NUTRIENT UPTAKE BY HYBRID POPLAR IN COMPETITION WITH WEED SPECIES UNDER GROWTH CHAMBER AND FIELD CONDITIONS USING THE SOIL SUPPLY AND NUTRIENT DEMAND (SSAND) MODEL A Thesis Submitted to the College of Grdute Studies nd Reserch in Prtil Fulfillment of the Requirements for the Degree of Doctor of Philosophy in the Deprtment of Soil Science University of Ssktchewn Ssktoon, Cnd By Bchitter Singh Copyright Bchitter Singh, Jnury, 2008, All rights reserved
PERMISSION TO USE In presenting this thesis in prtil fulfillment of the requirements for doctorte degree from the University of Ssktchewn, I gree tht the Librries of this University my mke it freely vilble for inspection. I further gree tht permission for copying this thesis in ny mtter, in whole or in prt, for scholrly purposes my be grnted by the professor or professors who supervised my thesis in work or, in their bsence, by the Hed of the Deprtment or the Den of the College in which my thesis work ws done. It is understood tht ny copying or publiction or use of this thesis or prts thereof for finncil gin shll not be llowed without my written permission. It is lso understood tht due recognition shll be given to me nd to the University of Ssktchewn in ny scholrly use which my be mde of ny mteril in my thesis. Request for permission to copy or to mke other use of mterils in this thesis in whole or in prt should be ddressed to: Hed of the Deprtment of Soil Science University of Ssktchewn 51 Cmpus Drive Ssktoon, Ssktchewn, Cnd S7N 5A8 i
ABSTRACT Success of hybrid poplr plnttions will rely on the efficient mngement of nutrients nd weeds. Reltively little is known bout the root uptke chrcteristics of hybrid poplr nd weeds, their belowground interctions nd prticulrly, the quntittive understnding of nutrient uptke using mechnistic models under weed-competing conditions. Therefore, the objectives of this study were to investigte the effects of dndelion nd quckgrss on the growth of hybrid poplr, to estblish their root uptke chrcteristics nd to quntify their nutrient uptke using the soil supply nd nutrient demnd (SSAND) model. In pot study, hybrid poplr stem height, root collr dimeter, shoot nd root biomss, root length, nd N, P nd K uptke significntly decresed in the presence of dndelion nd quckgrss weeds. Similr weed competition effects on growth of hybrid poplr were lso observed in the field t the Psture nd Alflf sites where hybrid poplr ws grown with nd without weeds for 50, 79 nd 100 dys. In hydroponic experiment, I mx vlues for NH 4 -N, NO 3 -N, P nd K vried significntly mong hybrid poplr seedlings nd dndelion nd quckgrss weed species nd ws gretest for dndelion followed by hybrid poplr nd then quckgrss. The K m vlues were lowest for quckgrss compred to the other plnt species for ll of the nutrients. Simultion results from the SSAND model for the pot study showed tht N uptke ws underpredicted in hybrid poplr by 58 to 73%, depending upon soil type nd weed tretment. Incorportion of N minerliztion s model input improve the hybrid poplr N uptke predictions by 24 nd 67% in the Psture nd Alflf soil, respectively, when grown without weeds. SSAND model underestimted P uptke by 84-89% nd overestimted K uptke by 28 to 59% for hybrid poplr depending upon the soil type nd weed tretment. In the field, N uptke by hybrid poplr ws in close greement to mesured N uptke in the control tretment. N uptke ws gretly underestimted for both hybrid poplr nd weeds in the weed tretment. Including chnging wter content gretly improves the N uptke by hybrid poplr nd weeds in weed tretments. Results from this study suggest weed control is n essentil prctice to estblish successful hybrid poplr plnttions. Also, SSAND model cn be n effective tool for predicting the nutrient uptke under two plnt species competing environment if ll the processes of nutrient supply re dequtely described in the model. ii
ACKNOWLEDGEMENTS I feel highly privileged to extend my sincere thnks nd deep sense of grtitude to my supervisor Dr Ken Vn Rees for his excellent guidnce, constructive criticism, nd genuine encourgement nd support, which not only shped my ides nd thoughts to fulfill the objectives of this thesis but lso my professionl pproch towrds scientific reserch. I extend sincere thnks to the members of my supervisory committee, Drs Dine Knight nd Drwin Anderson from Deprtment of the Soil Science, Steve Shirtliffe from Deprtment of Plnt Science nd Mrk Johnston from the Ssktchewn Reserch Council for their invluble input. I lso thnk my externl exminer, Dr Michel Kelly from Virgini Tech. I lso thnk Doug Jckson, Mike Steckler, Shwn McDonld nd Ashley Anholt for their help with the field nd lbortory work nd Bill Schroeder of AAFC-PFRA Shelterbelt Centre for the supply of hybrid poplr cuttings. I pprecite the support from the Mistik Mngement for llowing me to use their lnd bse for the field study. I would like to thnk my fellow Soil Science student for their continuous support nd encourgement. I cknowledge the finncil support from the Deprtment of Soil Science nd the Nturl Science nd Engineering Reserch Council of Cnd Strtegic Grnts Progrm. Words becme indequte to express my indebtedness, grtitude nd deep herted respect to my lte grnd mother, my prents nd my younger sister Mnpreet for their morl nd firm support, ever willing help, inspirtion nd constnt encourgement which lwys kept me focused on my objectives while myself being fr wy from my home. Finlly I owe everything of me to the cretor of this universe who wnts me to Mke this body the field, nd plnt the seed of good ctions. Wter it with the nme of the Lord, who holds the entire world in his hnds. Let your mind be the frmer; the Lord shll sprout in your hert (Sri Guru Grnth Shib, Pge 13). iii
