nd SOM incorportion into cly minerls t deeper soil lyers. In this study, the ctivity of rthropods ws importnt for mixing orgnic mtter t different soil lyers. The trnsformtion of litter to SOC cn be illustrted s in Figure 6.1. Crbon dioxide Crbon dioxide Microbil decomposition Humus formtion Termites nd erthworms Archnids Residue frgmenttion Centipedes, millipedes etc. Figure 6.1 Process of litter trnsformtion in litter decomposition in griculturl systems Accordingly, the smpling time of the study (spring) might lso ffect the mounts of C in the different SOC pools in both croplnds nd forests. Shortge of orgnic mtter for soil fun is often demonstrted by the consumption of crops in the field (mostly by termites) minly towrds the end of the growing seson. Litter nd the resulting humic substnces re decomposed, minly by the bulk soil micro-orgnisms tht comprise bcteri, fungi nd soil meso- nd mcro-fun, 145

156 Synthesis resulting in respirtions nd subsequent soil CO 2 efflux nd ltertions to the soil chemicl nd physicl properties. Senevirtine (2003) suggested inocultion of soil fun into litter to enhnce decomposition, thus improving nutrient storge. Activities of inocultion re however, not necessry in smllholder res under study s the litter is lwys indequte for the decomposers minly during the cropping seson when climtic conditions re fvourble. The nturl forests re often subjected to nnul fires mking possibility of chrcol frgments floting with the flf nd olf since they re within the sme densities. It is therefore possible tht chrcol derived from nnul burnings contributed to flf nd olf C in ll sites. The overlpping densities mke it impossible to seprte the chrcol from plnt mterils in light frctions. The flf nd olf reflect the extent of ltertions imposed on the soil by mngement ctivities nd cn thus be used s sensitive indictors of the qulity of soil. The light frction C ccounted for the lowest proportion of the totl soil C in ll tillge (6.6%) nd forest systems (4.2%) on sndy soils, but the proportion incresed nd stbilised in the clyey soils (Tble 2.3). Although most studies of SOC re bsed on long term ssessments, the light frction C is thought to be n erly indictor of soil qulity improvement nd C sequestrtion becuse it is more sensitive to lnd-use nd mngement prctices thn totl SOC (Six et l., 2002). In support of this, Soon et l. (2007) showed light frction C significntly responded to tillge fter 4 yers, wheres the tillge effects on totl SOC ws not pprent until the 12 th yer. In the present study, I found tht the response of the light frction C storge ws not more sensitive thn the response of totl soil C. The clyey nd sndy soils re in res with long dry period chrcterised by slow decomposition resulting greter flf. The greter mounts of flf in sndy soils show importnce of flf in soils of limited sorption cpcity. The lck of bseline dt on SOC pools mkes it difficult to guge the effects of lnd mngement ctivities on SOC pools. However, if we consider the nturl forest (chpter 2) s the nturl stte, we observe tht conservtion tillge prctices nd fertility mendments increse SOC nd TON in MHF. Ripping nd direct seeding show n increse of 5% ech on sndy soils while they increse C by 5 nd 7% respectively in clyey soils when compred with conventionl tillge. 146

157 Chpter 6 Furthermore, DS on clyey soils hs difference of 1% with the djcent nturl forest while on sndy soils differences with nturl forest re up to 9%. The min wy of enhncing stble SOM pool is to increse litter incorportion nd humus formtion rtes (de Mores Sà & Séguy, 2008). The mximum level of C stbilistion under conventionl tillge is likely to be lower thn tht under RP nd DS becuse of fster decomposition nd lower soil ggregtion (Reicosky & Archer, 2007). The stbility of SOM is demonstrted when orgnic mtter resists further trnsformtion or degrdtion (Sollins et l., 1996). The MHF controlled the behviour of ll soils. The lck of significnt differences between CT, RP nd DS on clyey soils my be ttributed to reltively sufficient C nd N supply by the crops during the growing seson, the mounts of cly nd silt nd possible existence of Fe nd Al oxi-hydroxides (lthough these were not ssessed in the study). The cropping seson is chrcterised by rpid depletion of orgnic mtter likely to hve corresponding effects on the increse in MHF C nd N. The ccumultion of greter quntities of MHF C nd N in both sndy nd clyey soils could be result of the trnsformtion of frgmented litter into humus by erthworms nd termites. Once plnt residues enter the soil minerl horizon, they go through microbil degrdtion nd, re simultneously stbilised through interctions with soil minerl prticles. The mngement options tht increse SOC should increse productivity, profitbility nd promote sustinbility of lnd re. Although residue retention is criticl for the success of conservtion tillge prctices, locl institutionl rrngements during the long dry period re not fvourble (fields re communlly grzed by nimls nd left overs re consumed by termites nd other micro fun. For this reson residues re supplemented by grss t the beginning of the cropping seson to mintined the recommended Mg h -1. Furthermore, conservtion tillge prctices hve lower short term gins despite the insignificnt SOC stocks mong the tillge systems (especilly on clyey soils), conservtion tillge prctices hve lower short term gins in crop yields thn conventionl tillge (Thierfelder et l., 2012; Thierfelder & Wll, 2012) with better gins t three yers i.e. fter ccumultion of residues nd consequently SOC. The low production mkes it difficult for frmers to understnd the benefits of conservtion tillge prctices if ssocited with some loss in the initil yers. 147

158 Synthesis Smllholder frmers grow crops for subsistence nd csh income nd they my not be willing to loose production for reson. Therefore, to fcilitte fster doption of reduced nd no tillge prctices, there is need for subsidies in the initil yers to compenste frmers for opportunity costs foregone. The future is lso uncertin given the possibilities of climte chnge effects with no gurntee for good hrvest in the next seson. Incresed C stocks nd better crop yields mke conservtion tillge prctices superior only fter the initil yers. In sndy soils, better moisture retention nd better mngement mke DS more superior thn the other two prctices. The reduction in crop yields in initil yers under conservtion tillge my reduce the chnces of frmers shifting from conventionl to conservtion tillge. In order to mtch yields in conventionl tillge prctices, conservtion tillge prctices need more N fertiliser in the initil yers (Rusinmhodzi et l., 2011). Furthermore, since SOC is n environmentl good/service, there my be need for incentives, either in the form of direct government subsidies or credits from n emissions trding mrket to stimulte positive uptke of technologies tht increse SOC stocks. Such regultory mesures will prepre the ground for doption of other prctices which in the sme wy s CA cn be hmpered by unfvourble institutionl rrngements. In developing countries, the strong theoreticl bsis for SOC sequestrtion is prtilly supported by limited number of field studies. In ddition, generl lck of reserch in this re is currently preventing more quntittive ssessment of the potentil of soil C sequestrtion in both griculturl nd forest soils. Smllholder frmers cn only benefit from CA prctices in the presence of government support (e.g. for conservtion tillge equipment nd supportive policies) nd this will result in chievement of the gols of C sequestrtion, soil conservtion nd eventully better crop production (FAO, 2010). 6.3 Effects of fertilistion on SOC nd TON storge Poor soil fertility coupled with errtic rinfll constrin food production nd sustinbility of smll holder frming systems. The inherently infertile soils hve been identified to hve potentil for enhncing SOC sequestrtion with n 148

159 Chpter 6 estimted globl sequestrtion potentil of Gt C yr -1 with corresponding positive impcts on crop yields (Ll, 2004). There is however, need for continuous replenishment of the lbile C pools through prctices tht increse C inputs nd stbilise lbile C pools (e.g. the return of crop residues nd the ppliction of mnure nd fertilisers). Techniques tht hve been proposed for improving soil fertility include legume intercropping, green mnuring, groforestry (e.g. improved fllows), inorgnic nd orgnic fertilisers (Vnluwe et l., 2010). Addition of mnure nd inorgnic fertilisers hs often been recommended to increse SOM nd enhnce soil C sequestrtion (Gulde et l., 2008). Severl scholrs hve highlighted the benefits of combining mnure with inorgnic fertiliser for improving SOM nd its frctions (Rudrpp et l., 2006; Purkysth et l., 2008). Appliction of N fertiliser nd cttle mnure reduces depletion of SOC nd TON stocks in cropping systems (Chpter 3) nd show tht mendments reduce depletion of SOC stocks. Mnn (1986) showed tht conversion of ntive vegettion to croplnds hd impcts rnging from loss of 70% to gins of up to 200% in SOC stocks depending on soil type, fertilistion tillge system, cropping history nd smpling depth with the gretest chnges occurring in the first 20 yers. When conventionl tillge is the only vilble option, the ppliction of nitrogen fertiliser (chpter 3) is more beneficil for incresing C stock in sndy soils. Nitrogen fertilizer cn increse soil orgnic mtter in soils tht hve nitrogen deficiency but emissions from CO 2 relesed from fossil fuel combustion during the production nd trnsport cn reduce the net mount of crbon sequestered in fertilised systems. The fertilisers cn lso be lost through run off into nerby strems nd wter bodies where it my hve significnt negtive ecologicl effects. In some cses the use of N fertiliser without orgnic mendments leds to minerlistion of SOM nd loss of SOC (Mnley et l. (2002). This ws however true for clyey soils where the ppliction of mnure plus N fertiliser ws more fvourble thn N fertiliser lone. Fctors such s soil moisture nd soil texture re lso importnt in fcilitting chnges in micro climte resulting in more rpid minerlistion of C in soils tht hve high initil C stock (Mnn, 1986). Blnchrt et l. (2007) showed tht on sndy soils, fertiliser ppliction cuses n initil nitrogen flush erly in 149

