Effects of agricultural land uses on Phosphorus fractions and aggregation of wetland soils in East Africa

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1 Effects of griculturl lnd uses on Phosphorus frctions nd ggregtion of wetlnd soils in Est Afric H. Kmiri 1, 2, H. Richrts 2, C. Kreye 2 nd M. Becker 2 1 Krtin University, School of Agriculture nd Biotechnology. P.O Krtin Keny 2 Dep of Plnt Nutrition, Institute of Crop Science nd Resources Conservtion (INRES), University of Bonn, Krlroert- Kreiten-Str. 13, Bonn, Germny Astrct Conversion of nturl wetlnds to cropped lnd cn hve significnt impcts on the trnsformtion of soil ggregtes nd ssocited nutrients, including soil phosphorus (P). However, little is known out the chnges in P frctions in wetlnd systems, or the fte of ggregte frctions due to conversion of unused wetlnd res to cropped systems. We hypothesized tht conversion of nturl wetlnd systems will result to chnges in the size of P pools nd ggregte frctions ssocited with SOM nd tht these chnges will e determined y the soil type nd lnd use. Our ojectives were to evlute the effects of lnd use chnges on: - (1) the proportions of soil P frctions; (2) the size nd proportion of wter stle ggregtes in two different wetlnds in Est Afric. The wetlnds comprise n inlnd vlley in the highlnds-krtin (1700 msl) nd floodplin in the lowlnds-korogwe (320 msl). Lnd use tretments included the nturl unused sites which were dominted y Cyperus spp; the cropped fields under neroic nd eroic soil conditions nd ndoned sites under fllow vegettion. Results indicted tht conversion from nturl systems to eroic, neroic or ndoned fllow ffected soil P frctions nd the stility of soil ggregtes. The floodplin soils responded sensitively to intensified lnd use with pronounced decreses in soil ggregte stility, the vilility of lile P-frctions, nd the supplying cpcity of stle P-frctions. The inlnd vlley showed higher contents of ville P nd stle P, while the stle P stocks in the floodplin were pprently depleted quickly. This my imply tht the conversion of wetlnd systems to cropped systems could hve significnt ltertions of soil P frctions nd the ggregte frctions ssocited with orgnic mtter unless strtegies to increse the orgnic mtter level nd hence improved ggregtion nd minerlizle nutrients re put into considertion. Key words: lile P, orgnic mtter, stle P, wetlnds soil. Introduction Aggregte frctions ssocited with soil orgnic mtter (SOM) nd phosphorus (P) pools re likely to chnge with chnges in lnd use or with the intensifiction of cropping in wetlnd res. A reduction in SOM content, which reflects high rte of ggregte turnover, cn e due to fster SOM minerliztion nd oxidtion of soil orgnic cron, smller quntities of orgnic inputs, nd / or more esily decomposed orgnic inputs in mnged systems s compred to nturl systems (Brdy nd Weil, 2002). In wetlnd systems, drying nd wetting of wetlnd soils induces oxidtion nd leds to irreversile chnges in its SOM frctionl composition. The soil ggregte sizes ssocited with orgnic mtter include the mcro-ggregtes (>0.25mm dimeter) nd the micro-ggregtes (<0.25mm dimeter). Chnges in these frctions sizes re likely to result to chnges in the SOM content nd hence the minerlizle soil nutrients. Rewetting of the drined soils is ccompnied y enhnced relese of P due to chnges in redox potentil of P dherent chemicl elements such s Fe (Kirk, 1984). Sequentil P nlysis cn revel chnges in P forms which depend on soil type, climtic conditions nd crop mngement prctices. The sequentil chemicl extrction procedures hve een nd still re widely used to divide extrctle soil P into different inorgnic nd orgnic frctions (Tiessen nd Moir, 1993). Severl studies hve lso relted these different P frctions to plnt growth nd nutrient uptke (Guo et l., 2000) Octoer Nkuru, Keny.

