Ground Covers in Fraser Fir (Abies fraseri [Pursh] Poir.) Production Systems: Effects on Soil Fertility, Tree Morphology and Foliar Nutrient Status

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1 Europ.J.Hort.Sci., 75 (6). S , 21, ISSN Verlg Eugen Ulmer KG, Stuttgrt Ground Covers in Frser Fir (Aies frseri [Pursh] Poir.) Production Systems: Effects on Soil Fertility, Tree Morphology nd Folir Nutrient Sttus A. R. Wilson 1), P. Nzokou 1) nd B. Cregg 2) ( 1) Deprtment of Forestry, Michign Stte University, nd 2) Deprtment of Horticulture, Michign Stte University) Summry The effect of three ground covers [lff (Medicgo stiv), Dutch white clover (Trifolium repens) nd perennil rye (Lolium perenne) on soil fertility, tree morphology, nd tree nutrient sttus in Frser fir-cover crop intercropping system ws investigted to ssess the impcts of cover crop species nd mngement prctice on the sustinility of the cropping system. Ground cover were mowed every 21 dys nd returned to the ground s green mulch. Prmeters monitored included soil moisture, soil orgnic cron nd nitrogen, soil minerl nitrogen, cover crop iomss nd nutrient content, nd tree morphologicl nd nutrient chrcteristics. Results otined indicted tht the species selection ffected the mount of green mnure produced nd its totl nitrogen nd phosphorus content. Soil orgnic cron nd nitrogen were stle in the upper soil profile due to the reltively smll quntity dded with the cropping system compred to the totl stock of soil C nd N. Minerl nitrogen decresed over the seson due to the synchrony of cover crop decomposition, minerliztion nd nutrient uptke y trees in the upper profile. Species selection nd mngement prctices did not ffect tree reltive height growth nd 1-needles weight, ut mngement prctices ffected reltive dimeter growth, indicting competition for soil moisture. Tree folir nutrient concentrtions were not significntly ffected y cover species, ut were much lower in some of the mngement prctices, suggesting competition for nutrients. We conclude tht creful mngement should e used to cpitlize on cover crop enefits nd void competition for soil nutrients nd moisture. Key words. cropping system green mnure soil orgnic cron soil orgnic nitrogen plnttion mngement soil moisture Introduction Frser fir (Aies frseri [Pursh] Poir.) is commonly grown species for Christms tree production or short rottion silviculture for its morphologicl nd esthetic chrcteristics (NZOKOU nd LEEFERS 27). Trditionl production prctices include high rtes of inorgnic fertilizer ppliction throughout the growing seson in ddition to frequent hericide spryings nd pesticide use (NZOKOU nd LEEFERS 27). These prctices re costly to growers nd detrimentl to the environment, ut re necessry for frmers to mintin high soil nitrogen to ensure mximum growth nd desired tree colour. However, when excess orgnic nitrogen exists in the soil due to imlnces of plnt uptke nd immoiliztion, the negtively chrged nitrte ions ecome vulnerle to leching (THOMSEN 25). Intensive fertilizer regimens disrupt production systems innte ility to lnce nutrients (ALTIERI 1999). An lterntive to conventionl production methods is intercropping of cover crops into the growing system to function s green mnures, which decreses expenses in eliminting the need for frequent inorgnic fertilizer inputs (SIRRINE et l. 28). Returning cut cover crop to plots through regulr mulching provides constnt flow of orgnic nutrient input to the soil (PARDO et l. 29), which cn lter soil cron to nitrogen rtios nd influence microil nitrogen moiliztion. Acute effects of cover crop ddition include initil spikes in ville nitrogen followed y decreses due to minerliztion, llowing soil nitrogen to e stored until mde ville for plnt use (PARDO et l. 29). While using cover crops cn e dvntgeous to production systems, intercropping develops complex ecosystem in which competitive interctions my occur. Where elow ground competition occurs, plnts compete for ville soil resources, minly wter nd minerl nutrients, cusing decrese in growth to the out-competed species (CASPER nd JACKSON 1997). Severl studies on the use of ground covers in vrious griculturl cropping systems hve een conducted. However, their use in plnttion forestry nd short rottion silviculture is reltively new nd there is little pulished evidence on the effect of ground cover on such production systems. We hypothesized tht when ground covers re mixed with trees oth fcilittive nd compet-