160 Synthesis the cropping seson (bout 20 dys), with net N minerlistion ccompnied by net nitrifiction fvouring loss of N erly in the cropping seson (bout 20 dys). The N flush is ccompnied by incresed microbil biomss t the onset of the riny seson which is followed by rpid decy of orgnic mtter in soils tht hve limited cpcity to protect their orgnic mtter. The ccelerted decomposition during the cropping seson consequently results in more soil CO 2 releses into the tmosphere. However, incresed decomposition rtes my stimulte greter soil N vilbility, leding to higher net primry production (NPP) with potentil to increse C inputs into the soil through rhizodeposition nd litter fll thus offsetting incresed soil C loss (Gelmn et l., 2013). Soils might then gin more C thn they lose due to such fvourble environmentl conditions in N-rich environment. The higher N content might lso stimulte initil litter decomposition but lso suppresses humus decy in some stges, leding to stbilistion of SOM in minerl-ssocited frctions. Results support the hypothesis tht ppliction of N fertiliser increses N vilbility cusing incresed NPP nd subsequently litter production nd thus soil C inputs (Figure 3.2). Rudrpp et l. (2006) nd Brr et l. (2013) found considerble build-up of SOM frctions under fertiliser being gretest under combintion of mnure nd N fertiliser. Fvourble moisture nd tempertures during the cropping seson promotes greter minerlistion of flf orgnic mtter resulting in less ccumultion of LFC nd N in fertiliser nd mnure tretments on clyey soils. The flf C nd N is governed by the degree to which temperture nd moisture conditions constrin decomposition of ccumulted SOM (Biederbeck et l., 1994). The ddition of N fertiliser nd mnure cn significntly improve flf C t 0 10 cm nd t cm depths when compred with the control nd N fertiliser only. In clyey soils, the control tretment hd the lowest flf C t ll depths levels due to low productivity cpcity, inputs coming from better crop growth fter ddition of fertiliser nd mnure. The olf ws more under fertiliser t 0 10 nd cm in clyey soils while in sndy soils there ws no sttisticl difference t the three depths nd lso mong tretments. Incresed flf nd olf N under N fertiliser my be result of priming effect of fertiliser on fresh orgnic inputs, which stimultes microbil ctivity nd eventully decomposition. 150

161 Chpter 6 However, the use of both fertiliser nd cttle mnure my limit the number of frmers prticipting in crbon sequestrtion ctivities, minly becuse of the prohibitive costs of inorgnic fertilisers nd the lck of drught power. The poor will not be ble to contribute unless some enbling economic nd institutionl policy strtegies re designed. Existing crbon mrkets re likely to hve only modest impcts on the poor, even if reltively high crbon prices re offered. It is highly unlikely tht poor frmers will prticipte in crbon sequestrtion projects s they re likely to be constrined by the sme ttitudinl, economic nd institutionl fctors tht hve inhibited their socio-economic improvement in the pst. There ws strong positive liner reltionship between LF C nd MHF C on clyey soils wheres sndy soils hd no liner reltionship. It is possible tht flf nd olf in both sndy nd clyey soils my be exhusted nd need to be replced through continuous cover. The liner reltionship on clyey soils showed decresing intercept representing decline of lbile SOC with the remining levels being lrgely reclcitrnt C (Figure 3.5 nd Tble 3.3). The inherent infertility of outfields cused the soils to rech new low equilibrium minly in the control tretments wheres ppliction of N fertiliser on outfields indicted stte close to equilibrium level. The dded fertiliser cn fcilitte rpid decomposition of vilble SOM which is in turn tken up by the crops. There is lwys limit to sequestrtion potentil of soil, such tht once prticulr mount of C hs been sequestered, the soils hve limited cpcity to serve s C sinks. It is for this reson tht degrded soils nd ecosystems re thought to hve the highest potentil for C sequestrtion (Ll, 2004). Similrly, Powlson et l. (2012) nlysed dt from long term experiments t Rothmsted reserch in UK nd found tht soils under control tretment remined stble over the experimentl period. They ttributed this to the ttinment of new low equilibrium which is mintined by the cropping regimes. Soils with the sme properties could exhibit different levels of mximum C sequestrtion potentil even under similr mngement prctices. In such cses, the cpcity for soil C sequestrtion, vries considerbly mong sites on sme soil type due to initil levels of lbile SOC nd the bility of mngement prctices to stbilise greter mounts of orgnic inputs (Mnn, 1986). In support of this, Chn et l. (2008) showed tht soil C stocks my increse over periods of 151

162 Synthesis yers only until the equilibrium level is chieved nd where mngement prctices re not ltered but rther improved. Even under such conditions, there is finite limit to C sequestrtion potentil beyond which the soils re less ble to function s C sinks. The MHF is bsed on sorption of orgnic C to minerl surfces nd clyey soils hd more MHF thn LF. The greter MHF ws due to the protection of MHF cused by stronger sorption nd reduced desorption of orgnic C to the minerl surfces (Kögel-Knbner, et l., 2008). In this cse, the MHF becomes less biovilble s the flf becomes more vilble energy/nutrient source. 6.4 C:N rtios in different lnd mngement prctices Crbon to nitrogen rtio is n importnt indictor of soil qulity. Results showed sndy soils hving higher C:N rtio thn clyey soil (Chpter 2) under different tillge tretments. Fertility tretments using inorgnic fertiliser nd mnure showed different trend with clyey soils hving higher C:N rtios thn sndy soils. A similr trends ws reported by Zingore et l,. (2007) while reporting on the initil soil conditions of the Murew study re. Other studies hve reported higher C:N rtios in sndy soils thn lomy soils nd clyey soils. Highest C:N rtios were found in forest floor L lyer (Tble 4.1). The C:N rtios of the top 10 cm ws higher thn the lower depths in conventionlly tilled tretments thn reduced nd no tillge tretments. Tillge tretments hd significnt short term effects on C:N rtios wheres the nine yers of cultivtion under different fertility tretments did not cuse ny significnt differences in C:N rtios within ech soil type. The differences in C:N rtios by depth were not evident in fertility tretments (Chpter 3) which hd C:N rtios between 10 nd 23. Density seprtion studies hve shown higher C:N rtio in flf thn olf nd MHF with differences in C:N of up to seven units in tillge nd fertility tretments (Tble 2.2 nd 4.1). On verge, the C:N rtio of flf ws 24 (stndrd devition [SD] 4) nd tht of olf ws 21 (SD 3), indicting higher men nd vrince in flf. The MHF hd consistently lower C:N rtios (men 14, SD 3). Other studies, on griculturl sites (Poirier et l., 2005), clcreous forest soils (Rovir & Vllejo, 2003), lowlnd tropicl forest nd pstures Mrin-Spiott et l., 152

163 Chpter 6 (2008) showed distinct decline in C:N from flf to olf, in ccord with current models. The C:N rtios were however higher for the flf under N fertiliser which could be result of incresed N in microbil biomss. Higher C:N rtios in the flf re possibly result of existence of higher mounts of crbohydrtes (Dll & Henry, 1988; Golchin et l., 1994) minly in form of unprocessed plnt residues nd sugrs (Odes, 1972). The results support current models showing declining C:N rtio from flf to olf then MHF s orgnic C is oxidised by heterotrophic microbes during decomposition, whilst N is somewht preserved in the microbil biomss becuse of microbil N demnd (their biomss hs much lower C:N rtio thn plnt biomss). The C:N rtio of olf is intermedite between tht of MHF nd tht of flf. Results support tht MHF is dominted by microbilly processed compounds nd/or N-rich compounds, such s peptides, with little inclusion of high C:N plnt detritus (Chn et l., 2008). Pine plnttion soils tended to hve higher C:N rtios in flf thn griculturl soils nd nturl forests, perhps becuse the conifers contin source mterils with wider rnges of chemicl compositions thn griculturl soils (e.g. (Kölbl & Kögel -Knbner, 2004). In ddition to the preferentil degrdtion of C- rich, lbile substrtes, selective preservtion of high C:N compounds seems necessry to ccount for the unit higher C:N rtios in flf in these soils. 6.5 Potentil of plnttion forests to mitigte climte chnge Agriculture nd overgrzing cuse mjor losses of woody vegettion cover (IPCC, 2007) s people expnd cropping res to meet the demnd for food in ddition to their need for wood energy. As nturl forest shrink, there is need to conserve the remining forest res nd focus on fst growing plnttions species to relieve pressure through promotion of privte plnting of fst growing species minly on degrded res under fvourble climtic zones. In forest ecosystems, C storge is minly result of higher ccumultion of biomss in woody tissue corresponding to bove ground biomss production which is fster thn growth nd turnover rtes of the belowground biomss. Atmospheric C retined in plnt biomss increses the crbon reservoir of vegettion (Schulze, 2006). 153