2 Though much reserch hs een done on plnt ville P nd totl P frctions in cultivted soils in the study regions (Ndkidemi nd Semok, 2006) little informtion exist on how soil P frctions chnge under wetlnd conditions when lnd uses re chnged. Thus, this study demonstrte how lnd use chnges ffect the size of ggregte frctions nd thus the orgnic mtter nd nutrient (P) relese nd cycling. We used sequentil P extrction method to determine potentil differences in the form nd quntity of different inorgnic nd orgnic soil P frctions in two wetlnd soils (inlnd vlley cly soil nd floodplin sndy cly) s ffected y lnd use chnges from uncultivted wetlnd res to cropped fields nd fllow plots). In ddition y crrying out ggregte stility determintion, the ggregte frctions most ffected y conversion of nturl wetlnd systems to res of cultivtion re explored. Mterils nd methods Study res The study re comprised two wetlnds in Est Afric differentited y climtic conditions. Thus, floodplin in the hot nd humid lowlnds of Korogwe in Tnzni nd n inlnd vlley in the cool highlnds of Krtin, Centrl Keny were selected. Detiled descriptions of the studied wetlnd environments re presented in Tle 1. The floodplin in the lowlnd lies t n ltitude of 320 m. The soils re minly the sndy clys (Fluvisols) nd some pockets of Vertisols. Annul rinfll verges less thn 700 mm with verge dily temperture of 35 C. The site is clssified s dry Svnnh (FAO, ) nd dominted y plm grdens, rinfed nd irrigted rice. The inlnd vlley in the highlnds of centrl Keny is locted t n ltitude of 1700m with n nnul verge rinfll of 1450 mm nd verge dily temperture of 23 C. The region flls under the Forest zone nd dominted y susistence frming of mize, ens, vegetles nd tro (Colocsi esculent). Soil types re minly cly nd some cly loms (Jetzold et l., 2006). Lnd use selection nd soil smpling The selected lnd uses comprised (i) the Nturl or unused wetlnd fields dominted y nturl wetlnd vegettion of Cyperus ppyrus in the lowlnd floodplin nd Cyperus exlttus in the highlnd inlnd vlley; (ii) the Aneroic cropped fields under permnent to sesonl soil wetness grown to crops such s tro in the inlnd vlley nd rice in the lowlnd floodplin; (iii) Aeroic fields under sesonlly wet soil moisture nd grown to field crops such s vegetle, mize nd ens; nd (iv) Fllow plots which comprised lnd previously under cultivtion ut now ndoned or under grzing. Tle 1: Description of the selected study wetlnds in Est Afric - floodplin in Korogwe Tnzni nd n inlnd vlley in Centrl Keny Chrcteristic Floodplin-Korogwe Inlnd vlley- Krtin Loction Pngni plins-tnzni Centrl Keny Longitude ( 0 ) '28"E '57"E Ltitude ( 0 ) '29"S '58"S Altitude msl Annul rinfll (mm) < Temperture ( C) AEZ 1 Dry svnnh Forest Length of growing period (dys) Soil order 2 Fluvisols, Vertisols Gleysols-Fluvisols Prent mteril Alluvil sediments Volcnic Texturl clss Sndy cly, cly cly, cly lom 1 : FAO, ( ); 2 : FAO-UNESCO, (1997) 2

3 The cultivted fields were mnged for rice or vegetle production for the lst 5-10 yers. Use of inorgnic fertilizers ws linked to cropped fields in eroic nd neroic lnd uses. Fertilizer ppliction ws generlly irregulr with rtes rnging from 20 to 42 kg N nd 10 to 25 kg P h -1 yer -1 (Jetzold et l., 2006; MOA-URT, 2006). The commonly used fertilizers were ure, triple super phosphte (TSP) nd di-mmonium phosphte. Fllow periods t the study sites rnged from 2 months to severl yers. These fllow sites were dominted y grss nd weeds such s Glinsog spp, Chenopodium spp nd Echinoclo spp. The fllow sites hd een previously cultivted for more thn 10 yers nd lter ndoned due to poor productivity. The cultivted