2 27 Wilson et l.: Ground Covers in Frser Fir Production Systems itive interctions occur with the response depending on the ground cover mngement prctice. The ojectives of this study re to 1) explore the effects three ground cover species nd their mngement on Frser fir morphology nd folir nutrient sttus, nd 2) to determine the impct of cover crop mngement on soil fertility. Mterils nd Methods Site description The experiment ws conducted t the Tree Reserch Center in Sndhill on the cmpus of Michign Stte University. The field coordintes re N ltitude nd W longitude. Wether conditions were reltively mild with rinfll firly well distriuted throughout the growing seson. The field ws surrounded y 3-wire electric fence to reduce deer rowsing. The soil is Mriett fine sndy lom tht is clssified s modertely well drined with high ville wter cpcity nd medium surfce runoff. Mriett soils re suject to erosion nd tend to hve poor tilth. Plnt mterils nd field mngement Plnt mterils used included Frser fir s tree crop nd three cover crops species two legumes [Dutch white clover (Trifolium repens) nd lflf (Medicgo stiv)) nd one grss (perennil rye (Lolium perenne)]. Frser fir trnsplnts were mchine plnted in My 27. Dutch white clover, lflf (SS 1 rnd) nd perennil rye (VNS) seeds were purchsed from Michign Stte Seeds (Grnd Ledge, Michign) nd hnd rodcsted t the eginning of the 27 seson. The seeding rtes were 28 kg h 1 for clover nd lflf nd 13 kg h 1 for rye. Cover crops were regulrly mowed during the growing sesons in 27 nd 28 to reduce competition with trees. Ech plot ws.8 h in size nd contined 35 trees (5 rows 7 trees). Perimeter trees functioned s uffer etween plots to void confounding effects etween tretments, nd the interior fifteen trees (3 5) were used for dt collection. Mngement prctices included comintions of nding nd no-nding tretments of ech species of cover crop, nd control or re-ground () tretment where ll weeds were removed y ppliction of kg h 1 of glyphoste with CO 2 powered ckpck spryer (R & D Spryers, Bton Rouge, LA). Bnding consisted of mintining re 52 cm nd centred round the tree row. This ws chieved y pplying directed spry of glyphoste with custom designed MANKAR spryer (George F. Ackermn Compny, Curtice, OH) t rte of 3 L h 1 three times during the seson. The summry of the seven tretments includes; re-ground (), nded perennil rye (), non-nded perennil rye (), nded flf (ALF+B), non-nded lflf (ALF+noB), nded white clover (), nd non-nded white clover (). All tretments were replicted 3 times in rndom design. Cover crop iomss nd nutrient ccumultion Mechnicl mowing ws performed every three weeks (5 cm ove the ground) to control cover crop growth, minimize cover competition with crop trees, nd dd green mnure to the soil. During ech mowing, cover crop iomss returned ws determined y collecting iomss from rndomly selected.55 m 2 qudrnts in ech plot. The green iomss collected in ech tretment ws weighed nd oven-dried t 65 C nd the dried iomss determined. Su-smples from ech tretment were used for chemicl nlyses. Soil nitrte nd mmoni Composite soil smples were collected from depths of 15 cm nd 15 3 cm, within one foot of trees t two rndom loctions within ech plot using soil uger. Smples were plced in zip-lock gs nd trnsported to the lortory where they were refrigerted until nlysis. Susets of soil specimens were extrcted using potssium chloride. The extrct otined fter filtrtion of the soil solution ws nlysed y segmented flow nlysis to determine soil NH 4 + nd NO 3. Totl cron nd totl nitrogen nlysis Soil smples were oven-dried t 15 C for 48 h nd sieved to pss through 2 µ m mesh efore nlysis. Totl orgnic cron (TOC) nd totl orgnic nitrogen (TON) were determined y dry comustion using CHN utonlyser (Leco, St Joseph, MI). Soil moisture Soil moisture content ws mesured throughout the seson using Field Scout TDR 3 Soil Moisture Proe (Spectrum Tech Inc, Plinfield, IL). Moisture redings were tken t four rndomly selected loctions in ech of the plots, within one foot of tree. Tissue nlyses Folir tissues from trees were collected in ech plot for nutrient nlysis. Susets of cover crop iomss smples collected during ech mowing were lso processed for nutrient nlysis. Ech tissue type ws oven dried t 6 65 C nd ground into fine powder. Approximtely.3 g of mteril ws plced into 75 ml digestion tue nd cid digested with sulfuric cid (4.5 ml) nd hydrogen peroxide (1.5 ml). A digestion lnk ws included for verifiction. Smples were pre-digested for two hours nd plced into lock digester (AIM6 Block Digestion System) t 34 ± 1 C for het digestion under progrmmed temperture schedule. Totl nitrogen ws determined s descried y CHRISTIANSON nd HOLT (1986). Aliquots from the digested solution were uffered nd chlorinted fter dilysis to form chemicl complex mesured t 66 nm on SAN++ segmented flow nlyser (Sklr Inc. Atlnt, GA). The totl phosphorous content determintion ws sed on the mmonium heptmolydte nd potssium ntimony (III) rection tht form under cidic environment n ntimony-phospho-molydte complex mesured t 88 nm on the SAN++ segmented flow nlyser. Tree growth nd mss of 1 needles Tree height nd root collr dimeter (RCD) were mesured in erly My nd lte August. Height nd dimeter