164 Synthesis Forests nd woodlnds provide essentil mterils for locl consumption (wood fuel being the primry energy source), trde nd export. In ddition, they provide environmentl nd culturl services. Forests nd woodlnds re often importnt scred nd buril sites in Afric. The use of wood fuel is likely to continue in both rurl nd urbn res s it remins the most relible, ffordble nd ccessible source of energy for poor households. Furthermore, the frequent power cuts mke the use of woodfuel unvoidble. If the vlue of bove nd below ground C sequestrtion is dded to the bove vlues, the vlue of forests increses nd cn dequtely support the livelihoods of forest dependent communities. The benefits of C sequestrtion in forests is well documented nd their role in biodiversity conservtion is n dded dvntge. Therefore, investing in forest plnttions nd/or conservtion cn llow communities to benefit from crbon trding. In forest ecosystems, C is stored in biomss, forest floor nd soils. The forest floor lyer is minly composed of woody tissue (e.g. twigs, brk) leves, flowers, fruits, mosses, lichens nd fungi wheres below-ground inputs re from ded roots nd their ssocited mycorrhizl hyphe nd root exudtes. Some studies showed tht roots nd mycorrhiz were lso more importnt elements for the ccumultion of below ground SOM thn boveground lef litter. Exotic monocultures (including pine forests) exhibit superior bility in sequestering C bove nd below ground (Shn et l., 2001). The build- up of SOM in forested ecosystems depends on the mount of litter, its chemicl composition, the rtes nd mechnisms of decomposition, nd climtic fctors (Berg et l., 1995). The fte of ll forest floor mteril is eventully to become below-ground SOM input nd this cn be fcilitted by bioturbtion nd cultivtion. Although the living fine root biomss constitutes only smll frction of the totl stnd biomss, the contribution of fine roots to totl stnd biomss, nd totl soil C inputs cn be substntil. When the sme type of vegettion grows on the sme site for long time, n equilibrium stte is ttined through litter input nd its ssocited rte of decomposition. Ultimtely, n orgnic lyer tht is typicl of prticulr site is eventully formed (Evns et l., 2001). Wild fires re thret to forests nd woodlnds, cusing enormous destruction to both flor nd fun. However, fire plys n importnt role in 154

165 Chpter 6 determining the distribution nd composition of some vegettion types nd is responsible for the extensive occurrence of grsslnds in Southern Afric. In tropicl svnnhs (woodlnds, bush lnds nd grsslnds) fire is common tool used to convert forestlnd to griculture nd forest plnttions. During the burning, C is lost from the forest floor, wheres losses from the minerl soil lyers depends on fire intensity, frequency nd thickness of litter lyers. In ll cses, fire cn become source of CO 2 emission to the tmosphere nd lter net ssimiltion rtes of the stnding vegettion cusing decrese in the supply of orgnic mtter nd root-derived substrtes to the soil (Duguy et l., 2007). Not only is CO 2 lost but other soil nutrients re lso either voltilised or trnsformed into shes resulting in net loss of nutrients through wind blow, erosion or leching. In other nturl systems, fires of low intensity re used to consume the under storey nd prt of the forest floor lyers to prevent fuel build-up (Ferrn et l., 2005). The fire events consume ll forest floor mteril the severity of which depends on the intensity of the fire. High intensity fires cn extend deeper into the minerl soil lyers (Duguy et l., 2007) cusing some ltertion of soil physicochemicl properties (Nery et l., 1999) nd ffecting soil nutrient fluxes nd consequently C stocks. During nd fter fire, in the short term soil fertility usully increses due to higher nitrogen nd phosphorous vilbility in the soil solution. Incresed ph nd concentrtion of bse ctions temporrily enhnce soil respirtion cusing higher nitrogen minerliztion nd nitrifiction. The processes cuse some res s in the one yer old stnd not to show drstic decline in soil C nd N stocks (Chpter 4). The mgnitude of the nutrient flush depends on the temperture nd durtion of the fire nd the mount of orgnic mtter burned. In the 0 60 cm depth, the flf, olf nd MHF C nd N were significntly lower in the miombo woodlnd nd pine stnds thn in the moist forest. Generlly, LF, olf nd MHF C nd N stocks declined consistently with incresing soil depth. Significnt differences in LF C, olf C nd MHF C were between the moist forest nd the one yer old pine stnd t 0 20 cm depth, especilly the top 10 cm, wheres there ws less chnge below 30 cm, indicting tht lbile frction losses due to forest trnsition minly occurred in the surfce soils. This suggests tht conversion of miombo woodlnds nd moist forest to pine plnttions 155

166 Synthesis significntly reduces flf C, which my be ttributed to combintion of fctors including quntity of litter mterils, microbil ctivity nd mngement disturbnces, which would chnge gretly with the forest conversion. Results suggest tht net C storge is likely to occur in forested lnd nd the ultimte fte of the wood products determines the finl ssessment of C sequestrtion potentil. For exmple, if the rottion ge is extended or if the forest is not hrvested, the C is sequestered for longer periods of time. Sequestrtion cn lso be long lived if trees re processed to swn timber nd used in construction of houses nd these cn lst for decdes. On the other hnd, sequestrtion is short lived if trees re burned for firewood, or processed into pper. Tree residues cn lso be recycled into orgnic mendments for use in cropping systems. 6.6 Modelling soil C stocks in Zimbbwen gro- ecosystems Future impct of lnd mngement prctices cn effectively be predicted by the use of observtions in combintion with models providing mens of evluting the chnging prctices in the future. In the RothC soil C model decomposition processes re represented in three rbitrry SOM pools, which only vguely relte to mesurble SOC frction, with specific turnover rtes nd n dditionl cly decomposition modifiction fctor. Simultion modelling using the RothC model, showed good greement between the simulted nd observed SOC for 2010 in ll tretments on sndy soils except the nturl forest where the C ws underestimted by 30%. On clyey soils, fertility tretments showed better greement with smll devitions thn tillge tretments nd nturl forest. Highest mesured SOC ws found in nturl forests on clyey soils whilst lowest ws in control tretments of both tillge nd fertility tretments. There my be mny weknesses nd limittions of SOM models, since most were prmeterised under prticulr mngement or climtic regions. Idelly SOM is ffected by complex interctions which models do not ccount for such s prent mteril, ph, time, litter qulity nd biot mngement. Despite this shortfll, the modelling pproch gives guideline of C dynmics under prticulr mngement system. The filure of RothC to ccount for some of these fctors 156

167 Chpter 6 my ffect the ccurcy of predictions. Furthermore, the theoreticl comprtments/pools of RothC model re difficult to mtch with mesurble pools mking it difficult to initilize the models nd vlidte model-clculted results for the individul pools (Kutsch et l., 2009). Despite this disprity, there ws good reltionship between mesured minerl ssocited hevy frction nd modelled HUM + IOM wheres the LF hd wek reltionship with RPM. Under dry lnd conditions on the sndy soils of West-Afric, simultion model predictions using the CENTURY nd RothC models suggested tht conversion to no-tillge will result in smll increses in soil C contents ( t h -1 yer -1 ) (Frge et l., 2007). Only prctices tht entiled n incresed input of orgnic mtter, for exmple through groforestry or mnure, were predicted to result in lrge increses in soil C. Although it is often difficult to seprte the effects, increses in modelled SOC re probbly due to incresed biomss production nd retention in conservtion tillge systems (RP nd DS) rther thn reduced or no-tillge supporting the rguments of Corbeels et l. (2006). 6.7 Conclusion Any technology or prctice tht increses the photosynthetic input of crbon nd/or slows the return of stored crbon to CO 2 through respirtion, erosion or fire hs potentil to store crbon nd cn be potentil C sink. Significnt mounts of SOC cn be stored in this wy, through rnge of prctices, depending on locl environmentl conditions. Substntil mounts of vegettive C cn lso be stored in perennil plntings including gro-forestry systems. The use of conservtion tillge, nd use of mnure nd nitrogen fertiliser contribute to improving soil C sttus in cropping systems lthough the benefits of N fertiliser cn offset by higher N 2 O emissions from soils nd CO 2 from production of fertiliser. Results of this study indicte tht C storge vries with tillge nd fertility mendments rnging from 15.3 to 32.0 nd Mg h -1 on clyey nd sndy soil respectively. The difference in SOC content between ntive forests nd croplnds (50%) on sndy soils ws less thn the difference between ntive forests nd croplnds in clyey soils (16%). Although sndy soils showed weker greenhouse gs mitigtion potentil (shown by low potentil of C storge) when 157