fields were mnged for rice or vegetle production for the lst 5-10 yers. Use of inorgnic fertilizers ws linked to cropped fields in eroic nd neroic lnd uses. Fertilizer ppliction ws generlly irregulr with rtes rnging from 20 to 42 kg N nd 10 to 25 kg P h -1 yer -1 (Jetzold et l., 2006; MoA-URT, 2006). The commonly used fertilizers were ure, triple super phosphte (TSP) nd di-mmonium phosphte. Fllow periods t the study sites rnged from 2 months to severl yers. These fllow sites were dominted y grss nd weeds such s Glinsog spp, Chenopodium spp nd Echinoclo spp. The fllow sites hd een previously cultivted for more thn 10 yers nd lter ndoned due to poor productivity. The cultivted fields were mnged for rice or vegetle production for the lst 5-10 yers. Use of inorgnic fertilizers ws linked to cropped fields in eroic nd neroic lnd uses. Fertilizer ppliction ws generlly irregulr with rtes rnging from 20 to 42 kg N nd 10 to 25 kg P h -1 yer -1 (Jetzold et l., 2006; MOA-URT, 2006). The commonly used fertilizers were ure, triple super phosphte (TSP) nd di-mmonium phosphte. Fllow periods t the study sites rnged from 2 months to severl yers. These fllow sites were dominted y grss nd weeds such s Glinsog spp, Chenopodium spp nd Echinoclo spp. The fllow sites hd een previously cultivted for more thn 10 yers nd lter ndoned due to poor productivity. Three replictes of the sme lnd use were selected within minimum distnce of 100 m from ech other. In ech lnd use type nd repliction, su plot of 10m * 20 m ws demrcted nd top soil ws smpled from ± 0-15 cm depth. In totl, 9 uger holes were smpled in W shpe in ech su plot using hnd-held uger, 5-cm in dimeter. The composite smples were ir-dried, ground nd sieved to pss through 2mm sieve for susequent soil nlysis. Generl chrcteristics of the soils from the wetlnd lnd uses re presented in Tle 2. Tle 2: Generl chemicl properties of the whole soil under different lnd uses in the floodplins nd inlnd vlleys (smpled during lnd use selection period) Soil chrcteristics Lnd use ph Floodplin Totl C (g kg - 1 ) Totl N (g kg - 1 ) Aville P (mg kg - 1 ) Nturl Aneroic Aeroic Fllow Inlnd vlley Nturl Aneroic Aeroic Exchngele K (cmol kg -1 ) 3

4 Fllow : Olsen P Soil nlysis Phiosphorus frctiontion. Sequentil P frctiontion ws crried out using the modified Hedley procedure (Tiessen nd Moir, 1993). Sequentil P procedure uses series of incresingly ggressive extrctnts to remove lile inorgnic nd orgnic P (P i nd P o) pools followed y more stle orgnic nd inorgnic P forms. At ech stge, ound P ws ssumed to e removed selectively from specific types of compounds contined in the soil. In order to verify the P content of the frctions, it ws necessry lso to determine totl P for ech smple. The following P frctions were determined sequentilly: Resin-P, 0.5M NHCO 3 extrctle P, 0.1M NOH extrctle P, dil. HCl nd conc. HCL. The frctions recovered y this procedure correspond to the following soil P pools (Hedley et l., 1982): (i) Resin Pi: form of soil inorgnic P from which plnts normlly drw their supply; (ii) Bicronte-Pi: extrcts dditionl inorgnic P tht is ville to plnts which is dsored on surfces of crystlline compounds; (iii) NOH-Pi: prtilly dissolves Fe nd Al phosphtes nd desors Pi from the surfces of sesquioxides nd (iv) HCL-Pi: stle P or occluded P held t the internl surfces of soil ggregtes nd is thought to e ville only on long-term sis. The P frctions were then grouped to (i) stle pool comprising the NOH extrctle P, the dil. HCl nd the conc. HCL nd (ii) lile P frctions of the resin nd NHCO 3 -P. Aggregte frction determintion y wet sieving. Soil ggregte size