3 Wilson et l.: Ground Covers in Frser Fir Production Systems 271 growth were clculted s the difference etween the finl nd initil mesurements. Reltive height growth (RHG) nd reltive RCD growth were clculted y dividing the height nd root collr growth y the initil vlue for ech tree. During ech growth mesurement, needle smples were rndomly collected from trees within ech plot nd the verge mss of one hundred needles ws determined for ech tretment. Dt nlysis Two-wy nlysis of vrince (ANOVA) ws used to test the effects of the groundcover mngement system on the growth of the Frser fir nd nutrient concentrtions of plnts nd soil. The mens were seprted y n LSD-test for lph α<.5. Person's correltion coefficient ws clculted to detect reltionships etween tree growth nd nutrient sttus. All sttisticl nlyses were performed using Systt 13 sttisticl softwre (Systt Softwre, Inc., Chicgo, IL). Biomss ccumultion (x1 3 kg h -1 ) ALF+B ALF+noB Results Cover crop iomss ccumultion Accumulted iomss ws gretest in tretments with leguminous cover crop species, which produced more green mnure thn rye tretments (Fig. 1). Alflf tretments without nds (ALF+noB) cumulted the gretest mount of iomss (4.33 kg h 1 ), while perennil rye tretments ccumulted the lest ( kg h 1 ). Cover crop species significntly ffected iomss ccumultion (P.1). Except for the perennil rye, tretments without nding generlly produced higher cumulted mounts of iomss, ut the effect of the mngement strtegies (nding vs. no nding) were not sttisticlly significnt (P.37). Cover crop nitrogen nd phosphorous content As expected, the nitrogen content of the ground cover iomss returned to the ground through mowing ws greter in legume tretments compred to the grss tretment (Fig. 2). The cumulted nitrogen content ws gretest in (151 kg h 1 ), followed y ALF+noB (141 kg h 1 ). Perennil rye tretments returned the lest with 56 nd 52 kg h 1 for nd respectively. Similr to iomss ccumultion, cover species selection ffected N ccumultion (P.1), ut mngement strtegy did not (P.17). The N ccumultion vried cross the seson (P.3) due to higher iomss production erly in the spring. Ptterns of phosphorous content differed from N content with ALF+noB returning higher mounts of P (13.4 kg h 1 ) due to higher P concentrtion. White clover tretments were intermedite nd perennil rye tretments returned the lest mount of P (6.1 kg h 1 nd 6.7 kg h 1 for nd respectively). Phosphorus content did not vry over time (P.14). Soil orgnic cron, nitrogen nd C/N rtio At the eginning of the growing seson (My), totl orgnic cron (TOC) ws similr mong ll tretments Fig. 1. Averge cumultive cover crop iomss returned to respective tretment with mowing in the 29 growing seson. Lels: ALF+B = lflf with nding, ALF+noB = lflf with no nding; = perennil rye with nding; = perennil rye with no nding; =white clover with nding; = white clover with no nding. for smples from the 15 cm depth (Tle 1). At the end of the seson indicted TOC ws lower (P.5) in thn the other tretments. Otherwise, TOC did not differ mong plots. Similrly, totl orgnic nitrogen (TON) vlues did not vry mong tretments on either smpling dte, lthough TON tended to decrese slightly over time (Tle 1). Similr trends with few exceptions were found for smples from the deeper profile (15 3 cm) for TOC nd TON. Significnt differences in TOC nd TON were found for compred to other tretments, except for TON under perennil rye (P.5). TOC nd TON vlues from 15 3 cm were generlly lower thn those otined from smples from the upper soil profile. Erly in the seson, C/N t depth of 15 cm vried widely mong tretments rnging from 8.6 to 1.7. Lte in the seson, C/N vlues t 15 cm vried less, rnging from 1.4 to 1.9 depending on the tretment. Similr trends were oserved t 15 3 cm with erly seson C/N vlues rnging from 9.4 to 11.5 nd lte seson vrying from 1.4 to Overll C/N did not vry throughout the growing seson (P.6). Soil nitrte nd mmonium concentrtion Nitrte concentrtions t 15 cm were similr in ll tretments t the eginning of the growing seson nd tended to decrese in ll tretments except nd tretments. However, the difference etween tretments ws not significnt in the lte seson smpling (Tle 1). At 15 3 cm smpling depth, nitrte ws lower thn in the upper profile. Nitrte vlues for (16.7±5.9 ug g 1 ) were lower thn those in tretments (54.±13. ug g 1 ).