168 Synthesis compred with clyey soils, results suggest tht the no tillge system (DS) is the best mngement for C storge on sndy soil whilst the conventionl tillge (CT) system is the worst. When conventionl tillge is the only vilble option, the ppliction of nitrogen fertiliser (Chpter 2) is more beneficil for incresing C nd N stock. Significnt differences in C nd N storge in plnttion stnds were shown between the different ge clsses nd nturl forest stnds t 0 10 cm nd cm. However, when smpling deeper in the profile (below 30 cm), differences were not evident becuse of the incresing mgnitude of rndom vrition ssocited with greter soil depth. Seprtion of SOM into discrete frctions hs been successfully used to isolte chnges in the structure nd function of SOM pools in response to lnd mngement ctivities. The ppliction of density frctiontion hs been limited in tropicl soils of Afric. Three distinct SOM pools (free light frction (flf), occluded light frction (olf) nd minerl ssocited hevy frction (MHF)) were isolted by density frctiontion procedure for sndy nd clyey soils in smllholder frming systems nd on Pinus ptul ge series. In ll tretments the stocks of C nd N were in the order olf<flf<mhf. Ech frctionl component ws higher under RP nd DS thn CT nd higher under N Fert + mnure thn N fertiliser nd control on clyey soil except olf which ws higher under control thn mnure. On sndy soils the three frctions were higher under DS thn RP nd CT nd higher under N fertiliser thn N Fert + mnure nd control. The flf contributed less to totl soil mss, hs high C concentrtion nd wider C:N rtios reltive to the olf nd MHF, chrcteristics consistent with generl trends in other findings. In conceptul models of SOM pools, the MHF is ssumed to be more stble pool. Results show tht, in cropping systems, plnted forests nd nturl forests, soils cn be isolted into distinct SOM frctions tht re distinguished by their contributions to totl soil mss, C nd N storge. The flf is of recent origin, while the MHF is more reclcitrnt pool of SOM nd the olf is intermedite. Reltive to the flf, the olf ws the smllest frction with respect to totl soil mss nd hving higher C:N rtios thn MHF but lso hving high C nd N concentrtions. This shows the extent of physicl protection within ggregtes nd suggests tht stbility is function of the 158

169 Chpter 6 position of SOC within the soil mtrix. If well protected, the olf is less ltered by microbil processing when compred to the flf. We cn infer from these ptterns tht mostly, the MHF nd to lesser degree, the flf nd olf ll drive soil properties in the upper soil lyers becuse of their contribution to soil mss nd C concentrtions, wheres t lower depths, the bulk soil ws most similr to the dominting MHF. The verticl distribution of flf nd olf showed similr depth trends in soil mss, C concentrtion, C content, nd C:N rtios. The C nd N concentrtion nd contribution to totl soil mss decresed with incresing soil depth wheres there ws no generl depth trend in the C:N rtio. Distribution of flf nd olf ws consistent with their conceptul nd functionl roles of being reltively lbile pools of SOM tht re free nd occluded within the soil mtrix nd tht re poorly decomposed. While the flf nd olf re criticlly importnt in the context of orgnic mtter cycling nd site productivity, their contribution to totl C nd N storge is smll (verge 6%). These pools re lso much esier to mnipulte using selected mngement prctices thn the MHF. It is much esier to lter the rte nd nture of inputs nd controls on outputs in the upper soil lyers thn mnipulting deeper soil lyers. Cses where there is less SOC nd TON in light frctions, suggest tht the light frction inputs were trnsformed rpidly into MHF leding to more physiclly protected C nd N in MHF. Although flf nd olf re often more sensitive to lnd mngement prctices thn TOC, in some soils there my be no significnt difference between MHF due to ccruls in MHF (Jstrow, 1996) resulting in rpid response of MHF to mngement. In the 1, 10 nd 25 yer old plnttion stnds, the C:N rtio decresed with depth, while in the 20 nd 30 yer old stnds, the C:N rtio decresed in both the free nd occluded light frctions. Fctors such s the preferentil leching nd desorption of C from deep soil long with other processes my lso ply role nd ccount for some of the vribility in C nd N over the ge sequence. Differences in the deeper minerl soil lyers, in prticulr the cm depth intervl, re minly driven by the MHF frction. The MHF ccounted for over 98% of the soil mss t 0-30 cm nd over 85% of SOC below 30 cm. This shows tht the contribution of the MHF to totl C nd N storge lso occurs 159

170 Synthesis below 20 cm nd yet depths beyond 20 cm re often overlooked in most studies exmining chnges in C nd N storge in response to lnd mngement. There is lso need to obtin more dt on long-term effects of different tillge systems on C nd N dynmics for vrious gro ctivities in order to gin full understnding of the C nd N cycling. Understnding soil qulity cn help in the design of crop nd soil systems for griculturl sustinbility nd should focus on further investigtion under more holistic pproch. Chrcol nd wood fuel use, logging, poor griculturl nd lnd use prctices will ultimtely, continue to threten forests unless lterntive energy sources re utilised, lterntive livelihoods re sought, nd sustinble griculturl methods re employed on frmlnds. Urbn nd peri-urbn energy demnd increses fuelwood nd chrcol prices nd demnd eventully leding to deforesttion unless lterntive energy supply is provided. Finlly, the discussions in his thesis suggest tht cultivtion, tree plnting nd forest conservtion for crbon sequestrtion cn become profitble if hrmonised with other services for soil conservtion, wter qulity, wildlife hbitt, biodiversity nd the environment. Although hectre of forest my sequester more C thn hectre of crop lnd, tree plnttions cnnot be mrkedly expnded on croplnd or woodlnds without generting some costs to society. Opportunities for expnding crbon sequestrtion in tree plnttions exists in mrginl lnds where rinfll is dequte. In Afric, REDD+ pilot projects hve demonstrted tht climte chnge mitigtion through forest crbon pyments cn enhnce the incomes of the rurl poor, s well s incresing opportunities for dpttion nd growth. Generlly, pyments for REDD+ nd other ecosystem services hve gret potentil in light of the diversity of schemes tht re likely to emerge nd the diversity of services likely to be obtined including potentil positive impct on the environment. REDD+ hs bility to sve public nd privte sector funds by promoting diversity of benefits, improving people s livelihoods nd hving potentil to reduce conflicts. Since C sequestrtion is best viewed s prt of environmentl benefits or costs, ctivities normlly ccrue to society s externlities. The focus on the monetry vlution nd pyment for environmentl services cn contribute to the ttrction of politicl support for soil conservtion. Developing countries therefore, need to 160

171 Chpter 6 formulte enbling economic nd institutionl lnd mngement policies tht hve positive impcts on poverty llevition, food security nd environmentl sustinbility. The current inititives for GHG mitigtion show the need for synergy between sustinble development, climte chnge nd environmentl integrity s mens for relistion of the mitigtion potentil of the griculturl sector. 161

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200 References Vlek, P. L. G., Fillery, I. R. P. & Burford, J. R Accession, trnsformtion, nd loss of nitrogen in soils of the rid region. 58, von Lützow, M., Kögel-Knbner, I., Ekschmitt, K., Fless, H., Guggenberger, G., Mtzner, E. & Mrschner, B SOM frctiontion methods: Relevnce to functionl pools nd to stbiliztion mechnisms. Soil Biology nd Biochemistry, 39, von Lützow, M., Kögel-Knbner, I., Ludwig, B., Mtzner, E., Fless, H., Ekschmitt, K., Guggenberger, G., Mrschner, B. & Klbitz, K Stbiliztion mechnisms of orgnic mtter in four temperte soils: Development nd ppliction of conceptul model. Journl of Plnt nutrition nd Soil Science, 171, Von Lützow, M. & Kӧgel-Knbner, I Temperture sensitivity of soil orgnic mtter decomposition- wht do we know? Biology nd Fertility of Soils, 46, von Lützow, M., Kӧgel-Knbner, I., Ekschmitt, K., Mtzner, E., Guggenberger, G., Mrschner, B. & Fless, H Stbiliztion of orgnic mtter in temperte soils: mechnisms nd their relevnce under different soil conditions review Europen Journl of Soil Science, 57, von Lützow, M., Leifeld, J., Kinz, M., Kögel-Knbner, I. & Munch, J. C Indictions for soil orgnic mtter qulity in soils under different mngement. Geoderm, 105, Wlker, S. M. & Desnker, P. V The impct of lnd use on soil crbon in Miombo Woodlnds of Mlwi. Forest Ecology nd Mngement, 203, Wllce, E. S. & Freedmn, B Forest floor dynmics in chronosequence of hrdwood stnds in centrl Nov Scoti. Cndin Journl of Forest Reserch, 16, WATCO Overviwe nn Mngement Pln of the Pine Esttes. Wttle Compny Limited. Wendling, B., Jucksch, I., Mendonç, E. S. & Alvreng, R. C 'Orgnic-Mtter Pools of Soil under Pines nd Annul Cultures',. Communictions in Soil Science nd Plnt Anlysis, 41: Whlen, J. K. & Chng, C Mcroggregte chrcteristics in cultivted soils fter 25 nnul mnure pplictions Soil Science Society of Americ Journl, 66, White, R. E Principles nd Prctice of Soil Science: The Soil s Nturl Resource, 4th Edition, Blckwell Science Ltd., Cornwll, Gret Britin. pp