TABLE OF CONTENTS PERMISSION TO USE... i ABSTRACT... ii ACKNOWLEDGEMENTS... iii TABLE OF CONTENTS... iv LIST OF TABLES... vi LIST OF FIGURES... viii 1 INTRODUCTION... 1 1.1 References... 4 2 LITERATURE REVIEW... 5 2.1 Plnt Competition... 5 2.1.1 Aboveground nd belowground competition... 5 2.1.2 Competition in mnged ecosystems... 8 2.1.3 Weed competition in hybrid poplr plnttions... 8 2.2 Root Uptke Chrcteristics... 11 2.2.1 Mesurement of nutrient uptke kinetics... 11 2.2.2 Fctor ffecting nutrient uptke kinetics... 12 2.2.3 Estimted vlues of N, P nd K uptke kinetics for tree species nd weeds... 14 2.3 Nutrient Uptke Modeling... 14 2.4 Verifiction of Simultion Models in Tree Species... 21 2.5 References... 23 3 GROWTH OF HYBRID POPLAR AS AFFECTED BY DANDELION AND QUACKGRASS COMPETITION... 32 3.1 Introduction... 32 3.2 Mterils nd Methods... 33 3.2.1 Soils nd soil preprtion... 33 3.2.2 Tretments nd experimentl design... 35 3.2.3 Plnting nd mintennce of pots... 35 3.2.4 Smple collection nd nlyses... 36 3.2.5 Sttisticl nlyses... 37 3.3 Results... 37 3.3.1 Aboveground growth prmeters of hybrid poplr nd weeds... 37 3.3.2 Belowground growth prmeters of hybrid poplr seedling nd weed species... 43 3.3.3 Nutrient uptke by hybrid poplr nd weed species... 47 3.3.4 Soil solution chemistry... 50 3.4 Discussion... 50 3.5 References... 56 4 NUTRIENT UPTAKE KINETICS FOR HYBRID POPLAR AND TWO COMPETITIVE WEED SPECIES... 60 4.1 Introduction... 60 4.2 Methods nd Mterils... 61 iv
4.2.1 Plnt mteril nd growth conditions... 61 4.2.2 Depletion experiment nd uptke kinetics... 62 4.2.3 Processing of plnt mteril... 63 4.2.4 Sttisticl nlyses... 63 4.3 Results... 64 4.3.1 Plnt root nd shoot prmeters... 64 4.3.2 Nutrient uptke kinetics... 64 4.3.3 Plnt tissue nutrient concentrtions... 68 4.4 Discussion... 68 4.4.1 Kinetics of NH 4 -N nd NO 3 -N Uptke... 68 4.4.2 Kinetics of P nd K uptke... 73 4.4.3 Implictions for belowground competition... 73 4.5 REFERENCES... 76 5 NUTRIENT UPTAKE OF HYBRID POPLAR IN COMPETITION WITH WEEDS USING THE SOIL SUPPLY AND NUTRIENT DEMAND (SSAND) MODEL... 80 5.1 Introduction... 80 5.2 Mterils nd Methods... 81 5.2.1 Estimtion of model input prmeters... 81 5.2.2 Nutrient uptke modeling... 84 5.3 Results... 86 5.4 Discussion... 91 5.5 References... 102 6 MODELLING NUTRIENT UPTAKE FOR HYBRID POPLAR AS AFFECTED BY WEED COMPETITION IN THE FIELD... 106 6.1 Introduction... 106 6.2 Mterils nd Methods... 107 6.2.1 Site description nd experimentl design... 107 6.2.2 Mesurements nd smplings... 108 6.2.2.1 Hybrid poplr... 108 6.2.2.2 Weeds... 109 6.2.3 N uptke modeling... 109 6.2.3.1 Soil prmeters... 110 6.2.3.2 Plnt prmeters... 112 6.2.4 Sttisticl nlysis... 112 6.3 Results... 112 6.3.1 Soil moisture... 112 6.3.2 Effects of weeds on hybrid poplr growth... 114 6.3.3 N uptke modeling... 117 6.3.4 Sensitivity nlysis... 120 6.4 Discussion... 124 6.4.1 Hybrid poplr- weeds competition... 124 6.4.2 N uptke predictions for hybrid poplr in control tretment... 124 6.4.3 N uptke predictions under weed competition conditions... 126 6.5 References... 128 7 GENERAL DISCUSSION AND SUMMARY... 130 APPENDIX A... 133 v
LIST OF TABLES Tble 2.1. Hybrid poplr height, dimeter t brest height (DBH), nd wood volume on September 30 ech yer in response to five groundcovers, Mlheur Experiment Sttion, Oregon Stte University, Ontrio, OR (dpted from Shock et l., 2002).... 10 Tble 2.2. I mx nd K m vlues for NH 4 -N nd NO 3 -N in vrious tree species nd weeds. Note the difference in units for reported I mx vlues ( nd b represent µmol cm -2 s -1 nd µmol g -1 h -1, respectively).... 15 Tble 2.3. I mx nd K m vlues for P nd K in vrious tree species nd weeds... 16 Tble 2.4. Nutrient uptke models nd their chrcteristics... 19 Tble 3.1. Physicl nd chemicl chrcteristics of the Psture nd Alflf soils.... 34 Tble 3.2. Effect of weed competition on seedling survivl (%) of hybrid poplr grown with different densities of dndelion nd quckgrss weeds in Alflf nd Psture soils t hrvest-i (47 nd 49 DAP for Psture nd Alflf soil, respectively) nd hrvest-ii (97 nd 105 DAP for Psture nd Alflf soil, respectively). Tretments were SHP= single hybrid poplr, SHP + 1QG = single hybrid poplr + one quckgrss, SHP + 3QG = single hybrid poplr + three quckgrss, SHP + 4D = single hybrid poplr + four dndelion, SHP + 8D = single hybrid poplr + eight dndelion plnts per pot... 38 Tble 3.3. Probbility levels from the three-wy nlysis of vrince for hybrid poplr growth nd nutrient uptke prmeters. Hybrid poplr seedlings were grown for 47 nd 97 dys in the Psture soil nd 49 nd 105 dys in the Alflf soil with different densities of dndelion nd quckgrss weeds. Effect of source of vrition on hybrid growth nd nutrient uptke prmeters is considered significnt t p < 0.05... 40 Tble 3.4. Men hlf-distnce (cm) between ll roots in the tretments grown in the Psture nd Alflf soils t hrvest-i (47 nd 49 DAP for Psture nd Alflf soil, respectively) nd hrvest-ii (97 nd 105 DAP for Psture nd Alflf soil, respectively). Tretments were SHP= single hybrid poplr, SHP + 1QG = single hybrid poplr + one quckgrss, SHP + 3QG = single hybrid poplr + three quckgrss, SHP + 4D = single hybrid poplr + four dndelion, SHP + 8D = single hybrid poplr + eight dndelion plnts per pot.... 46 Tble 3.5. Men shoot concentrtion nd uptke for hybrid poplr grown with different densities of dndelion nd quckgrss weeds in the Alflf nd Psture soils t hrvest-ii (97 nd 105 DAP for Psture nd Alflf soil, respectively). Tretments were SHP= single hybrid poplr, SHP + 1QG = single hybrid poplr + one quckgrss, SHP + 3QG = single hybrid poplr + three quckgrss, SHP + 4D = single hybrid poplr + four dndelion, SHP + 8D = single hybrid poplr + eight dndelion plnts per pot... 48 vi
Tble 4.1. Selected root nd shoot prmeters for plnt species grown in the hydroponic solution experiment... 65 Tble 4.2. Men N, P nd K tissue concentrtions for hybrid poplr, dndelion nd quckgrss t the end of depletion experiment... 71 Tble 5.1. Rnge of soil nd plnt input prmeters used in SSAND model... 85 Tble 5.2: Liner regression of predicted (y) vs observed (x) nutrient uptke for weeds.89 Tble 6.1. Rnge of plnt nd soil input prmeters used for N uptke modeling in field study... 111 Tble 6.2. Shoot nd root growth prmeters of hybrid poplr in control nd weed tretment for the Psture nd Alflf site fter 50, 79 nd 100 dys fter plnting (DAP).... 114 Tble 6.3. Growth nd N uptke by weeds in the weedy tretment t ech hrvest for the Psture nd Alflf site.... 116 Tble 6.4. Liner regression of predicted (y) vs. observed (x) N uptke by hybrid poplr in control nd weedy tretments.... 118 Tble 6.5. Liner regression of predicted (y) vs. observed (x) N uptke by weeds in weed tretment.... 119 vii