4 272 Wilson et l.: Ground Covers in Frser Fir Production Systems Cumultive totl phosphorous content (kg h -1 ) Cumultive totl nitrogen content (kg h -1 ) (A) ALF+B ALF+noB (B) ALF+B ALF+noB At the 15 3 cm profile, mmoni did not vry cross the seson. Soil moisture With few exceptions, soil moisture content ws similr in nd nded tretments for ll three ground cover species (Fig. 3). consistently hd lower moisture content compred to, nd, nd tended to hve higher soil moisture content. For mesurements conducted in July (low rinfll), ALF+noB nd hd significntly lower soil moisture content. As expected, mngement prctice (nding or no nding) hd significnt effects on soil moisture (P.), ut the ground cover species lso ffected (P.4) soil moisture content. Folir weight nd tree growth Needle msses in ll tretments incresed from My to August (Fig. 4) ut did not vry mong tretments. Both ground cover crop species (P.36) nd mngement prctices (P.44) did not ffect the needle iomss. There ws no tretment effect on RHG (P.43), RRCD growth (P.) ws significntly ffected (Fig. 5). RRCD ws generlly lower in tretments without nds compred to nded tretments tht generlly displyed RRCD vlues similr to tretments. Folir nutrient concentrtions Folir N concentrtions were similr in ll tretments in June nd July, ut tretments vried in ALF+noB,, nd for August smples (Fig. 6). At the end of the seson, nded tretments generlly hd higher folir N concentrtions compred to, nd non-nded tretments hd lower N concentrtions. As consequence, the mngement prctice hd sttisticlly significnt effect on the folir N concentrtions (P.). Surprisingly, the ground cover species did not significntly ffect folir N concentrtions (P.76). Folir P concentrtions displyed trends very similr to folir N concentrtions. Concentrtions were sttisticlly similr in oth June nd July, ut differences were oserved t the end of the growing seson when nded tretments hd higher P concentrtions compred to non-nded tretments. The mngement strtegies hd significnt effect on P concentrtions (P.) while the ground cover species hd no significnt effect (P.93). Fig. 2. Cumultive (A) nitrogen nd (B) phosphorous returned in green mnure during the 29 growing seson. Lels: ALF+B = lflf with nding, ALF+noB = lflf with no nding; = perennil rye with nding; = perennil rye with no nding; = white clover with nding; = white clover with no nding. Ammoni concentrtions lso vried in the 15 cm profile for the erly seson smpling with higher vlues for (233.8±56.4 ug g 1 ) compred to tretments (41.2±17.6 ug g 1 ). However, in the lte seson smpling, there ws no difference etween tretments. Discussion Cover crop species ffected the totl dry weight of green mnure returned to the system with the two legumes (lflf nd white clover) returning significntly more iomss thn the grss species (perennil rye). This oservtion is in greement with previous studies conducted on iomss quntifiction (CLINE nd SILVERNAIL 21). Genetic differences nd the ility to fix tmospheric N dictte why the two legumes were le to grow lrger. Biomss production ws directly relted to the quntity of orgnic nitrogen nd phosphorus returned to the system nd helped improve the totl nutrient udget. As

5 Wilson et l.: Ground Covers in Frser Fir Production Systems 273 Tle 1. Totl soil cron, nitrogen, nitrtes nd mmoni concentrtions in erly (My 29) nd lte (August 25) growing seson s relted to the ground cover species nd mngement prctice. Tretment My 29, 29 August 25, 29 TOC (mg g 1 ) TON (mg g 1 ) C/N NO 3 (µ gg 1 ) NH4 + (µ gg 1 ) TOC (mg g 1 ) TON (mg g 1 ) C/N NO 3 (µ gg 1 ) NH4 + (µg g 1 ) Smpling depth 15 cm 17.5 ± ± ± ± ± ± ± ± 22.5 ALF+B 17.8 ± ± ± ± ± ± ± ± 29.8 ALF+noB 18.1 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 26.5 Smpling depth 15 3 cm 12.5 ± ± ± ± ± ± ± ± 3.1 ALF+B 12.6 ± ± ± ± ± ± ± ±.3 ALF+noB 14.3 