clsses were ssessed y wet sieving technique nd the mount of the mcro nd micro ggregte frctions determined s percentge of the initil soil weight (Tisdll nd Odes, 1982). Air dried soil (<2 mm dimeter) ws seprted into four size frctions thus, the lrge mcro-ggregtes (>0.5mm dimeter), smll mcroggregtes (0.25mm-0.5 mm dimeter), micro-ggregtes ( mm dimeter), nd silt + cly ssocited prticles (<0.053 mm dimeter). Sttistic nlysis. A rndomized experimentl design ws utilized with four lnd uses, two wetlnd types nd three field replictions. A one wy ANOVA model ws used to determine differences etween individul frctions in lnd uses. Significnt tretment comprisons were sed on Tukey test t P=0.05 (Brymn nd Crmer, 2001). Results The results of this study portryed tht wetlnd conversion from nturl system to cropped fields highly ltered the soil ggregte frctions nd the mounts nd proportion of lile nd stle soil P pools compred to the initil nturl wetlnd system. Effect of lnd use chnges on soil P frctions Both floodplin nd inlnd vlley soils showed low contents of esily ville P (resin- nd NHCO 3 extrctle) nd high contents of stle P (NOH-P nd HCl-P). Lile P pools. In the floodplin soils, the lile frctions responded sensitively to intensive cultivtion nd the stle supply of P of the NOH frction ws lso reduced. The mount of ion-exchnge resinextrctle P rnged etween 33 mg P kg -1 nd 189 mg P kg -1, while the mount of NHCO 3 extrctle P ws lower etween 2 mg P kg -1 nd 66 mg P kg -1 (Figure 1). In the inlnd vlley soils the chnges were less distinct, ut soils from intensive cultivtion showed higher P mounts thn the uncultivted soil in the nturl system. The mounts of resin-extrctle P rnged from mg P kg -1 while NHCO 3-extrctle P rnged etween 35 mg P kg -1 nd 202 mg P kg -1. Stle P pools. The stle pool comprised the NOH nd the HCl extrcted P. The NOH pool contined more P thn ny other frction in the two wetlnd soils (Figure 1) nd ws highest in the inlnd vlley nd lowest in the floodplin. This pool ws differentited etween the lnd uses wherey 4

5 it ws highest in the nturl nd the cultivted plots. The gretest impct on the stle frctions ws oserved under eroic cultivtion in the drined vegetle soils. The NOH-P rnged etween 143 to 1442 mg P kg -1 in the floodplin nd 959 mg P kg -1 in the nturl to 1629 mg P kg -1 in the neroic cultivtion plots in the inlnd vlley. The very stle or highly occluded P (HCl extrcted P) ws high in ll the lnd uses nd rnged etween 428 to 567 mg P kg -1 in the inlnd vlley nd from 108 to 399 mg P kg -1 in the floodplin soil. Totl P nd P stocks. The totl mounts of P showed differences etween floodplin nd inlnd vlley, with higher mounts in the uncultivted soils in the floodplin nd reverse trend in the inlnd vlley, indicting effects of soil mendments under intensive cultivtion. Soil P stock vried etween the wetlnds nd the lnd use types. The P stock ws lmost twice s much in the inlnd vlley (rnged etween 900 to 1700 g P m -2 ) thn in the floodplins (Figure 1). The unused nd ndoned res hd higher nd significntly different P stocks (P < 0.05) compred to cropped fields in the floodplins while in the inlnd vlleys, the cropped sites hd on verge higher P stocks. () Lile nd stle P frctions Phosphorus frctions (mg P kg -1 ) Floodplin NOH-P Dil. HCl-P Conc. HCl-P Resin-P NHCO 3 -P Inlnd vlley 0 Nturl Aneroic Aeroic Fllow Nturl Aneroic Aeroic Fllow () P stocks 5