LIST OF FIGURES Figure 2.1. Schemtic description of nutrients supply in soil to the root surfce... 18 Figure 3.1.Effect of weed competition on () stem height nd (b) root collr dimeter of hybrid poplr grown in the Psture nd Alflf soils t hrvest-i (47 nd 49 dys fter plnting for the Psture nd Alflf soil, respectively) nd hrvest-ii (97 nd 105 dys fter plnting for the Psture nd Alflf soil, respectively). Tretments were SHP= single hybrid poplr, SHP + 1QG = single hybrid poplr + one quckgrss, SHP + 3QG = single hybrid poplr + three quckgrss, SHP + 4D = single hybrid poplr + four dndelion, SHP + 8D = single hybrid poplr + eight dndelion. Error brs represent one stndrd devition. The brs with the sme letter within ech soil nd hrvest re not significntly different t p 0.05... 41 Figure 3.2. Dry shoot biomss of () hybrid poplr seedlings nd (b) weed species grown in the Psture nd Alflf soils t hrvest-i (47 nd 49 dys fter plnting for Psture nd Alflf soil, respectively) nd hrvest-ii (97 nd 105 dys fter plnting for Psture nd Alflf soil, respectively). Tretments were SHP= single hybrid poplr, SHP + 1QG = single hybrid poplr + one quckgrss, SHP + 3QG = single hybrid poplr + three quckgrss, SHP + 4D = single hybrid poplr + four dndelion, SHP + 8D = single hybrid poplr + eight dndelion. Error brs represent one stndrd devition. The brs with the sme letter within ech soil nd hrvest re not significntly different t p 0.05.... 42 Figure 3.3. Fresh root biomss of () hybrid poplr seedlings nd (b) weed species grown in the Psture nd Alflf soils t hrvest-i (47 nd 49 dys fter plnting for Psture nd Alflf soil, respectively) nd hrvest-ii (97 nd 105 dys fter plnting for Psture nd Alflf soil, respectively). Tretments were SHP= single hybrid poplr, SHP + 1QG = single hybrid poplr + one quckgrss, SHP + 3QG = single hybrid poplr + three quckgrss, SHP + 4D = single hybrid poplr + four dndelion, SHP + 8D = single hybrid poplr + eight dndelion. Error brs represent one stndrd devition. The brs with the sme letter within ech soil nd hrvest re not significntly different t p 0.05.... 44 Figure 3.4. Root length of () hybrid poplr seedlings nd (b) weed species grown in the Psture nd Alflf soils t hrvest-i (47 nd 49 dys fter plnting for Psture nd Alflf soil, respectively) nd hrvest-ii (97 nd 105 dys fter plnting for Psture nd Alflf soil, respectively). Tretments were SHP= single hybrid poplr, SHP + 1QG = single hybrid poplr + one quckgrss, SHP + 3QG = single hybrid poplr + three quckgrss, SHP + 4D = single hybrid poplr + four dndelion, SHP + 8D = single hybrid poplr + eight dndelion. Error brs represent one stndrd devition. The brs with the sme letter within ech soil nd hrvest re not significntly different t p 0.05.... 45 viii
Figure 3.5. Totl N, P nd K (root + shoot) uptkes for hybrid poplr seedling (, c nd e) nd weed species (b, d nd f) grown in the Psture nd Alflf soils t hrvest-i (47 nd 49 dys fter plnting for Psture nd Alflf soil, respectively) nd hrvest-ii (97 nd 105 dys fter plnting for Psture nd Alflf soil, respectively). Tretments were SHP= single hybrid poplr, SHP + 1QG = single hybrid poplr + one quckgrss, SHP + 3QG = single hybrid poplr + three quckgrss, SHP + 4D = single hybrid poplr + four dndelion, SHP + 8D = single hybrid poplr + eight dndelion. Error brs represent one stndrd devition. The brs with the sme letter within ech soil nd hrvest re not significntly different t p 0.05... 49 Figure 3.6. Chnges in soil solution nitrogen nd potssium concentrtion in () Psture nd (b) Alflf soil t hrvest-i (47 nd 49 dys fter plnting for Psture nd Alflf soil, respectively) nd hrvest-ii (97 nd 105 dys fter plnting for Psture nd Alflf soil, respectively) under vrious densities of hybrid poplr seedling nd weed species. Tretments were SHP= single hybrid poplr, SHP + 1QG = single hybrid poplr + one quckgrss, SHP + 3QG = single hybrid poplr + three quckgrss, SHP + 4D = single hybrid poplr + four dndelion, SHP + 8D = single hybrid poplr + eight dndelion. Error brs represent one stndrd devition. The brs with the sme letter within ech soil nd hrvest re not significntly different t p 0.05.... 51 Figure 4.1. I mx nd K m vlues for nitrte nd mmonium uptke by hybrid poplr, quckgrss nd dndelion grown in hydroponic nutrient solution. Error brs represent one stndrd devition. Brs with the sme letter within ech nutrient re not significntly different (p 0.05). Dt with * indictes significnt difference (p <0.05) between nitrte-n nd mmonium-n within ech plnt species... 66 Figure 4.2. I mx nd K m vlues for phosphorus nd potssium uptke by hybrid poplr, quckgrss nd dndelion grown in hydroponic nutrient solution. Error brs represent one stndrd devition. Brs with the sme letter within ech nutrient re not significntly different (p > 0.05)... 67 Figure 4.3. Michelis-Menten curves for () nitrte nd (b) mmonium uptke for hybrid poplr, quckgrss nd dndelion... 69 Figure 4.4. Michelis-Menten curves for () phosphorus nd (b) potssium uptke for hybrid poplr, quckgrss nd dndelion... 70 Figure 5.1. SSAND model predicted nd observed nitrogen uptke by hybrid poplr grown without weed competition, without ( ) nd with ( ) nitrogen minerliztion incorportion in the model, for () Psture nd (b) Alflf soil... 87 Figure 5.2. SSAND model predicted nd observed nitrogen uptke by hybrid poplr grown with weed competition, without ( ) nd with ( ) nitrogen minerliztion incorportion in the model, for () Psture nd (b) Alflf soil... 88 ix
Figure 5.3. SSAND model predicted nd observed phosphorus uptke by hybrid poplr grown () without nd (b) with weed competition for the Psture ( ) nd Alflf ( ) soil.... 90 Figure 5.4. SSAND model predicted nd observed potssium uptke by hybrid poplr grown () without nd (b) with weed competition for the Psture ( ) nd Alflf ( ) soil.... 92 Figure 5.5. Sensitivity nlysis for mmonium uptke by hybrid poplr grown without weed competition in Psture ( nd c) nd Alflf (b nd d) soil. Ech prmeter ws chnged individully while ll other prmeters were held constnt. Results re expressed reltive to mmonium uptke under initil conditions. Abbrevitions were L v = root length density, r o = root rdius, I mx = mximum nutrient influx rte, K m = Michelis- Menten constnt, V o = wter influx rte nd C li = initil soil solution concentrtion, K d = Solid-liquid prtition coefficient... 