201 References Willims, M., Ryn, C. M., Rees, R. M., Smbne, E., Fernndo, J. & Grce, J Crbon sequestrtion nd biodiversity of re-growing miombo woodlnds in Mozmbique Forest Ecology nd Mngement, 254, Woomer, P. L The impct of cultivtion on crbon fluxes in woody svnns of Southern Afric. Wter, Air, nd Soil Pollution, 70, World Bnk World Development Indictors. Wshington DC: The World Bnk. ext/ddpqq/member.do?method=getmembers&userid=1&queryid=137.. World Bnk Crbon sequestrtion in griculturl soils. Report No GLB. World Fctbook Zimbbwe. Wu, T. Y., Schoenu, J. J., Li, F. M., Qin, P. Y., Mlhi, S. S., Shi, C. & Xue, F. L Influence of cultivtion nd fertilistion on totl orgnic crbon frctions in soils from Loess Plteu of Chin. Soil nd Tillge Reserch, 77, Xu, M., Lou, Y., Sun, X., Wng, W., Bniymuddin, M. & Zho, K Soil orgnic crbon ctive frctions s erly indictors for totl crbon chnge under strw incorportion. Biology nd Fertility of Soils, 47, Yni, R. D., Currie, W. S. & Goodle, C. L Soil crbon dynmics fter forest hrvest: An ecosystem prdigm reconsidered. Ecosystems, 6, Yng, Y., J. Guo, J., Chen, G., Yin, Y., Go, R. & Lin, C Effects of forest conversion on soil lbile orgnic crbon frctions nd ggregte stbility in subtropicl Chin. Plnt Soil. DOI /s Yng, Y. & Luo, Y Crbon:nitrogen stoichiometry in forest ecosystems during stnd development. Globl Ecology nd Biogeogrphy, 20, Yin, Y. & Ci, Z Equilibrium of orgnic mtter in hevy frction for three long term experimentl field soils in Chin. Pedosphere, 16 (2), Yin, Y. F., Ci, Z. C. & Lu, J. L Reltionship between light nd hevy frctions of orgnic mtter for severl griculturl soils in Chin. Journl of Environmentl Sciences, 17, Yokozw, M., Shirto, Y., Skmoto, T., Yonemur, S., Nki, M. & Ohkur, T Use of the RothC model to estimte the crbon sequestrtion potentil of orgnic mtter ppliction in Jpnese rble soils. Soil Science nd Plnt Nutrition, 56, Yoo, G. & Wnder, M. M Tillge effects on ggregte turnover nd sequestrtion of prticulte nd humified soil orgnic crbon. Soil Science Society of Americ Journl, 72,

202 References Zk, D. R., Tilmn, D., prmenter, R. R., Rice, C. W., Fisher, F. M., Vose, V., Milchuns, D. & Mrtin, W. C Plnt production nd soil microorgnisms in lte successionl ecosystems: continentl -scle study. Ecology, 75, Zhng, P., Zhng, T. & Chen, N. L Verticl distribution ptterns of soil orgnic crbon nd totl nitrogen nd relted ffecting fctors long northern slope of Qilin Mountins. Chinese Journl of Applied Ecology, 20, Zimbbwe s Fourth Ntionl Report to the Convention on Biologicl Diversity Ministry of Environment & Nturl Resources Mngement. Republic of Zimbbwe. Zingore, S., Mnyme, C., Nymugft, P. & Giller, K. E Long-term chnges in orgnic mtter of woodlnd soils clered for rble cropping in Zimbbwe. Europen Journl of Soil Science, 56, Zingore, S., Murwir, H. K., Delve, R. J. & Giller, K. E Soil type, mngement history nd current resource lloction: Three dimensions regulting vribility in crop productivity on Africn smllholder frms Field Crops Reserch, 101, Zingore, S., Murwir, H. K., Delve, R. J. & Giller, K. E. 2007b. Influence of nutrient mngement strtegies on vribility of soil fertility, crop yields nd nutrient blnces on smllholder frms in Zimbbwe Agriculture Ecosystems nd Environment, 119, Zingore, S., Nymngr, J., Delve, R. J. & Giller, K. E Multiple benefits of mnure: The key to mintennce of soil fertility nd restortion of depleted sndy soils on Africn smllholder frms grdients on Africn smllholder frms. Nutrient Cycling in Agroecosystems, 80, Zotrelli, L., Alves, B. J. R., Urquig, S., Torres, E., Sntos, H. P., Pustin, K., Boddey, R. M. & Six, J Impct of tillge nd crop rottion on ggregte-ssocited crbon in two Oxisols Soil nd Tillge Reserch, 95,

203 Summry SUMMARY Climte chnge dversely ffects humn livelihoods nd the environment through ltertion of tempertures, rinfll ptterns, se level rise nd ecosystem productivity. Developing countries re more vulnerble to climte chnge becuse they directly depend on griculture nd nturl ecosystem products for their livelihoods. Mitigtion of climte chnge impcts includes prctices tht cn store crbon (C) in soil nd biomss thus, reducing concentrtions of tmospheric crbon dioxide (CO 2 ) nd other greenhouse gsses. In ddition, plnted nd nturl forests tht store lrge mounts of C, cn become key resources for mitigting nd reducing vulnerbility to climte chnge, whilst infertile griculturl soils require lrge mounts of chemicl nd/or orgnic fertilisers to improve productivity. Incresing wreness bout climte chnge mitigtion hs led to relistion of need for sustinble lnd mngement prctices nd promoting soil C sequestrtion to reduce the greenhouse effects. The C storge potentil of griculturl soils is compounded by conventionl tillge prctices, covering lrge res with only smll portions of fields dedicted to conservtion frming prctices. Mintining soil nd crop productivity under these griculturl systems becomes mjor chllenge especilly in rin-fed rid nd semi-rid regions, chrcterised by long nnul dry spells. Conservtion tillge prctices, such s no-till nd reduced tillge, hve been reported to increse soil orgnic crbon (SOC) stocks in griculturl systems s they reduce soil disturbnce, wheres conventionl tillge hs been criticised for cusing soil C losses, ccelerting soil erosion nd displcing of soil nutrients, despite benefits, such s reduced soil compction, weed control nd preprtion of fvourble seedbed, which hve been reported under conventionl tillge. The identifiction of pproprite griculturl mngement prctices is criticl for relistion of the benefits of Soil C sequestrtion nd reducing emissions from griculturl ctivities. This thesis ws plnned to improve our understnding on how tillge, fertilistion, tree plnting or nturl forest conservtion cn enhnce C 193

204 Summry sequestrtion nd thus mitigte climte chnge. The min gol ws to quntify the influence of tillge, fertilistion nd plnttion forestry prctices on C nd N dynmics in bulk soil nd density seprted soil orgnic mtter (SOM) frctions reltive nturl forest. Tillge tretments under reduced tillge (RP), no tillge (DS) nd conventionl tillge (CT) were compred with nturl forests (NF) in sndy Hplic Arenosols nd clyey Rhodic Ferrlsols. Impcts of fertilistion were ssessed from three fertility tretments; unfertilised control (control), nitrogen fertiliser (N Fert) nd nitrogen fertiliser plus cttle mnure (N Fert + mnure) in conventionlly tilled fields on Arenosols (sndy soil) nd Luvisols (clyey soil) long two soil fertility grdients. Similrly, C nd N storge in tree frming ws studied using Pinus ptul chronosequence. Soil smpling followed rndomised complete block design with four replictions in griculturl systems nd two replictes in ech plnttion ge stnds nd nturl forest. Sodium polytungstte (density 1.6 g cm -3 ) ws used to isolte orgnic mtter into free light frction (flf), occluded light frction (olf) nd minerl ssocited hevy frction (MHF). Crbon n N were nlysed by dry combustion nd C nd N stocks clculted using bulk density, depth nd C nd N concentrtion. The RothC model ws used to mtch density seprted frctions with conceptul model pools for griculturl nd nturl forest soils. Findings from tillge studies showed significntly lrger C nd N stocks in nturl forests thn tillge systems despite the open ccess use of the nturl forests. The C nd N stocks were significntly lower in sndy thn clyey soils. At 0 10 cm depth, SOC stocks incresed under CT, RP nd DS by 0.10, 0.24, 0.36 Mg h 1 yr 1 nd 0.76, 0.54, 0.10 Mg h 1 yr 1 on sndy nd clyey soils respectively over four yer period while N stocks decresed by 0.55, 0.40, 0.56 Mg h 1 yr 1 nd 0.63, 0.65, 0.55 Mg h 1 yr 1 respectively. Under previling climtic nd mngement conditions, improvement of residue retention could be mjor fctor tht cn distinguish the potentil of different mngement prctices for C sequestrtion. Among the fertility tretments, there were significntly higher SOC nd TON stocks under N Fert nd N Fert + mnure t 0-10 cm soil depth in Luvisols. Although this effect ws not significnt t cm nd cm depth. On Arenosols, N Fert hd highest C nd N t ll depths except t 0-10 cm. The storge 194