6 1800 Floodplin soil Inlnd vlley soil Phosphorus stock (g P m -2 ) NoH-P Dil. HCL-P Conc. HCL-P Resin-P NHCO 3 -P 0 Unused Aneroic Aeroic Andoned Unused Aneroic Aeroic Andoned Figure 1: Effects of lnd use types on soil P frctions (lile nd stle) nd P stocks in wetlnd soil from floodplin nd inlnd vlley wetlnd in Est Afric. Lel points followed y sme letter (s) shows tht the totl P is not significntly different etween the lnd use types y Tukey test (p=0.05) Phosphrus frctions s percentge of totl P. As percentge of totl phosphorus, the lile phosphorus in resin nd icronte extrcts constituted less 30% of the totl phosphorus pool in ll the lnd uses (Figure 2). The eroic cultivted plots hd the highest shre of the lile pool (26% nd 21% in the floodplin nd inlnd vlley respectively) in reltion to the totl P pool. The nturl system nd the fllow sites hd the highest shre of the stle pools comprising the NOH nd the HCL inorgnic frctions nd were highest in the inlnd vlley nd lowest in the floodplin (rnge from 87% to 93%). P frctions percentge of Totl P (%) Floodplin Nturl Aneroic Aeroic Fllow Inlnd vlley Stle P Lile P Nturl Aneroic Aeroic Fllow Figure 2: Distriution of lile nd stle phosphorus pools s frction of the totl P s ffected y lnd use chnges in wetlnd soils of Est Afric 6

7 Effect of lnd use chnges on the proportion of ggregte frctions Chnges in lnd use significntly ffected the proportion of ggregtes cross the wetlnd soils nd within the lnd uses (Figure 3). The ggregte frctiontion showed the lrgest shre of ggregtes occurring in the iggest size frction of >0.5mm dimeter, while only smll shre of ggregtes ws found in the smllest size frction of <0.053mm dimeter, irrespective of the wetlnd type or the lnd use. In the floodplin soils the content of soil ggregtes >0.5mm dimeter rnged from 42-60% with the lrgest shre occurring in nturl system nd fllow lnd nd the lowest shres occurring in the cultivted soils. The percentge of medium-size ggregtes ( mm dimeter) rnged from 18-32% nd followed the trend of the lrge ggregtes with lrgest shres in uncultivted nd lowest shres in cultivted soils. The smll ggregtes followed reverse trend nd were most undnt in the cultivted soils. The microggregtes (<0.053mm dimeter) mde up only minor shre (4-5%). In the inlnd vlley soils the content of the ggregte frction >0.5mm ws higher thn in the floodplin, rnging from 53-66% nd hving lrger shres in the cultivted thn the uncultivted soils. The medium size ggregte frction ( mm dimeter) responded most sensitively to lnd use with shre of 25% in nturl system nd declining to 12% with intense eroic cultivtion. As in the floodplin, microggregtes mde up only smll shre with 5-8%. Aggregte frction size (%) Floodplin Inlnd vlley >0.5mm mm mm <0.053mm 0 Nturl Aneroic Aeroic Fllow Nturl Aneroic Aeroic Fllow Figure 3: Effect of lnd use chnges on the percentge distriution of ggregte size clsses in floodplin nd inlnd vlley soils Wter stle ggregtes. While the ggregte size did not provide conclusive picture, the shre of the wter stle ggregtes (WSA) showed distinct differences etween the wetlnd systems. While cultivtion of soils in the floodplin strongly ffected the mounts of WSA, the shre of WSA ppered unffected y lnd use in the inlnd vlley soils (Figure 4). The content of WSA within the respective soil ggregte frctions ws higher in ll frctions in the inlnd vlley soils (25-30%), with exception of the soil from the floodplin under nturl system, which ws significntly different from ll other soils (P < 0.05) nd showed the overll highest mounts with out 36% of WSA in ll three frctions nd strong decline under cultivtion. This decline fter conversion from nturl systems to cropped lnd in the lowlnd floodplins dominted y sndy cly soils ws on verge 65% in the 0.5mm dimeter, 69% in the 0.25mm dimeter nd 51% in the mm dimeter ggregtes. The