93 Figure 5.6. Sensitivity nlysis for nitrte uptke by hybrid poplr grown without weed competition in the Psture ( nd c) nd Alflf (b nd d) soil. Ech prmeter ws chnged individully while ll other prmeters were held constnt. Results re expressed reltive to nitrte uptke under initil conditions. Abbrevitions were L v = root length density, r o = root rdius, I mx = mximum nutrient influx rte, K m = Michelis-Menten constnt, V o = wter influx rte nd C li = initil soil solution concentrtion.... 94 Figure 5.7. Sensitivity nlysis for phsophorus uptke by hybrid poplr grown without weed competition in the Psture () nd Alflf (b) soil. Results were similr for hrvest-i nd hrvest-ii simultions. Ech prmeter ws chnged individully while ll other prmeters were held constnt. Results re expressed reltive to phosphorus uptke under initil conditions. Abbrevitions were L v = root length density, r o = root rdius, I mx = mximum nutrient influx rte, K m = Michelis-Menten constnt, V o = wter influx rte nd C li = initil soil solution concentrtion, K d = Solid-liquid prtition coefficient... 95 Figure 5.8. Sensitivity nlysis for potssium uptke by hybrid poplr grown without weed competition in the Psture ( nd c) nd Alflf (b nd d) soil. Ech prmeter ws chnged individully while ll other prmeters were held constnt. Results re expressed reltive to potssium uptke under initil conditions. Abbrevitions were L v = root length density, r o = root rdius, I mx = mximum nutrient influx rte, K m = Michelis-Menten constnt, V o = wter influx rte nd C li = initil soil solution concentrtion, K d = Solidliquid prtition coefficient... 96 Figure 6.1. Chnge in volumetric wter content t the Psture nd Alflf site during the study period... 113 Figure 6.2. Sensitivity nlysis of predicted () mmonium nd (b) nitrte uptke for hybrid poplr grown in control tretment t Psture site. Results re expressed reltive to nutrient uptke under the initil conditions. Ech prmeter ws vried individully while ll other prmeters were held constnt. Prmeters used for sensitivity nlysis re L v x
(root length density), C li (initil soil solution concentrtion), r o (root rdius), V o (wter influx rte), I mx (mximum nutrient influx rte), K m (Michelis-Menten constnt) nd K d (solid-liquid prtition coefficient)... 121 Figure 6.3. Sensitivity nlysis of predicted () mmonium nd (b) nitrte uptke for hybrid poplr grown in control tretment t Alflf site. Results re expressed reltive to nutrient uptke under the initil conditions. Ech prmeter ws vried individully while ll other prmeters were held constnt. Prmeters used for sensitivity nlysis re L v (root length density), C li (initil soil solution concentrtion), r o (root rdius), V o (wter influx rte), I mx (mximum nutrient influx rte), K m (Michelis-Menten constnt) nd K d (solid-liquid prtition coefficient)... 122 Figure 6.4. Sensitivity nlysis of predicted () mmonium nd (b) nitrte uptke for hybrid poplr grown in weed tretment (trends for similr for the Psture nd Alflf site). Results re expressed reltive to nutrient uptke under the initil conditions. Ech prmeter ws vried individully while ll other prmeters were held constnt. Prmeters used for sensitivity nlysis re L v (root length density), C li (initil soil solution concentrtion), r o (root rdius), V o (wter influx rte), I mx (mximum nutrient influx rte), K m (Michelis-Menten constnt) nd K d (solid-liquid prtition coefficient).... 123 xi
1 INTRODUCTION The concept of growing trees on frmlnd is not entirely new nd frmers hve prcticed groforestry for thousnds of yers. This prctice hs been dopted on very limited scle to meet some of the domestic demnds for fuelwood nd timber supply, fruit production, niml fodder, medicinl vlue nd lndscpe nd esthetic vlues (King, 1987). During the pst few yers, however, there hs been growing interest towrds the cultivtion of fst-growing tree species, prticulrly hybrid poplr, on lrge scle due to the incresing demnd for pulp, pper nd other wood products, to relieve the hrvesting pressure on ntive forests, to combt the incresing greenhouse gs emission thret (Liberloo et l., 2006), nd to diversify frm income (Yemshnov et l., 2005). Severl mngement prctices re required to successfully estblish tree plnttions nd, mong them, nutrient mngement is very importnt (Blndier et l., 2006; Thompson nd Pitt, 2003; Wgner et l., 2006). Nutrient mngement cn be ffected by vrious soil nd climtic fctors nd, furthermore, its dvntge to the trees will depend upon how effectively weeds re controlled in the plnttions (Blndier et l., 2006). Weed species competition for nutrients nd moisture in plnttions occurs prticulrly during the initil estblishing yers when tree species do not hve lrge nd deep root systems to ccess resources from deep soil lyers (Nmbir nd Snds, 1993). Therefore, it is very importnt to understnd the mechnisms which ffect nutrient uptke under weed-competing environment in order to mke weed nd nutrient mngement more vible economiclly. Nutrient uptke t the root surfce occurs from the soil solution. Severl soil processes such s buffering power, minerl wethering nd orgnic mtter minerliztion-immobiliztion determine the concentrtion of nutrients in the soil solution (Brber, 1995). Trnsport of nutrients in the soil to the root surfce is controlled by mss flow nd diffusion while uptke t root surfces is considered to 1
follow Michelis-Menten kinetics (Brber, 1995). These fctors cn be integrted into mthemticl models in order to quntify nutrient uptke nd this pproch hs been used successfully in simplified system of one plnt species (Brber, 1995; Rengel, 1993; Silberbush, 2002). However, under plnt competition environment, soil nd plnt fctors interct in more complex wy depending upon the root size nd uptke chrcteristics of competing plnt species. Smethurst nd Comerford (1993) using the COMP8 model predicted the phosphorus (P) nd potssium (K) uptke for slsh pine (Pinus elliottii Engelm. vr. elliottii) nd grss (Pnicum ciculre) which were grown in competition to ech other. They found tht some of the predictions were not ccurte, probbly becuse some of the processes responsible for supplying P nd K in the soil nd uptke t root surfce were not dequtely described in the model (Smethurst nd Comerford, 1993). Here in this study, ttempts re being mde to predict nutrient uptke by hybrid poplr in competition with weed species using the Soil Supply nd Nutrient Demnd (SSAND) model which cn incorporte soil minerliztion input nd chnging soil wter content. For this study, therefore, it ws hypothesized tht growth of hybrid poplr would decrese in the presence of weeds due to the competition for nutrients. The second hypothesis is tht including N minerliztion nd chnging soil wter content s input vribles in the SSAND model will improve predictions of N uptke. These hypotheses would be tested through series of experiments with the following objectives: 1) To investigte the effects of weeds on the growth of hybrid poplr under both growth chmber nd field conditions by exmining the below- nd boveground chrcteristics of hybrid poplr nd weeds. 