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 8.6 Vlues followed y the sme letters men no sttisticl difference using the Tukey's Honestly-Significnt-Difference Test. TOC, TON nd C/N mesurement in My nd August were nlysed using the repeted mesures mixed model. The model ws not significnt for TOC (P=.65), TON (P=.57) nd C/N (P=.6) Tretments: = re-ground, ALF+B = Alflf with nding, ALF+noB = Alflf with no nding, = Perennil rye with nding, = Perennil rye with no nding, = White clover with nding, = White clover with no nding. Reltive soil moisture (%moisture) ALF+B ALF+noB Jun26 Jun3 Jul2 Jul7 Jul15 Jul17 Jul21 Jul24 c c c c c cd d c c c Fig. 3. Volumetric soil moisture content June to July of cover crop tretments reltive to reground control. Brs represent the difference in soil moisture content etween re-ground control tretments nd respective cover crop tretments. Men seprtion y Tukey s Honestly-Significnt-Difference Test t ech smpling dte; tretments followed y the sme letter ( d) re sttisticlly similr. Lels: ALF+B = lflf with nding, ALF+noB = lflf with no nding; = perennil rye with nding; = perennil rye with no nding; = white clover with nding; = white clover with no nding. expected, species selection ws criticl in reltion to totl orgnic N contriution to the site. It hs long een demonstrted tht significnt mounts of N cn e dded y the inclusion of N fixing legumes to production systems (RENNIE nd KEMP 1983). This ws evident in this project with some of the legume tretments contriuting

6 274 Wilson et l.: Ground Covers in Frser Fir Production Systems Averge mss of 1 needles (g),5,4,3,2,1 ALF+B ALF+noB Fig. 4. Tree dried needle msses y cover crop tretment throughout the growing seson. Men seprtion y Tukey s Honestly-Significnt-Difference Test t ech smpling dte; tretments followed y the sme letter ( ) re sttisticlly similr for tht dte. Lels: ALF+B = lflf with nding, ALF+noB = lflf with no nding; = perennil rye with nding; = perennil rye with no nding; = white clover with nding; = white clover with no nding. three times s much orgnic N s the grss species. The green mnure returned dded to the soil orgnic pool tht cn ecome ville to the plnt through minerliztion (PARDO et l. 29) thus incresing the overll fertility of the site. The P contriutions of legumes were lso significntly higher thn those of the perennil rye system. Cover crops hve een reported to improve the P udget in the soil (CAVIGELLI nd THIEN 23). Residues of cover crops my recycle normlly unville phosphorous into forms more ville to the trget crop. The residue decomposing process improves the P vilility y relesing CO 2, which forms H 2 CO 3 in the soil solution, resulting in the dissolution of primry P contining minerls (SHARPLEY nd SMITH 1989). It is lso possile tht orgnic compounds relesed during decomposition processes my increse P vilility y locking P-dsorption sites (EASTERWOOD nd SARTAIN 199). Effect of cover crops on soil fertility The short-term effect of the cover crop species nd mngement prctices on SOC, SON, nd soil nitrogen ions ws not very well defined. The ddition of orgnic residues is the only wy to increse soil orgnic mtter levels (FAGERIA 27). The effect of crop residue on soil orgnic mtter content is highly relted to the mount nd only wekly to the type of residue pplied (FAGERIA 27). The ddition of green mnure to the sites ws expected to hve significnt positive influence on soil physicl, chemicl nd iologicl properties. In this study Reltive RCD growth (mm mm -1 ) Reltive height growth (cm cm -1 ) ALF+B ALF+noB (A) (B) ALF+B ALF+noB Cover crop tretment Fig. 5. Reltive Frser fir height growth (A) nd reltive root collr dimeter growth (B). Tukey s Honestly-Significnt-Difference Test ws used to mesure vrince within tretments t ech smpling dte; tretments with the sme letter re sttisticlly insignificnt. Lels: ALF+B = lflf with nding, ALF+noB = lflf with no nding; = perennil rye with nding; = perennil rye with no nding; = white clover with nding; = white clover with no nding. with few exceptions, erly nd lte seson TOC nd TON were generlly stle in the upper soil horizon. This ws not surprising ecuse it hs een reported tht when totl stocks of C nd N in the soil re of sufficient size, the differences in C or N input from tretments compred to controls re smll (GRIFFIN nd PORTER 24). For exmple, COLLINS et l. (1992) compred continuous whet nd whet-pe rottions in plce for 25 yers, nd found

7 Wilson et l.: Ground Covers in Frser Fir Production Systems 275 () () 3 25 Jun 14 2 Jun 14 Folir N (mg g -1 ) Folir P (mg g -1 ) 1,5 1 5,5 ALF+B ALF+noB ALF+BALF+noB 3 Jul 7 2 Jul 7 Folir N (mg g -1 ) Folir P (mg g -1 ) 1,5 1,5 ALF+B ALF+noB ALF+B ALF+noB Folir N (mg g -1 ) Aug 24 Folir P (mg g -1 ) 2 1,5 1,5 Aug 24 ALF+B ALF+noB Cover crop tretment ALF+B ALF+noB Cover crop tretment Fig. 6. Chnges in folir nitrogen (A) nd phosphorous (B) s ffected y cover crop species nd mngement. Tukey s Honestly-Significnt-Difference Test ws used to mesure vrince within tretments t ech smpling dte; tretments with the sme letter re sttisticlly insignificnt. Lels: ALF+B = lflf with nding, ALF+noB = lflf with no nding; = perennil rye with nding; = perennil rye with no nding; = white clover with nding; = white clover with no nding. no significnt effect on SOC nd SON. While we did not oserve sustntil chnges in C/N rtios, previous studies hve oserved increses in C/N rtios oveground nd root iomss especilly dds to soil nitrogen nd cron nks over time (PUGET nd DRINKWATER 21). A possile explntion of the short-term stility in orgnic prmeters is the C/N rtio of the green mnure used. The C/N rtio of the residue plys n importnt role in the immoiliztion of soil N ecuse the plnt tissue is primry sink for C nd N (FAGERIA 27).

8 276 Wilson et l.: Ground Covers in Frser Fir Production Systems Legume residues in our study contined considerle mounts of N nd certinly reltively low C/N rtio leding to fster decomposition, minerliztion nd relese of N, compred to perennil rye tretments. C/N vlues were generlly much higher in the upper horizon, indictive of superior fertility compred to deeper horizons. High C/N vlues llow slow moiliztion of nitrogen (SNAPP nd BORDEN 25) s microil popultions grow, llowing orgniclly fertilized tretments to relese nitrogen t stedy rtes s opposed to spontneous rtes oserved in trditionlly fertilized tretments (POUDEL et l. 22). Our higher C/N vlues could e explined y helthy soil microil popultion, which led to rpid decomposition nd turnover of nutrients. Tretments incorporting cover crop ll showed decreses in minerl N (NH 4 + -N nd NO 3 -N) over the growing seson in the upper soil profile while tretments showed increses for oth ions. This cn e due to comintion of fctors including synchrony etween cover crop decomposition, minerliztion nd uptke y trees, or incresed leching elow the tested upper soils profile. Decreses in minerl N hve een previously linked to greter demnd for this nutrient y higher plnts nd microes tht grow vigorously during the seson, s well s leching, runoff nd erosion (SINGH et l. 1989; SRIVASTAVA 1992). This cn e especilly true for cover crops with highly degrdle residues such s legumes where the minerliztion rte of plnt-derived C nd N cn e s high s 8 % (GRIFFIN nd PORTER 24). Effect of cover crop mngement on tree morphology nd folir nutrient concentrtion In our study, the cover crop mngement prctice nd species selection hd no significnt impct on height growth nd needle dry weight, suggesting the tree-cover crop production system ws pproprite for promoting growth nd supplying nutrients to the tree. Such results hve een reported in severl studies focused on other griculture crops. For