soils from the inlnd vlley were more homogenous nd showed ll the sme trend with the lowest contents of WSA in the iggest frction (0.5 mm dimeter) nd the highest in the smllest frction (0.053 mm dimeter). The differences etween the lnd uses were miniml, nd etween the nd 0.25 mm dimeter ggregtes. Unlike the floodplin soils, the decline in WSA in inlnd vlley soils fter conversion from nturl system to cultivted system ws less thn 6% etween the cultivted nd the nturl systems. This could e ssocited with stiliztion nd protection of the ggregtes y the cly prticles. 7

8 % Wter stle ggregtes Floodplin Inlnd vlley 0.5mm 0.25mm 0.053mm 0 Nturl Aneroic Aeroic Fllow Nturl Aneroic Aeroic Fllow Figure 4: Effect of lnd use chnges on the percentge distriution of wter stle ggregtes in soils from floodplin nd inlnd vlley wetlnd in Est Afric Discussion Results from this study hve demonstrted tht conversion of nturl systems to either eroic or neroic cultivtion sustntilly ffected the soil P frctions nd the proportions of the ggregtes ssocited with orgnic mtter. Soil P frctions in wetlnd soils The results of the P-frctiontion demonstrted tht the pools of plnt ville P (lile frctions) in oth floodplin nd inlnd vlley re very smll. The lrgest frction turned out to e the NOHextrctle P, which minly contins modertely stle Al- nd Fe-ound inorgnic P. Lile P represent the most ville form of P hence these pools re likely to represent the recent inputs from P fertilizer, rinfll, or P minerlized from orgnic mtter (Negss nd Leinweer, 2009). This rguement proly explins the reltively higher P in soils from the the eroic cultivted fields. The NHCO 3 extrctle frction contins Fe nd Al ound P which is ssocited with morphous compounds. The P contined in this frction is usully unstle in wetlnd soils nd is likely to chnge with fluctuting redox conditions (Kirk, 2004; Wright, 2009). From the present study, this pool hd the lowest P content of ll the frctions. The orgnic mtter content of the nturl undistured plots re likely to hve enhnced microil ctivity which in turn converted P to inorgnic forms which could explin the high proportion of P pools in the nturl undistured res. On the other hnd, lnd use conversion from nturl systems in reduced conditions to eroic cultivtion my lso enhnce the minerliztion of orgnic mtter nd therefore orgnic P relese (Negss nd Leinweer, 2009). The stle pools (NOH nd HCl extrcted P) contins the C-ound nd the highly occluded P pools. In the present study these frctions hd the highest P vlues. This ws proly due to ccumultion of residul P (Smeck, 1995; Wright, 2009). Similr findings hve een reported y Nwoke et l., (2003) in inlnd vlley soils of West Afric nd y Solomon et l., (2002) in the su humid highlnds of Ethiopi. This frction hs een found to e n importnt source of ville P in the tropicl soils not mnged with inorgnic P fertilizer (Beck nd Snchez, 1994). Orgnic mtter is known to contriute significntly to P vilility in wethered soils y enhncing the CEC nd reducing P fixtion (Brer, 1984; Okleo et l., 1991). Thus, P held in the stle pool is long term source of lile P therefore minerliztion of the stle pools my contriute sustntilly to the supply of ville P to these soils (Brer, 1984; Wright, 2009). Intensive fertiliztion nd mngement 8