205 Summry of C nd N on Luvisols, followed: control < N Fert < N Fert + mnure wheres Arenosols hd control < N Fert + mnure < N Fert. Compred with control, N Fert nd N Fert + mnure enhnced flf C on homefields nd outfields by 19%, 24% nd 9%, 22% on Luvisols nd 17%, 26% nd 26%, 26% respectively on Arenosols. Homefields on Luvisols, under N Fert nd N Fert + mnure hd similr equilibrium levels, which were 2.5 times more thn control. Forests ply mjor role in regulting the rte of increse of globl tmospheric CO 2 storing C in soil nd biomss lthough the C storge potentil vries with forest type nd plnt species composition. In this reserch, storge of C nd N were highest in moist forest nd lowest in the Miombo woodlnd. In both nturl nd plnted forests, bove ground tree biomss ws the mjor ecosystem C pool followed by forest floor s humus (H) lyer. The minerl soil hd 45%, 31% nd 24% of SOC stored t the 0 10, nd cm soil depths respectively. Stnd ge ffected C nd N storge significntly hving n initil decline fter estblishment recovering rpidly up to 10 yers, fter which it declined nd incresed gin by 25 yers. Averge soil C mong the Pinus comprtments ws 12 kg m -2, being highest t 10 yers nd lowest in the 1 yer old stnds. Orgnic N ws lso highest t 10 yers nd lest t 25 yers. The proportionl mss of flf nd olf in Miombo woodlnds ws similr while the other stnds hd higher flf thn olf. The highest LF ws in the moist forest. In the Pinus ptul stnds the flf C contributed between 22 25, the olf C contributed 8 16 nd MHF C contributed between to totl SOC. Crbon in MHF nd olf incresed with depth while the flf decresed with depth in ll except the 1 nd the 10 yer old stnds. Conversion of depleted Miombo woodlnds to pine plnttions cn yield better C gins in the short nd long run whilst moist forests provide both crbon nd biodiversity. Where possible moist forests should be conserved nd enrichment plnting done in degrded res to sustin them nd if possible the forests cn be considered s prt of future projects on reduced emission from deforesttion nd degrdtion (REDD+). It is believed tht REDD+ cn promote both conservtion nd socio economic welfre, including poverty llevition by bringing together the development of the forest nd climte chnge link in Africn forests nd woodlnds. The focus on the monetry vlution nd pyment for environmentl services cn contribute to the ttrction of politicl support for soil 195

206 Summry conservtion. Developing countries therefore, need to formulte enbling economic nd institutionl lnd mngement policies tht hve positive impcts on poverty llevition, food security nd environmentl sustinbility. Soil C models re used to predict impcts of lnd mngement on C storge. The RothC 2.63 model ws used for estimting SOC stock under selected lnd mngement prctices on the clyey nd sndy soils of Zimbbwe. There is greter potentil to store more C in clyey soils thn sndy soils nd in prctices tht receive more orgnic inputs. Results show tht the RothC model pool of HUM + IOM is relted to the mesured MHF from density frctiontion nd tht the model cn be used to estimte SOC stock chnges on Zimbbwen griculturl nd forest soils. The reltionship between equilibrium levels estimted by the RothC model nd those estimted using the Lngmuir eqution ws good. A 1.5 C rise in temperture ws found to cuse the A nd B systems on clyey soils to sequester more C. The results lso show tht, when holding ll the other fctors constnt, the model is sufficiently sensitive to rise in tempertures with sndy soils reching n equilibrium much erlier thn clyey soils. The modelling pproch represents one of the most promising methods for the estimtion of SOC stock chnges nd llowed us to evlute the chnges in SOC in the pst period on the bsis of mesured dt. However, since the dt were obtined from short term experiments (4 9 yers), further ground vlidtion cn be hmpered by the lck of long-term experimentl trils in the southern Africn region. The deficiency of dequte experimentl sites lso limits further work on model uncertinties. The understnding soil qulity nd dynmics however, helps to design sustinble griculturl systems, while chieving the urgently needed win-win sitution in enhncing productivity nd sequestering C. 196

207 Smenvtting SAMENVATTING Klimtverndering heeft een negtief effect op het welzijn vn mensen en op het milieu door vernderingen vn tempertuur, neerslgptronen, zeespiegelstijging en ecosysteem productiviteit. Ontwikkelingslnden zijn kwetsbrder voor klimtverndering omdt ze meer fhnkelijk zijn vn lndbouw en ntuurlijke ecosysteem producten voor levensonderhoud. Tegengn vn de gevolgen vn klimtverndering omvtten mtregelen zols het opsln vn koolstof (C) in biomss en bodem wrmee de concentrties vn tmosferische CO 2 en ndere broeiksgssen verlgd worden. Bovendien, plntges en bossen die grote hoeveelheden C opsln kunnen belngrijke mogelijkheden bieden om de kwetsbrheid door klimtverndering te verminderen. Drnst vereist onvruchtbre lndbouwgrond grote hoeveelheden norgnische of orgnische meststoffen om de productiviteit te verbeteren. Een toenemende bewustwording m.b.t. klimtverndering heeft geleid tot de relistie dt er behoefte is n duurzme lnd mngementprktijken en de bevordering vn opslg vn C in de bodem om de effecten vn broeiksgssen te verminderen. Het C-opslg potentieel vn lndbouwgrond is fgenomen door conventionele grondbewerkingsmethoden die betrekking hebben op grote gebieden. Alleen op kleine relen worden duurzme lndbouwmethoden toegepst. Behoud vn bodem en gews productiviteit onder conventionele lndbouwsystemen wordt een grote uitdging met nme in de regen-gevoede droge gebieden die gekenmerkt worden door lnge jrlijkse droge perioden. Duurzme grondbewerkingsmethoden, zols zonder-ploegen en gereduceerde bodembewerking kunnen de bodem orgnische koolstof (SOC) voorrden verhogen in omdt ze de verstoring vn de bodem verminderen. Conventionele grondbewerking wordt bekritiseerd door het veroorzken vn bodem C verliezen, het versnellen vn de bodemerosie en het verlies vn nutriënten ondnks de voordelen zols verminderde bodemverdichting, onkruidbestrijding en toenme vn nutriëntbeschikbrheid n het begin vn het groeiseizoen. De vststelling vn goede lndbouwmethoden is crucil voor de verwezenlijking vn de 197

208 Smenvtting voordelen vn de bodem C opslg en drmee de vermindering vn emissies ls gevolg vn lndbouwctiviteiten. Dit proefschrift heeft ls doel om onze kennis te verbeteren m.b.t. hoe grondbewerking, bemesting, het plnten vn bomen en bosmngement kunnen bijdrgen n de verbetering vn C opslg in de bodem. Het voornmste doel ws het kwntificeren vn de invloed vn grondbewerking, bemesting en plntge bosbouwprktijken op C en N dynmiek in de bodem en op fzonderlijke bodem orgnische stof frcties. Verschillende grondbewerkingsmethoden, zols verminderde grondbewerking (RP), geen grondbewerking (DS) en conventionele grondbewerking (CT) werden vergeleken met ntuurlijke bossen (NF) in Hplic Arenosols en Rhodic Ferrlsols. Effecten vn bemesting werden beoordeeld door middel vn drie behndelingen; onbemest (controle), stikstof toediening (N Fert) en stikstof toediening plus orgnische (vee) mest (N Fert + mest) in conventioneel bewerkte velden op Arenosols (zndige grond) en Luvisols (kleichtige bodem) lngs twee bodemvruchtbrheidsgrdiënten. Koolstof en N opslg in plntges werd ook bestudeerd met behulp vn Pinus ptul opstnden met oplopende ouderdom. Grondmonsternme ws gebseerd op een gerndomiseerde volledig blokontwerp met vier herhlingen in de lndbouwsystemen en twee herhlingen per plntge opstndleeftijd en in de oorspronkelijke bossen. Ntrium polytungstte (dichtheid 1.6 g cm -3 ) werd gebruikt voor het isoleren vn bodem orgnisch mteril in de vrije lichte frctie (flf), beschermde lichte frctie (olf) en de minerlgebonden frctie (MHF). Koolstof en N werden genlyseerd door droge verbrnding in een CN nlyser en C en N voorrden berekend n de hnd vn bodemdichtheid, diepte en C en N concentrtie. Het RothC model werd gebruikt om gemeten bodem orgnische stoffrcties te vergelijken met conceptuele bodemkoolstofvoorrden in lndbouw- en bosbodems. Bevindingen uit grondbewerkingsstudies lieten nzienlijk grotere C en N voorrden zien in ntuurlijke bossen dn in lndbouwsystemen ondnks de open toegng en het gebruik vn ntuurlijke bossen. De C en N voorrden wren beduidend lger in zndige dn in kleiige bodem. Op 0 10 cm (diepte), SOC voorrden stegen onder CT, RP en DS met 0.10, 0.24, 0,36 Mg h 1 yr 1 en 0.76, 0.54, 0,10 Mg h 1 yr 1 op zndige en kleiige bodems respectievelijk over een periode vn vier jr terwijl N voorrden respectievelijk dlden met 0,55, 0.40, 198