exmple, CTAHR (29) reported improved tree growth nd reduced relince on fertilizers when using living mulches with fruit trees. Similrly in other studies, nnul mulching chieved higher soil N nd wter content, resulting in vigorous tree growth (COSTELLO 21). However, there re lso severl reports on competition for nutrients nd moisture leding to depressed growth (WALSH et l. 1996). Competition ws evidenced in our dimeter growth response vrile especilly in tretments without nds where slow growth ws ssocited with low moisture content in cover crop tretments. While cover crops hve the potentil to compete with primry crops for wter, groundcover cn sor wter in the soil, reducing the vulnerility of nitrte to leching (PEDERSEN et l. 29) nd cn reduce soil tempertures proportionl to height of cover (WALSH et l. 1996) to prevent drying of soils. When incorporting cover crops in production systems, creful mngement prctices should e utilized to cpitlize on the enefits of intercropping without risking reduced growth s oserved in tretments lcking nds. Even so, dding cler nds etween trees nd cover crops reduced competition. During the pek of the growing seson (July), decreses in tree folir N nd P were oserved proly due to shifts in the nutrient lloction towrds developing secondry xylem nd phloem tissue. However, it is lso possile tht nutrient uptke y plnts ws lower in this month due to reduced rinfll in July (6.7 mm) s previously reported y PARDO et l. (29) in semirid conditions. The choice of cover crop species hd no influence on the tree folir nutrient concentrtion. Nevertheless, ecuse the mngement prctice significntly ffected this prmeter, it is likely tht competition for nutrients ws occurring etween the cover crop nd trees. Our results lso suggest tht intercropping cover crop species with the ddition of nds cn help llevite competition for nutrients, resulting in folir nutrient concentrtions equl or superior to those of trditionl production prctices. Conclusion This study investigted the effect of the choice of ground cover species nd mngement prctice on green mnure production nd nutrient content, soil chrcteristics, tree morphology nd folir nutrient concentrtion. Species selection significntly ffected the mount of green mnure produced nd its totl N nd P content. Soil orgnic cron nd nitrogen were stle in the upper soil profile nd unchnged in the deeper soil profile. We hypothesized tht the insignificnt vrition in soil orgnic nutrients is due to the reltive smll size of the quntities dded compred to the totl stock of C nd N in the soil. Soil nitrte nd mmoni were highly vrile with no cler seprtion relted to either the cover crop species or the mngement prctice. This ws ttriuted to either the synchrony etween the cover crop decomposition, minerliztion, nd nutrient uptke y trees in the upper profile, or to the leching of excess nutrients elow the smpling profile. The tretment effects on morphologicl prmeters nd folir nutrient concentrtion suggest some level of competition for soil moisture nd nutrients. Further studies re needed to evlute tretment effects on other mcronutrients. Our conclusion is tht creful mngement is necessry to cpitlize on the enefits of cover crop nd void competition for soil nutrients nd moisture. Acknowledgements This project ws conducted with funding support from the MSU project GREEEN nd the Michign Christms Tree Assocition. References ALTIERI, M.A. 1999: The ecologicl role of iodiversity in groecosystems. Agriculture, Ecosystems nd Environment 74, CASPER, B.B. nd R.B. JACKSON 1997: Plnt competition underground. Annu. Rev. Ecol. Syst. 28, CAVIGELLI, M.A. nd S.J. THIEN 23: Phosphorous iovilility following incorportion of green mnure crops. Soil Sci. Soc. Am. J. 67, CHRISTIANSON, C.B. nd L.S. HOLT 1986: Rpid digestion procedure for the determintion of totl Nitrogen nd Nitrogen-15 content of soils. Soil Sci. Soc. Am. J. 53, CLINE, G.R. nd A.F. SILVERNAIL 21: Residul nitrogen nd kill dte effects on winter cover crop growth nd nitrogen content in vegetle production system. HortTechnology 11,