2) To quntittively describe the N, P nd K uptke chrcteristics of hybrid poplr seedlings, nd dndelion nd quckgrss weed species. 3) To predict nutrient uptke by hybrid poplr in competition with selected weeds by incorporting N minerliztion nd chnging soil moisture content in the SSAND model under growth chmber nd field conditions. 2
This thesis is comprised of six chpters, which provide the detils of vrious experiments conducted to meet the overll objectives nd to test the proposed hypotheses. Chpter 2 is literture review which includes vrious spects nd mechnisms of plnt competition: specificlly, belowground competition, root nutrient uptke chrcteristics nd nutrient uptke modeling. Chpter 3 exmines the competition effects of dndelion nd quckgrss weed species on boveground nd belowground growth of the hybrid poplr s well s nutrient dynmics in soil solution. Chpter 4 is solution culture study tht estblishes the vlues for N, P nd K uptke chrcteristics (I mx nd K m ) of hybrid poplr, dndelion nd quckgrss roots using Michelis-Menten kinetics. Chpter 5 ttempts to predict N, P nd K uptke by hybrid poplr grown without nd with weeds using the SSAND model fter incorporting the dt obtined from Chpters 3 nd 4, nd from some independent mesurements. Chpter 6 dels with the competition effects of weeds on the growth of hybrid poplr under field conditions nd subsequently predicts N uptke using the SSAND model. Chpter 7, finlly, summrizes the slient results from Chpters 3 to 6 to conclude this study in terms of meeting the objectives nd testing the hypotheses nd implictions for future reserch. 3
1.1 References Blndier, P., C. Collet, J.H. Miller, P.E. Reynolds, nd S.M. Zedker. 2006. Designing forest vegettion mngement strtegies bsed on the mechnisms nd dynmics of crop tree competition by neighbouring vegettion. Forestry 79:3-27. Brber, S.A. 1995. Soil nutrient biovilbility - mechnistic pproch. 2nd ed. Wiley, New York, USA. King, K.F.S. 1987. The history of groforestry. p. 1 11. In H. A. Steppler nd P. K. R. Nir (ed.) Agroforestry: A decde of development. ICRAF, Nirobi, Keny. Liberloo, M., C. Clfpietr, M. Lukc, D. Godbold, Z.B. Luos, A. Polle, M.R. Hoosbeek, O. Kull, M. Mrek, C. Rines, M. Rubino, G. Tylor, G. Scrsci- Mugnozz, nd R. Ceulemns. 2006. Woody biomss production during the second rottion of bio-energy Populus plnttion increses in future high CO 2 world. Glob. Chnge Biol. 12:1094-1106. Nmbir, E.K.S., nd R. Snds. 1993. Competition for wter nd nutrients in forests. Cn. J. For. Res. 23:1955-1968. Rengel, Z. 1993. Mechnistic simultion models of nutrient uptke: review. Plnt Soil 152:161. Silberbush, M. 2002. Simultion of ion uptke from the soil. p. 651-661. In Y. Wisel, et l. (ed.) Plnt Roots: The Hidden Hlf. Mrcel Dekker Inc., New York, USA. Smethurst, P.J., nd N.B. Comerford. 1993. Potssium nd phosphorus uptke by competing pine nd grss - observtions nd model verifiction. Soil Sci. Soc. Am. J. 57:1602-1610. Thompson, D.G., nd D.G. Pitt. 2003. A review of Cndin forest vegettion mngement reserch nd prctice. Ann. For. Sci. 60:559-572. Wgner, R.G., K.M. Little, B. Richrdson, nd K. McNbb. 2006. The role of vegettion mngement for enhncing productivity of the world's forests. Forestry 79:57-79. Yemshnov, D., D.W. McKenney, T. Htton, nd G. Fox. 2005. Investment ttrctiveness of fforesttion in Cnd inclusive of crbon sequestrtion benefits. Cn. J. Agric. Econ. 53:307-323. 4
2 LITERATURE REVIEW 2.1 Plnt Competition Competition is n importnt ecologicl process in both nturl nd griculturl plnt communities. Competition cn be boveground nd/or belowground (Grce nd Tilmn, 1990; Wilson, 1988). Aboveground competition is minly described s shoot competition where shoots of competing species compete for light, while belowground competition involves roots of different species growing in the sme volume of soil competing for nutrients nd wter (Csper nd Jckson, 1997; Grce nd Tilmn, 1990; Wilson, 1988). 2.1.1 Aboveground nd belowground competition Severl plnt nd soil fctors cn ffect competition mong plnt species. Aboveground competition is ffected by plnt morphologicl nd physiologicl trits such s lef re, plnt height nd shoot biomss, photosynthetic nd drk respirtion rtes, nd lef nitrogen (N) content (Blndier et l., 2006; Gudet nd Keddy, 1988), which directly or indirectly control plnt s bility to cpture the boveground light resource. Aboveground competition is reltively more importnt when soil resources re in sufficient supply nd roots re less competing (Wilson, 1988). Gerry nd Wilson (1995) reported tht the competitive responses of six plnt species were not influenced by the initil size of the plnts. They concluded initil plnt size my not confer competitive dvntge for light if plnts re limited by soil resources. Wilson (1988) reviewed 23 studies on plnt competition tht involved both below- nd boveground competition nd concluded tht belowground competition ws more intense thn shoot competition. Therefore, it cn be concluded tht in most ecosystems where the supply of soil nutrients nd wter is limiting, belowground competition cn be more importnt thn boveground competition. The importnce of belowground competition lso hs been reviewed comprehensively in the literture for plnts in generl (Cldwell nd Richrds, 1986; Csper nd Jckson, 1997; de Kroon et l., 2003) nd more specificlly 5