9 proly incresed P content in the lile frctions in the cultivted plots. Similr findings hve een reported y (Negss nd Leinweer, 2009) on incresed P retention in the minerl ssocited P pools in cultivted plots while lnd uses with miniml mngement nd no fertiliztion such s the nturl systems nd fllow retined more P in the stle frctions. Prticulrly the cultivted floodplin soils re very low in lile P while the P stock is lso much reduced. This mens tht floodplin soils under cultivtion show low P vilility nd shrinking P supplying potentil from the stle pools. Previous studies hve shown tht the nturl systems hve greter proportion of the totl P in orgnic forms while distured or cultivted fields usully hve more P in the inorgnic pools (Grhm et l., 2005; Sleque et l., 2004). However under the current lnd uses nd environmentl conditions such s wetting nd drying in the wetlnds soils, orgnic P my not e considered stle pool nd my eventully e lost from the fields. If the resin nd icronte frctions represent soil phosphorus tht is oth exchngele nd esily minerlizle, then the frction of the totl soil phosphorus pool tht is ville to plnts is minute frction of the totl phosphorus pool, either s n solute vlue or s percentge of totl phosphorus (Cross nd Schlesinger, 1995; Nwoke et l., 2003). The proportion of phosphorus in lile nd stle frctions is likely to vry etween the soil types, nd soil wethering intensity nd use intensity. In the lowlnd floodplins, P deficiency is widespred in rinfed rice fields (Mitim et l., 2009). Thus, these soils when put under cultivtion re likely to show low P vilility nd shrinking P supplying potentil from the stle pools. However, in the inlnd vlley, the plnt ville lile P nd stle P frctions re lrger, which indictes lesser vulnerility concerning the P supply. Therefore, the inlnd vlley soils prove less susceptile to griculturl use, lthough the incresing mounts of lile nd stle P under intensive cultivtion re very likely to e cused y high fertilizer ppliction, especilly under dringe for the input-intensive production of uplnd crops. Aggregte sizes nd stility The stility of the soil ggregtes is ffected strongly y nthropogenic interventions nd the sence of soil mngement prctices. Wetlnd soils re often minerl in sesonlly wet wetlnd res nd pety in permnently flooded wetlnds therefore resulting to differentition in the decomposition nd ggregte proportions etween the lnd uses with vrying soil moisture (Wright, 2009). In the present study the high proportion of lrge mcro-ggregtes >0.25mm dimeter oserved in the nturl systems could proly e due to the high ccumultion of vegettion mterils on the soil surfce. The ccumultion of the SOM in the soil hs often een relted to n increse in ggregtion (Bronick nd Ll, 2005, Krull, 2004). In neroic systems, flooding conditions inhiit ggregte rekdown ecuse microes slowly decompose the smll mounts of cron-rich inding gents tht re produced (Olk et l., 1996). On the other hnd in the eroic cultivtion, dringe nd tillge leds to incresed microil ctivity nd therefore higher rekdown of the ggregte frctions resulting to reduced proportions of ggregte frctions >0.25mm dimeter. These findings re in ccordnce with mny other studies which confirm the dverse influence of cultivtion on SOM content nd ggregte stility. For exmple Bronick nd Ll (2005) relized tht the mcro-ggregtes pools (>0.25mm dimeter) ws the mjor pool depleted s result of cultivtion. Incresed mngement intensity hs lso een found to increse the humifiction process nd the proportion of highly decomposle nd esily lost orgnic mtter (Leinweer et l., 1995). Thus, in the present study, the decline in the ggregte frctions ws oserved when the nturl system ws rought under cultivtion in the sndy cly soils of the floodplin. Aggregtes mm dimeter re more stle to rpid wetting nd re not destroyed y griculturl prctices, prtly ecuse they re smll, ut lso ecuse they contin severl types of inding gents (Six et l., 1998). This could proly explin the miniml differences etween the 0.053mm dimeter wter stle frctions in the inlnd vlley soils. Where soil is cultivted frequently like in the eroic cultivted plots in the present study, ggregtes re exposed to physicl disruption. This my expose the previously inccessile orgnic mtter to microorgnisms nd stimulte oxidtion nd loss of orgnic mtter. This 9