9 Wilson et l.: Ground Covers in Frser Fir Production Systems 277 COLLINS, H.P., P.E. RASMUSSEN nd C.L. DOUGLAS JR. 1992: Crop rottion nd residue mngement effects on soil cron nd microil dynmics. Soil Sci. Soc. Am. J. 56, COSTELLO, M.J. 21: Growth nd Yield of Cultivted Grpe with Ntive Perennil Grsses Nodding Needlegrss or Cliforni Brley s Cover Crops. HortScience 45, CTAHR 29: Perennil penut. Sustinle Agriculture Cover Crop Dtse. College of Tropicl Agriculture nd Humn Resources, University of Hwi I t Mno. cthr.hwii.edu/susting/covercrops/perennil_ penut.sp. Updted 2/1/9. EASTERWOOD, G.W. nd J.B. SARTAIN 199: Clover residues effectiveness in reducing orthophosphte sorption on ferric hydroxide coted soil. Soil Sci. Soc. Am. J. 54, FAGERIA, N.K. 27: Green Mnuring in crop Production. J. Plnt Nutr. 3, GRIFFIN, T.S. nd G.A. PORTER 24: Altering soil cron nd nitrogen sticks in intensively tilled two-yer rottions. Biol. Fertil. Soils 39, NZOKOU, P. nd L.A. LEEFERS 27: Costs nd returns in Michign christms tree production, 26. Michign Stte University Extension. MSUE Extension ulletin E-2999, Est Lnsing. PARDO, G., J. CAVERO, J. AIBAR nd C. ZARAGOZA 29: Nutrient evolution in soil nd dry cerel yield under different fertiliztion type in drylnd. Nutr. Cycling Agroecosyst. 84, PEDERSEN, A., K. THORUP-KRISTENSEN nd L.S. JENSEN 29: Simulting nitrte retention in soils nd the effect of ctch crop use nd rooting pttern under the climtic conditions of Northern Europe. Soil Use nd Mnge. 25, POUDEL, D.D., W.R. HORWATH, W.T. LANINI, S.R. TEMPLE nd A.H.C. BAN BRUGGEN 22: Comprison of soil N vilility nd leching potentil, crop yields nd weeds in orgnic, low-input nd conventionl frming systems in northern Cliforni. Agriculture, Ecosystems nd Environment 9, PUGET, P. nd L.E. DRINKWATER 21: Short-term dynmics of root nd shoot-derived cron from leguminous mnure. Soil Sci. Soc. Am. J. 65, RENNIE, R.J. nd G.A. KEMP 1983: N 2 -fixtion in field ens quntified y 15 N isotope dilution. II. Effect of cultivrs of ens. Agron. J. 75, SHARPLEY, A.N. nd S.J. SMITH 1989: Minerliztion nd leching of phosphorous from soil incuted with surfce-pplied nd incorported crop residue. J. Environ. Qul. 18, SINGH, J.S., A.S. RAGHUBANSHI, R.S. SINGH nd S.C. SRIVASTAVA 1989: Microil iomss cts s source of plnt nutrients in dry tropicl forest nd svnn. Nture 338, SIRRINE, J.R., D.K. LETOURNEAU, C. SHENNAN, D. SIRRINE, R. FOUCH, L. JACKSON nd A. MAGES 28: Impcts of groundcover mngement systems of yield, lef nutrients, weeds, nd rthropods of trt cherry in Michign, USA. Agriculture, Ecosystems nd Environment 125, SNAPP, S.S. nd H. BORDEN 25: Enhnced nitrogen minerliztion in mowed or glyphosphte treted cover crops compred to direct incorportion. Plnt Soil 27, SRIVASTAVA, H.S. 1992: Multiple functions nd forms of higher-plnt nitrte reductse. Phytochemistry 31, THOMSEN, I.K. 25: Nitrte leching under spring rley is influenced y the presence of ryegrss ctch crop: Results from lysimeter experiment. Agriculture, Ecosystems nd Environment 111, WALSH, B.D., S. SALMINS, D.J. BUSZARD nd A.F. MACKENZIE 1996: Impct of soil mngement systems on orgnic dwrf pple orchrds nd soil ggregtes stility, ulk density, temperture nd wter content. Cn. J. Soil Sci. 76, Received My 17, 21 / Accepted Octoer 4, 21 Addresses of uthors: A.R. Wilson nd P. Nzokou (corresponding uthor), Deprtment of Forestry, Michign Stte University, 126 Nturl Resources Bld., Est Lnsing, MI 48824, nd B. Cregg, Deprtment of Horticulture, Michign Stte University, A214 Plnt nd Soil Sciences Bldg., Est Lnsing, MI 48824, e-mil (corresponding uthor): nzokoup@msu.edu.