209 Smenvtting 0.56 Mg h 1 yr en 0,65, 0.55 Mg h 1 yr 1. Onder het huidige klimt en bodembeheer zou verbetering vn het behoud vn gewsresten een belngrijke fctor zijn die het potentieel vn verschillende C-opslgmethoden zou kunnen verbeteren. Bemestingsproeven liet nzienlijk hogere SOC en TON voorrden zien onder N Fert en N Fert + mest op 0-10 cm (diepte) in Luvisols. Hoewel dit effect niet significnt ws op cm en cm (diepte). In Arenosols, N Fert gf de hoogste C en N wrden op lle diepten behlve op 0-10 cm. De opslg vn C en N in Luvisols volgde vn lg nr hoog: control < N Fert < N Fert + mest. Terwijl in Arenosols de volgorde ws: controle < N Fert + mest < N Fert. In vergelijking met controle, N Fert en N Fert + mest vergrootte flf C op homefields en outfields met 19%, 24% en 9%, 22% op Luvisols en 17%, 26% en 26%, 26% respectievelijk op Arenosols. Homefields op Luvisols, onder N Fert en N Fert + mest hdden vergelijkbre FLF niveus die 2,5 keer groter wren dn in de controle velden. Bossen spelen een belngrijke rol bij het reguleren vn de stijging vn mondile tmosferische CO 2 door het opsln vn C in bodem en biomss, hoewel het C-opslg potentieel per bos type vrieert. In dit onderzoek, opslg vn C en N wren het hoogst in vochtige bossen en het lgste in het Miombo-bos. In zowel ntuurlijke ls ngeplnte bossen, de bovengrondse biomss bevtte de grootste C voorrd gevolgd door de humus (H) lg vn de bosbodem. De minerle bodem hd 45%, 31% en 24% SOC opgeslgen op 0 10, en cm bodemdiepte. Bosopstnd leeftijd beïnvloed nzienlijk. N een nvnkelijke dling n inititie volgde een toenme vn C en N opslg rond de leeftijd vn 10 jr met een verdere stijging tot 25 jr. Gemiddelde bodem C onder de Pinus percelen ws 12 kg m -2 n tien jr met de lgste C wrden n 1 jr. Orgnische N ws ook het hoogste n 10 jr en het minste n 25 jr. De hoeveelheid C in flf en olf ws ongeveer gelijk in Miombo-bos, terwijl in de ndere bosopstnden meer flf dn olf nwezig ws. De hoogste LF ws in het vochtige bos. In de Pinus ptul opstnd, flf bevtte tussen 22 25, de olf 8 16 en MHF tot het totl SOC. Koolstof in MHF en olf steeg met diepte, terwijl de flf met diepte fnm behlve in de 1- en 10-jrige opstnden. Conversie vn verrmd Miombo-bos nr Pinus plntges kn betere C-opslg op de korte en lnge termijn opleveren terwijl vochtige bossen zowel meer koolstof 199

210 Smenvtting opslg en biodiversiteit bieden. Wr mogelijk moeten vochtige bossen worden behouden en ngetste gebieden zouden deels ngeplnt kunnen worden en indien mogelijk zouden de bossen kunnen worden beschouwd ls onderdeel vn toekomstige projecten die gericht zijn op verminderde uitstoot door het voorkomen vn ontbossing en degrdtie (REDD+). In het lgemeen kn gesteld worden dt REDD+ zowel de kwliteit vn het milieu en socil economische welvrt kn bevorderen, met inbegrip vn rmoedebestrijding, door het smenbrengen vn de ontwikkeling vn het bosgebieden en het tegengn vn klimtverndering. Andcht voor monetire wrdering en de betling voor ecosysteemdiensten kn bijdrgen n de toenme vn politieke steun voor bodembescherming. Ontwikkelingslnden moeten drom economische en institutionele beleidsregels opstellen voor lndgebruik met het doel positieve effecten te hebben op rmoedebestrijding, voedselzekerheid en ecologische duurzmheid. Bodem C modellen worden gebruikt om de gevolgen vn vernderend lndgebruik op C-opslg in de bodem te voorspellen. Het RothC 2.63 model werd gebruikt voor het schtten vn SOC voorrd onder geselecteerde vormen vn lndgebruik op kleiige en zndige bodems vn Zimbbwe. Er is een groter potentieel om C op te sln in kleiige bodems dn in zndige bodems en onder lndgebruik wrmee meer orgnische stof wordt ingebrcht. Resultten tonen n dt de in het RothC model gebruikte C voorrden HUM + IOM gerelteerd zijn n de gemeten MHF voorrd, en dt het model gebruikt kn worden om vernderingen in de SOC voorrd in Zimbbwnse lndbouw- en bosbodems te schtten. De relties tussen evenwichtsniveus gescht door het RothC-model en die gescht met behulp vn de vergelijking Lngmuir wren goed. Een gesimuleerde stijging vn de tempertuur met 1.5 C veroorzkte in systemen op kleiige bodems een toenme vn C-opslg. De resultten toonden ook n dt wnneer lle ndere fctoren constnt gehouden worden, het model voldoende gevoelig is t..v. een stijging vn de tempertuur wrbij met zndige bodems een nieuw evenwicht eerder bereikt wordt dn met kleiige bodems. Het gebruik vn modellen vertegenwoordigt een vn de meest veelbelovende methoden voor het schtten vn vernderingen in SOC en geeft de mogelijkheid om vernderingen in SOC voorrd te evlueren op bsis vn gemeten dt. Echter, ngezien de 200

211 Smenvtting bodem C dt werden verkregen uit korte termijn experimenten (4 9 jr) is lgere termijn voorspelling beperkt door het ontbreken vn lnge termijn meetreeksen in de regio zuidelijk Afrik. Ook het tekort n geschikte experimentele proefvelden beperkt verdere werkzmheden m.b.t. modelonzekerheden. Meer begrip vn de kwliteit en dynmiek vn de bodem is vn belng bij het ontwerpen vn duurzme lndbouwsystemen wrbij zowel het verbeteren vn productiviteit en het vstleggen vn bodem C een win-win situtie oplevert. 201

212 Acknowledgements ACKNOWLEDGEMENTS I wnt to thnk the Dutch government for their fellowship support through The Netherlnds Fellowship Progrmmes/PhD progrmme (grnt wrd CF 6677/2010). This opportunity mde me ride bicycle in Wgeningen, which I hd ruled out of my life. My promotor, Professor Rik Leemns, professor of Erth System Sciences t Wgeningen University, provided mny vluble comments on the thesis nd importnt lessons on lnguge mnipultion wtch out how you use the words which nd tht. I cherished this nd ll the other lessons nd I m thnkful for his support. I m indebted to my dily supervisor Dr Mrcel R Hoosbeek, Associte Professor of Soil Biogeochemistry for his continuous guidnce nd help tht culminted in the completion of this PhD thesis nd to his fmily. Tin nd the kids who were ccommodting me in their lives. I lso extend my grtitude for ssistnce from Professor Justice Nymngr especilly for the encourging words nd help throughout the process. Thnk you to Professor Pvel Kbt for your confidence t the beginning of the PhD process. I would lso like to thnk Professor Dvid Powlson from Rothmsted Interntionl for vluble comments nd literture for one of the ppers. Thnk you to ll stff nd fellow PhD students in the Erth System Science group for being friendly nd supportive during my sty in Wgeningen. Specil mention to Olf Velling nd Wietse Frnssen, who both gve me my first-ever bike lessons, nd to Emm, Mrleen nd Michelle. My grtitude lso to Mr Jn vn Minnen for ssisting me when needed nd for the pncke mels. The lbortory work ws never going to be s exciting without the help nd ssistnce of Mr Eef Velthorst. The coffee breks relly relieved the pressure of long hours of soil frctiontion. Specil thnks to Ms Ri Cuperus nd Ms Silvi Hrtendorp for logisticl rrngements. My life in Wgeningen would not hve been better without kinsmn support from Zimbbwens nd fellow Africns t Wgeningen University. Specil mention to Violet Dudu, Bbr Msung, Croline Chipomho, Zechri Msnyiw, Rit Owusu-omnkwh nd Innocent Nykudy. 202