for forests nd woodlnds (Coomes nd Grubb, 1998; Nmbir nd Snds, 1993) nd groforestry systems (Schroth, 1998). All of these reviews mintin tht plnts in given ecosystem compete for brod rnge of soil resources, including wter nd ll essentil nutrients. Fctors influencing belowground competition my vry depending upon the soil physicl environment, nd the physiology nd genetics of plnts involved in competition. Vrious belowground plnt trits cn influence the bility of plnts for belowground competition such s root growth rte, root biomss, root rdius, root length density, surfce re, nd rooting depth (Csper nd Jckson, 1997; Schenk nd Jckson, 2002). Such fctors help the plnts in occupying greter volume of soil nd greter ccess to soil nutrients nd soil moisture. The importnce of root ttributes in plnt competition lso incresed under conditions where vilbility of soil resources to the plnt roots is limited by ion mobility, nd the soil processes which govern their mobility. Movement of nutrients to the roots is minly controlled by three processes i.e. root interception, mss flow nd diffusion (Brber, 1995). Root interception, generlly, is considered to be less importnt (Brber, 1995), but its significnce cnnot be ignored under conditions of high root density becuse of the greter physicl ccess of roots to the soil resources. Mss flow of wter, driven by plnt trnspirtion, crries dissolved nutrients to the roots nd the vilbility of nutrients t the root surfce depends upon the rte of wter movement to the roots nd the concentrtion of dissolved nutrients. This process is considered more importnt for the supply of nitrte to plnt roots. Wheres the movement of less mobile nutrients bound to the soil surfce, for exmple potssium nd phosphte ions, is controlled by the diffusion process by creting locl concentrtion grdient. Therefore, the supply of nutrients to plnt roots depends upon diffusion nd mss flow, which occur simultneously. In competition scenrio, plnts with lrge nd extensive root systems hve the dvntge to exploit greter soil resources becuse of greter root surfce re vilble for uptke nd reduced distnce to be trveled by less mobile nutrients to the root surfce. Once nutrients hve reched the root surfce, nutrient uptke by roots cn lso ply role in the competitive bility of plnts through its uptke kinetics. Plnt species with greter vlues of I mx (mximl influx t high concentrtion) nd lower K m 6
(concentrtion where influx rte is hlf of the I mx ) certinly will hve the competitive dvntge over the other competing plnt species, when the supply of nutrients from the soil is non-limiting (Aerts, 1999; Jungk nd Clssen, 1997). Also, the dvntge of superior kinetic prmeters for plnt will lso depend upon the mount of roots present for tht species in competing environment becuse these prmeters often re expressed on per unit root surfce re, root length or root weight bsis (Jungk nd Clssen, 1997). Therefore, competitive dvntge of superior kinetic prmeters for given plnt species my be msked in the presence of other plnt species which hve reltively lrger root system. Allelopthy is nother indirect mechnism of belowground plnt competition in which the growth of one plnt species is inhibited by nother plnt species through the relese of toxic chemicls from the roots (Grce nd Tilmn, 1990; Norby nd Kozlowski, 1980; Obid nd Qsem, 2005; Wrdle et l., 1998). Severl weed species re known to relese toxic llelopthic chemicls (Qsem nd Foy, 2001). The bility of quckgrss (Elymus repens) to produce llelopthic chemicls (Hgin, 1989; Korhmmer nd Hslinger, 1994; Schulz et l., 1994) nd dversely ffect the growth of other plnt species growth hs been reported (Kommedhl et l., 1957; Schulz et l., 1994). Likewise, llelopthic effects of certin nnul species on tree growth hve lso been reported by Smith et l. (2001). They reported tht tll fescue (Festuc rundince), bermudgrss (Cynodon dctylon) nd cutlef evening primrose (Oenother lcinit) lechte decresed pecn trunk weight by 22%, root weight by 17%, nd totl tree dry weight by 19%, compred to the control tretment. Mycorrhize cn lso ffect plnt competition, lthough the role tht mycorrhize ply in plnt competition is complicted nd needs to be understood more clerly (Allen nd Allen, 1990; Kernghn, 2005; Newmn, 1988; Wtkinson nd Freckleton, 1997). Mycorrhize hve been shown to increse plnt competition between plnt species (West, 1996). In growth chmber study, Mrler et l. (1999) reported tht the growth of Festuc idhoensis while competing with Centure mculos significntly decresed in the presence of rbusculr mycorrhizl fungi nd the growth of F. idhoensis ws 171% greter when rbusculr mycorrhizl fungi were not present. 7
In contrst, mycorrhize hve lso been shown to help in the nutrient shring between the competing plnt species thereby indirectly decresing plnt competition (Newmn, 1988). Mycorrhize my potentilly increse the pool of belowground resources by cpturing the quntities of inccessible nutrients tht would otherwise be unvilble to competing plnt species (Allen nd Allen, 1990). 2.1.2 Competition in mnged ecosystems In nturl ecosystems, the composition of plnt communities is minly decided by the competing bility of plnts for the existing resources in tht system (Grce nd Tilmn, 1990). However, in griculturl or mnged plnt systems, competition cn be ltered in fvour of the desired plnt species of economic importnce by eliminting unwnted plnt species (weeds) from the system by vrious methods. The presence of weeds in griculturl systems cn cuse economic losses through direct reductions in crop yields, cost of control nd reduced crop qulity (Zimdhl, 2004). Similr dverse effects of weeds hve lso been identified in forests nd intensively mnged tree plnttion systems (Blndier et l., 2006; Thompson nd Pitt, 2003; Wgner et l., 2006). Therefore, in mnged ecosystems, competition is often described s weed competition becuse of their negtive effects on the growth of the desired plnt species. Weeds re mjor impediment to the development of griculturl crops nd tree species which ffect their growth through vrious mechnism of below- nd boveground competition s described bove. 2.1.3 Weed competition in hybrid poplr plnttions Roots of 1-yr-old poplr trees cn spred horizontlly up to 2.7 m (Friend et l., 1991) nd more thn 60% of the totl root mss of 4-yr-old hybrid poplr clones occurred in the upper 0.36 m of the soil (Heilmn et l., 1994), which demonstrtes the shllow rooting nture of young poplr trees. Becuse of their shllow rooting, hybrid poplr encounters severe competition from weeds for soil resources, such s nutrients nd moisture, which ultimtely ffects the boveground tree biomss production. Poplr growth is sensitive to the mount of weed-free re round the tree, nd controlling competing vegettion is key to successful poplr estblishment nd production (Buhler et l., 1998; Heilmn et l., 1995). Nitrogen is nutrient tht often limits the 8