10 decline in orgnic mtter is usully ccompnied y decrese in the numer of wter-stle ggregtes (Tisdll nd Odes, 1982). The fllow soils indicte tht the ggregtion seems to improve gin fter cultivtion, ut not wter stility. In contrst to the floodplin soil stility in the inlnd vlley proved less sensitive. The Inlnd vlley soils pper more resilient to lnd use chnge nd my present in the long-term lrger potentil to sor the growing pressure on lnd for food crop production in Est Afric thn the floodplins Conclusion Lnd use chnges, prticulrly those involving soil dringe nd cultivtion led to chnges in chemicl nd physicl soil properties which ffected the vilility nd the pool size of different lile nd stle P-frctions nd ggregte stility. Lile P frctions comprised reltively smll percentge of the totl soil P while cultivted plots showed pprecily incresed P retention in the stle pools The stiliztion of mcro-ggregtes is controlled y mngement nd is decresed when nturl systems re converted to cropped fields The floodplin soils proved to e more vulnerle to intensive griculturl use thn the inlnd vlley soils. Phosphorus frctions nd ggregte size proportions nd stility declined with higher mrgin in the floodplin when compred to the inlnd vlley soils Acknowledgement This study ws conducted within the frmework of the Agriculturl Use nd Vulnerility of Smll Wetlnds of Est Afric Project supported y the Volkswgen Foundtion Hnnover Germny We re grteful to ll frmers who llowed us ccess to their wetlnd fields nd the dministrtive officers in Tnzni nd Keny for logisticl support. We cknowledge the support of lortory stff t Keny Agriculturl Reserch Institute nd t the Deprtment of Plnt Nutrition, University of Bonn Germny. References Brer, S.A. (1984). Soil nutrient iovilility. A mechnistic pproch. John Wiley nd Sons. New York. USA. Beck, M.A. nd Snchez, P.A. (1994). Soil phosphorus frction dynmics during 18 yers of cultivtion on Typic Pleudult. Soil Science Society of Americ Journl. 58: Brdy, N.C. nd Weil, R.R. (2002). The nture nd property of soils. 13 th edition, Person Eductionl. New Jersey, USA. Bronick, C.J. nd Ll, R. (2005). Soil structure nd mngement: review. Geoderm. 124: Brymn, A. nd Crmer, D. (2001). Quntittive dt nlysis with SPSS Relese 10 for Windows: A Guide for Socil Scientists. Routledge, London. UK. Cross, A. F. nd Schlesinger, W. H. (1995). A literture review nd evlution of the Hedley frctiontion: Applictions to the iogeochemicl cycle of soil phosphorus in nturl ecosystems. Geoderm. 64: FAO, ( ). Reports of the gro-ecologicl zones project. World Soil Resources. Food nd Agriculturl Orgniztion of the United Ntions. Report no 48. Vol 1-4. Rome, Itly FAO-UNESCO, (1997). Soil mp of the world, vol 1 legend, FAO-UNESCO. Rome, Itly. Grhm, S.A., Crft, C.B., McCormick, P.V. nd Aldous, A. (2005). Forms nd ccumultion of soil P in nturl nd recently restored pet lnds - upper Klmth Lke, Oregon, USA. Wetlnds. 25: Guo, F., Yost, R.S., Hue, N.V., Evensen, C.I. nd Silv, J.A. (2000). Chnges in phosphorus frctions in soils under intensive plnt growth. Soil Science Society of Americ Journl. 64: Hedley, M.J., Stewrt, J.W.B. nd Chuhn, B.S. (1982). Chnges in inorgnic nd orgnic soil 10

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