213 Acknowledgements Thnk you to Dr Christin Thierfelder t CYMMYT for providing the bseline dt for the tillge studies. Thnk you to ll the extension officers in Mdziw nd Hereford, nd the frmers for ccess to their plots nd soil, nd to those who provided vluble informtion in the form of bseline informtion. In prticulr, thnk you to Mr Willim Mukorowerw, Mr Nelson Guvkuv, Mr Joseph Nengre nd frmer Mr Chitte of Hereford. Thnk you to Wttle compny of Zimbbwe for llowing me to do the reserch nd for the hospitlity nd support provided by stff during our sty t Mutrzi Estte. In prticulr, I thnk Mr Lenin Mungnidze nd his stff t Nyng pine estte nd in prticulr Mr Kenneth Gohori, Mr Amen Mbunge, Ms kundi Kuvhey nd their forest inventory tem. It ws plesure being with you nd your endurnce for the long working hours without brek. Thnk you to Bindur University of Science Eduction (BUSE) for the study leve during my sty in the Netherlnds. Specil thnks to the deprtment of Environmentl Science for support nd ssistnce especilly for tking my teching lod when I ws wy. The field work would not be very exciting without Admore Murev, Timothy Gotor, Peter Mbender nd Bernrd Gurupir, especilly fter digging severl soil pits on hot dy. Thnk you, Mr. stnley Mpurzi, Mr Gylord Kdem, Mr Onwrd Seng, Mr Tobis Nhro nd Mr Kudkwshe Mpfk t BUSE. The mps were produced with vluble expertise of Mr Munyrdzi Musodz. Thnk you to Mr Srthiel Chip for the finl formtting of the document. I lso wnt to express my sincere grtitude to the climte food nd frming network (CLIFF) for sponsoring dditionl reserch on greenhouse gs emissions in griculturl systems, the proposl tht ws prt of my SENSE A2 ctivity. Most importntly I would like to sy big thnk you to my husbnd, my fmily nd friends who gve me their love, pryers, continuous support, encourgement nd bcking throughout the four yer period. Finlly but not lest, I would like to thnk nd sy ebenezer to those who supported me spiritully especilly, Pstor Chrles Msunungure nd the fmily of God church in Bindur nd Pstor Fri Mphos nd the AGP church in Wgeningen. 203

214 About the Author ABOUT THE AUTHOR Lizzie Mujuru (nee Chorum) ws born on 13 My 1965 in Wedz, Zimbbwe. She did her primry eduction t Mkondw primry nd Chemhnz primry schools in Wedz. She proceeded to secondry eduction t Sndringhm high school in Norton, Zimbbwe. In 1986 she becme one of the first two women to receive diplom in forestry from the Zimbbwe College of Forestry. Before moving on to university eduction, Lizzie spent seven yers s forestry extension officer under the Zimbbwen Forestry Commission. In 1996, she received her Bchelor in Forestry degree t Sokoine University of Agriculture in Tnzni funded by scholrship from DANIDA. Upon returning to Zimbbwe in 1996, she joined the Forestry Extension Services s provincil technicl officer for few months before moving to lecture t Zimbbwe College of Forestry. In 2003, she received her MSc degree in Tropicl Resource Ecology from the University of Zimbbwe with study focussing on environmentl determinnts of smll scle vegettion structure nd composition in ntive moist forest in Zimbbwe. In 2005, she joined the Bindur University of Science Eduction lecturing stff nd chired the Deprtment of Environmentl Sciences from 2009 to In 2008, she joined the Wtson Interntionl Scholrs of the Environment Progrmme t Brown University, USA nd ws wrded certificte in lnd-use science. The trining sessions creted desire to do PhD studies focusing on climte chnge nd lnd use. She strted her PhD reserch in 2010 under Erth System Science group t Wgeningen University funded by the Netherlnds fellowship progrmme. During this PhD period she lso received some reserch funds from the Climte Food nd Frming (CLIFF) network nd the Reserch Council of Zimbbwe. 204

215 About the Author Some Publictions And Conference Ppers 1. Mujuru, L. Gotor, T. Velthorst, E.J. Nymngr J. nd Hoosbeek M.R. (2014). Soil crbon nd nitrogen sequestrtion over n ge sequence of Pinus ptul plnttions in Zimbbwen Estern Highlnds. Forest Ecology nd Mngement Mujuru L., Murev A, Velthorst E nd Hoosbeek M.R. (2013) Lnd use nd tillge effects on soil orgnic mtter frctions in sndy nd clyey soils of Bindur nd Shmv districts in Zimbbwe. Geoderm Nymdzwo, G., Wut M., Chirind N., Mujuru L., Smith J.L. (2013). Greenhouse Gs Emissions from Intermittently Flooded (Dmbo) Rice under Different Tillge Prctices in Chiot Smllholder Frming Are of Zimbbwe. Atmospheric nd Climte Sciences, Gdziryi C. T., Kubiku F. N. M., Mupngw, J. F. Mujuru L., Chikuvire T. J. (2013). The Effect of Plnt Spcing nd Cutting Intervl on Growth of Moring Oleifer. Journl of Agriculturl Science nd Applictions 2, Jimu L., Ngoroyemoto N. nd Mujuru L. (2012) Structurl diversity of Prunus fricn in Zimbbwe. Africn Journl of Ecology. 51, Nyok, B.I. Tembni, M.C. Mdhibh, T.P. Mushonghnde, M. Gondo, P. Mchen, C. Chingrnde, Y.T. Ktivu, S. Mujuru, L. Mfurtidze R. nd Mudhefi A. (2012). Stte of Zimbbwe s Forest Genetic Resources country report. Ministry of Environment nd Nturl Resources Mngement, Republic of Zimbbwe, Hrre. pp 83. ISBN Mujuru L Diospyros lysiodes. In: Brink M. nd Achign-Dko E.G. (eds).plnt Resources of tropicl Afric. 16. Fibres. PROTA Foundtion. Wgeningen, Netherlnds/CTA. Wgeningen, Netherlnds. pp ISBN Chikuse J.L, Ktsvng C.A.T., Jimu L nd Mujuru L Impcts of outsourcing forestry opertions in the hyperinfltionry economic environment of Zimbbwe. Africn journl of griculturl reserch 7, Chimvurmbwe J. Musr J.P, Mujuru L. Gdziri C.T, Nykudy I.W., Jimu L., Ktsvng C.A.T., Mupngw J.F nd Chivhey R Effect of feeding grded levels of Adnsoni digitt (bobb) seed cke on performnce of broilers. Journl of niml nd plnt sciences 11,

216 About the Author 10. Chtng P., Kmnd M.T. Kudhlnde A, Imbyrwo-Chikosi V. E., Mujwo T. Mgdz C. H. D., nd Mujuru L. (2008). Effects of Lntn Cmr (L) invsion on the ntive vegettion of Gonrezhou Ntionl Prk, Zimbbwe. Southern Africn Journl of Eduction Science nd Technology 3, Mujuru L. nd Hmburg S. (2008). Biodiesel in Zimbbwe: Opportunities nd Pitflls. In: Mkungw, S.M., Chkeredz, S., Sk, A., Mwse, W. Sk, V., G.F. Slnje, A. D. Yye (eds). Minstreming Climte Chnge into Agriculturl nd Nturl Resources Mngement Eduction: Tools, Experiences nd Chllenges. Reviewed Ppers presented t ANAFE symposium on Tertiry Agriculturl Eduction, July 2008, Lilongwe, Mlwi Mujuru L. nd Kundhlnde A. (2007). Smll Scle vegettion structure nd composition of Chirind moist forest south est Zimbbwe. Africn Journl of Ecology 45, , Mujuru, L. Jimu, L. Nymugure, T. Kundhlnde, A. Gwenzi D. nd Ktsvng C.A.T (2007). The Role of certifiction in Biodiversity conservtion in commercil forest esttes of Zimbbwe: A cse study of Chimnimni Esttes. Pper presented t the interntionl conference on biodiversity nd sustinble mngement of nturl resources July Kigli. Rwnd. 206

217 About the Author Netherlnds Reserch School for the Socio-Economic nd Nturl Sciences of the Environment CERTIFICATE The Netherlnds Reserch School for the Socio-Economic nd Nturl Sciences of the Environment (SENSE), declres tht Lizzie Mujuru born on 13 My 1965 in Wedz, Zimbbwe hs successfully fulfilled ll requirements of the Eductionl Progrmme of SENSE. Wgeningen, 20 My 2014 the Chirmn of the SENSE bord the SENSE Director of Eduction Prof.dr.ir. Huub Rijnrts Dr. Ad vn Dommelen The SENSE Reserch School hs been ccredited by the Royl Netherlnds Acdemy of Arts nd Sciences (KNAW) 207

218 About the Author The SENSE reserch school declres tht Ms Lizzie Mujuru hs successfully fulfilled ll the requirements of the Eductionl PhD progrmme of SENSE with worklod of 45.8 including the following: SENSE PhD Courses Environmentl Reserch in Context (2011) Reserch Context ctivity: Writing Reserch proposl nd consultncy report on REDD+ ctivities in Africn woodlnds nd Svnnhs (2012) Other PhD nd Advnced MSc Courses Multivrite Anlysis (2010) Field Trining- Erth System Science (2010) Informtion Litercy nd EndNote Introduction (2011) Soil Ecology- tking globl issues underground (2011) REDD+SCIENCE+GOVERNANCE: Opportunities nd Chllenges (2012) Techniques of Scientific Writing (2012) Orl Presenttions Density frction nd its effect on orgnic crbon nd nitrogen in sndy nd clyey soils. Reserch nd Post grdute center t Bindur University. 26 August Bindur University. Effects of tillge on soil orgnic crbon nd nitrogen storge. Reserch nd intellectul expo, 6 September 2012, Hrre, Zimbbwe. Crbon nd nitrogen in gro ecosystems. Climte food nd frming network (CLIFF) Februry 2013, Copenhgen, Denmrk. SENSE Coordintor PhD Eduction Dr. ing. Monique Gulickx 208