growth of young poplr trees (Hnsen et l., 1988) nd weeds substntilly decrese the growth of poplr trees primrily through competition for N present in soil solution or vilble from fertilizers (Mclughlin et l., 1987). Filure to control weed competition will result in high tree mortlity nd growth reduction of >50% for surviving trees (Hnsen nd Netzer, 1985). Buhler et l. (1998) observed n increse in the height nd dimeter of stem, nd number of leves from 1 August to 6 September by 73, 104 nd 67%, respectively, where there ws continuous weed control. In comprison, trees with poor weed control grew less during the sme period (37, 45 nd 34% for the sme growth trits). In nother study in southwestern Michign, USA, Mrino et l. (1998) reported tht in the presence of weeds, poplrs were significntly shorter, hd smller bsl dimeter, initited fewer shorter brnches, nd hd fewer living brnches tht spred outwrd compred to the weed controlled trees. In nursery study t three different sites, Sixto et l. (2001) observed tht poplr tree height decresed 19 to 56% for non-weeded plots compred to the weeded plots during the first growing seson, depending upon the sites nd method of weed control used. In some groforestry systems, intercropping or use of ground cover crops is considered to be n effective mens for controlling weeds during estblishment of some woody crop plnttions including hybrid poplr plnttions (Willims nd Gordon, 1992). Intercropping cn lso generte short-term return from n herbceous griculturl crop during the erly nd unprofitble yers in tree plnttions (Burgess et l., 1996; Willims nd Gordon, 1992). Furthermore, groforestry hs become incresingly populr in res previously dominted by griculturl crops (Willims nd Gordon, 1992). However, studies with woody crop plnttions (Nmbir nd Snds, 1993) nd orchrds (Hogue nd Neilsen, 1987) hve shown the negtive effects of cover crops on tree growth, especilly in the first few yers fter estblishment. In three-yer study with hybrid poplr, Shock et l. (2002) reported tht intercropping or use of ground cover crops significntly reduced the tree height, dimeter t brest height nd wood volume (Tble 2.1). They concluded tht the first two yers of poplr plnttion growth re very criticl nd tht plnttions must be free from the weeds or groundcover in order to ensure helthy stnd. They observed tht the wood volume for 9
Tble 2.1. Hybrid poplr height, dimeter t brest height (DBH), nd wood volume on September 30 ech yer in response to five groundcovers, Mlheur Experiment Sttion, Oregon Stte University, Ontrio, OR (dpted from Shock et l., 2002). 10 Groundcover Tree Height (m) DBH (cm) Volume (m 3 h -1 ) 1997 1998 1999 1997 1998 1999 1997 1998 1999 Bre soil 2.94 5.97 8.61 1.9 7.3 11.1 0.18 4.72 15.6 Mowed weeds 2.28b 5.16bc 8.82 1.1c 5.1c 9.2b 0.05c 2.08c 11.6b Alflf 2.05b 4.47c 6.96b 1.0c 4.3c 7.0c 0.04c 1.41c 6.0c Whet 2.31b 5.10bc 8.10 1.2c 5.3c 9.1b 0.06c 2.21c 10.5b Sqush 2.71 5.58b 8.85 1.5b 6.1b 10.4 0.10b 3.18b 14.6 Within columns, mens followed by the sme letter re not significntly different ccording to Duncn s Multiple Rnge test. Whet ws not plnted in 1999, nd the plots were treted s mowed weeds tretment. Sqush ws not plnted in 1999, nd the plots were treted s bre-soil tretment.
the bre-soil plots ws 34% greter thn mowed weed plots, 88% greter thn whet plots, nd 260% greter thn the lflf plots by the end of the second yer. 2.2 Root Uptke Chrcteristics Mobility of nutrient ions in the soil to the root surfce is controlled minly vi mss flow nd diffusion processes. The bsorption of nutrients from soil solution by roots, once they rech the root surfce, is ffected by root prmeters such s rdius, density nd surfce re (Jungk, 2002) nd most importntly, by the physiologicluptke chrcteristics of the root which regulte the entry of nutrient ions t the root surfce (BssiriRd et l., 2000; Clrkson, 1985). 2.2.1 Mesurement of nutrient uptke kinetics Nutrient uptke kinetics re mesured t different externl ion concentrtions to determine ion influx t the root surfce. Usully, ion influx increses with incresing ionic concentrtion in the externl solution until sturtion kinetics re reched nd ion influx rte becomes independent of externl solution ion concentrtion. This reltionship of ion influx s function of externl solution ion concentrtions cn be described by Michelis-Menten kinetics by the following eqution (Nielsen nd Brber, 1978): I mx (C - C min ) I n = Km + (C - C min ) [2.1] -2 - where I n (µmol cm s1) is the nutrient ion influx, I mx (µmol cm -2 s -1 ) is the mximl nutrient influx rte t high concentrtions C, K m (µm) is the Michelis Menten constnt nd is the nutrient concentrtion where influx is 0.5 * I mx, nd C min (µm) is the nutrient concentrtion below which influx ceses (i.e., influx = efflux). There re number of methods which llow for the determintion of the Michelis-Menten kinetic prmeters for nutrient uptke. These methods involve using either excised or intct roots for nutrient uptke under rnge of externl nutrient solution concentrtions. For the excised root methods, root segments re plced into bthing solutions supplied with rdioctive trcers nd uptke is estimted from the ccumultion of the trcer in the root tissues (Epstein, 1972; Shock nd Willims, 1984). Use of this technique cn be ppeling due to the esy hndling of smples nd greter 11
repetition of experiments within short time period, but lekge of solute from the cut root end (Bryce nd Ap Rees, 1985) my cuse the uptke kinetic nlysis to be less ccurte. This limittion cn be overcome by mesuring the nutrient uptke kinetics by growing intct root systems in hydroponic solutions using the depletion method (Clssen nd Brber, 1974) or growing intct root systems in solutions where nutrient concentrtions re kept constnt during the growing period (Mullins nd Edwrds, 1988; Wild et l., 1979). The ltter pproch cn be criticized becuse the concentrtion of nutrients in the soil solution under field conditions does not remin constnt. Therefore, the depletion method my provide more relistic description of nutrient uptke kinetics in the field. In the depletion method, intct roots re llowed to deplete the nutrient from solution of known concentrtion over period of time for mesuring the uptke kinetics (Clssen nd Brber, 1974). Kinetic prmeters mesured by the depletion procedure hve been used successfully to predict nutrient uptke by plnts growing in the soil (Brber, 1995). 2.2.2 Fctor ffecting nutrient uptke kinetics Severl plnt nd environmentl fctors cn ffect nutrient uptke kinetics. Plnt fctors include crop vriety or crop cultivr, ge of roots or plnt nd the nutritionl sttus of the plnt. Environmentl fctors include the composition of the nutrient solution, rooting temperture nd the durtion of uptke experiment (Clrkson, 1985; Le Bot et l., 1998). Vritions in uptke kinetics prmeters mong different crop cultivrs hve been observed for NO 3 -N in red mple (Acer rubrum) tree seedlings (Kelly et l., 2000), whet (Triticum estivum) (Rodgers nd Brneix, 1988) nd potto (Solnum tuberosum) (Shrifi nd Zebrth, 2006), nd for P in whet (Egle et l., 1999), corn (Ze mys) (Nielsen nd Brber, 1978) nd spring brley (Hordeum vulgre) (Romer nd Schenk, 1998). Kelly et l. (2000) determined the NO 3 -N uptke prmeters for red mple in hydroponic system using two cultivrs tht differed in root production nd wter use efficiency. Estimtes of I mx vried between experiments with mens rnging from 1.5 x 10-5 to 5.9 x 10-5 µmol cm -2 s -1. Mens for K m rnged from 2.04 x 10-5 to 5.23 x10-5 µmol cm -3, while C min vlues were consistent t 0.001 µmol cm -3 (the limit of nlyticl detection) cross ll experiments. 12
Plnt ge lso ffects the nutrient uptke kinetic prmeters of plnt species. Nutrient influx ws observed to decrese with incresed plnt or root ge (Bht et l., 1979; Shrifi nd Zebrth, 2006; Wild nd Breeze, 1981). The reduction in clculted I mx s the plnts mtured my be ttributed to reduced uptke rte by older roots (Br- Yosef nd Kfkfi, 1971) nd/or to reduced proportion of the root system ctive in uptke (Robinson et l., 1991) nd lso to decresed demnd per unit root length due to incresed root length with plnt ge (Kuhlmnn nd Brrclough, 1987). Uptke kinetics of root cn vry in plnts which re grown with different nutrition or in plnts of different nutritionl sttus (Drew et l., 1984; Glss, 1977; Jungk et l., 1990; White, 1973). In study on soyben nd mize grown with different P nutrition to obtin plnts with different P sttus, Jungk et l. (1990) found tht I mx declined drsticlly with incresing % P in the plnt tissues, while K m did not chnge much. Similrly, inverse reltionships between the nutrient concentrtion in the root nd I mx vlues were found by Siddiqi et l. (1990) for NO 3 -N nd Glss (1977) for K. Adm et l. (2003) investigted the influence of root-zone temperture on the kinetics of net NO 3 -N uptke by using solution-grown "Autumn Flme" nd " Frnksred" (Red Sunset) rmets s representtives of red mple (Acer rubrum). Averged over both cultivrs in ll experiments, I mx estimtes were 120, 150, nd 170 nmol m -2 s -1 for the root-zone tretments tht hd tempertures of 14, 24, nd 34 C, respectively. Vlues of K m incresed with root-zone temperture nd verged 88, 140, nd 190 µm wheres C min decresed nd verged 66, 38, nd 18 µm for the 14, 24, nd 34 C tretments, respectively. They concluded tht it ws necessry to ccount for root- zone temperture when estimting NO 3 -N uptke. Kinetic prmeter vlues re lso ffected by the depletion curve obtined from the hydroponics depletion method (Clssen nd Brber, 1974). Use of prtil depletion curves could led to the erroneous kinetic prmeters nd subsequently erroneous uptke predictions for the nutrient of interest (Vn Rees, 1994). Therefore, it is lwys importnt to deplete the nutrient from the soil solution to sufficiently low concentrtion in order to obtin the correct kinetic prmeter vlues. Decresing the concentrtion in solution to some low extent is lso useful for clculting the C min where the net nutrient influx is zero (Bht, 1981). 13
2.2.3 Estimted vlues of N, P nd K uptke kinetics for tree species nd weeds Vrious studies hve been conducted to mesure the uptke kinetics for forest nd groforestry plnttion tree species nd some prllel informtion is lso vilble for the weed species which coexist in tree plnttion environments (Tble 2.2 nd Tble 2.3). Lrge vritions re observed in uptke kinetic vlues for NH 4 -N, NO 3 -N, P nd K mong the tree species due to one or severl resons s explined bove in Section 2.2.2. Comprison of kinetic prmeters between plnt species is sometimes difficult becuse the units for the uptke kinetic prmeters vry from study to study. Bsed upon the literture, vlues for root uptke chrcteristics for some tree nd weed species re reported in Tble 2.2 for NH 4 -N nd NO 3 -N nd in Tble 2.3 for P nd K. 2.3 Nutrient Uptke Modeling Avilbility of nutrients in the soil nd their cquisition by plnts is function of complex biotic nd biotic proce sses ssocited with the soil nd plnt chrcteristics, pst mngement prctices nd current nutrient ddition (Brber, 1995). Soil nd plnt ssocited fctors my interct in vrious wys in order to supply the nutrients to the plnt. In such complex intercting environment, therefore, it is difficult to ssess experimentlly the role of individul fctors in plnt nutrition. In order to understnd the complexity of these systems nd to evlute the role of individul soil nd plnt fctors in plnt nutrition, mechnistic nutrient uptke models re needed. The underlying processes which govern the supply of nutrients in the soil to the root surfce nd subsequent uptke hve been integrted into mechnistic nutrient uptke models (Rengel, 1993). To dte, vrious mechnistic nutrient uptke models hve been developed nd tested (Brber nd Cushmn, 1981; Reginto et l., 2000; Smethurst nd Comerford, 1993; Tinker nd Nye, 2000; Yni, 1994). This pproch hs been used very successfully in vriety of conditions nd for vriety of plnt species (Brber, 1995; Tinker nd Nye, 2000). The supply of soil nutrients to the root surfce using solutetrnsport theory nd the use of Michelis-Menten kinetics to describe the uptke t root surfce re common processes described in ll uptke models (Rengel, 1993; Silberbush, 2002). Mss flow nd diffusion re the two mjor soil processes which re included in the 14