Soil water content during and after plant growth influence nutrient availability and microbial biomass

Transcription

1 Journl of Soil Sin n Plnt Nutrition, 217, 17 ( 3), RESEARCH ARTICLE Soil wtr ontnt uring n ftr plnt growth influn nutrint vilility n miroil iomss Rn Xu 1, 2, Yuying Shn 1, Ptr Mrshnr 2 1 Stt Ky Lortory of Grssln Agro-osystms, Collg of Pstorl Agriultur Sin n Thnology, Lnzhou Univrsity, Lnzhou, 732, Chin. 2 Shool of Agriultur, Foo n Win, Th Univrsity of Ali, South Austrli, 55, Austrli Corrsponing uthor: ptr.mrshnr@li.u.u Astrt Two xprimnts wr onut to stuy th fft of soil wtr ontnt on soil rspirtion, miroil iomss n nutrint vilility in plnt n unplnt soil. In th first xprimnt, wht ws grown in pots for four wks in soil tht ws kpt ry twn on n four wks. In th son xprimnt, soil ws mintin t 5% of wtr-holing pity (WHC) for four wks n ithr plnt with wht or lft unplnt. Aftr rmovl from th pots, soil ws kpt t 5% WHC or quikly ri to 4, 3, 2 or 1% of WHC. Th soils wr inut four wks uring whih soil rspirtion, miroil iomss n nutrint vilility wr msur. In th first xprimnt, shoot n root iomss n miroil iomss ron wr highr in onstntly moist thn onstntly ry soil, ut th rvrs ws tru for vill N. In th son xprimnt, umultiv rspirtion ws two-fol highr in plnt thn unplnt soil n rs with wtr ontnt, with smllr rs in plnt soil. Miroil iomss ron on ys 5 n 1 ws highr t 1% thn t 5% WHC in plnt soil, ut not fft y wtr ontnt in unplnt soil. W onlu tht soil miros n mintin highr rspirtion t low wtr ontnt spit low iomss us tivity pr unit iomss is high. Kywors: Dry prio, miroil iomss, plnt, rspirtion, soil wtr ontnt 1. Introution Wtr is onsir s on of th most importnt ftors influning soil nutrint vilility n miroil tivity in trrstril osystms (Clrk t l., 29). It is th mium of nutrint trnsport in soil, so lthough nutrint n wtr sorption r inpnnt prosss, thy r inxtrily link to h othr (Vits, 1972). Low wtr vilility rus omposition n nutrint yling rt (Guntins t l., 213) us th thin wtr films roun soil prtils rus iffusion of nzyms n nutrints n thus sustrt supply for miros (Ppnik n Cmpll, 1981). To ountrt th strongly ngtiv wtr potntil in ry soil, som miros umult osmorgultory ompouns (.g. polyols, sugr lhys n mino is) (Hrris, 1981), this physiologil rspons vois 72

2 73 Xu t l. miroil hyrtion n th. Plnts r n importnt sour of sustrts for soil miros through xuts n root ris (Mrino t l., 215; Brg n Smll, 29). Thus, low wtr vilility my inirtly influn miroil tivity y ruing sustrt supply u to poor plnt growth. Whn wtr ontnt flututs uring plnt growth, plnt growth my rovr in moist prios suffiintly to ompnst for poor growth uring ry prios. But th fft of flututing wtr ontnt uring plnt growth on soil nutrint vilility n miroil iomss is poorly unrstoo. Diffrns in miroil iomss, omposition n tivity twn plnt n unplnt soil hv n stui xtnsivly. In prvious stuy, w impos wtr strss uring plnt growth n foun tht th fft of low soil wtr ontnt on miros ws xrt y low orgni C input s rsult of poor plnt growth (Xu t l., 216). Howvr, th fft of intrmittnt ry on plnt n miroil iomss is unlr. Furthr, it is not lr if th rspons of miros to low wtr vilility in plnt soil ftr plnt rmovl iffrs from tht of miros in unplnt soil. Th ims of this stuy wr to trmin (i) th fft of lngth n istriution of ry prio on plnt growth, miroil iomss n nutrint vilility (Exprimnt 1), n (ii) if th fft of low wtr vilility on miros n nutrint vilility iffrs twn plnt n unplnt soil (Exprimnt 2). Th hypothss wr tht 1) plnt growth n miroil iomss will rs with lngth of th ry prio with grtr fft if th ry prio is in th rly stgs of plnt growth, n 2) th fft of rsing wtr ontnt on soil rspirtion n miroil iomss will smllr in plnt soil thn unplnt soil. Th son hypothsis ssums tht miros with grtr supply of sily vill C r mor tolrnt to strss. 2. Mtrils n Mthos 2.1. Soil Th silt lom us in th xprimnts ws ollt on th Wit Cmpus of th Univrsity of Ali (Longitu E, Ltitu S) t -1 m pth. Th sit hs Mitrrnn limt with hot, ry summr n ol, wt wintr. Th soil proprtis r: 22% sn, 6% silt, 18% ly, wtr holing pity (WHC) 371 g kg -1, ph (1:5 soil wtr rtio) 5.6, EC (1:5).1 S m -1, totl orgni C 17 g kg -1, totl orgni N 1.5 g kg -1, ulk nsity 1.3 g m -1, vill P 1 mg P kg -1 n vill N 15 mg N kg - 1. Svrl smpls wr ollt rnomly n thn pool. Th soil is Rhooxrlf oring to US Soil Txonomy. Aftr stons n littr wr rmov, th soil ws ri t 4 C, thn siv through 2 mm siv n thoroughly mix for using it for th xprimnts Exprimntl sign Two xprimnts wr onut. For Exprimnt 1, soil wtr ontnt ws just to ithr 5% (-.78 Mp) WHC or 2% (-.7 MP) WHC. Th 5% WHC ws hosn us in prvious stuis with this soil, plnts grw wll t this wtr ontnt. Thn 4 g soil (ry wight quivlnt) ws fill in pots ( m) n plnt with pr-grmint wht ss (Tritium stivum L. v. Krihuff, 15 ss pr pot). Th xprimnt h nin trtmnts with four rplits pr trtmnt tht iffr in wtring rgim (Figur 1). In CW, th soil ws mintin throughout th four wks t 5% WHC. In CD th soil ws kpt t 2% of WHC for four wks. In th othr trtmnts, th soil ws t 2% WHC for t lst th first wk. Th trtmnt nms rfr to th numr n orr of wks whr th soil ws wtr to 5% Journl of Soil Sin n Plnt Nutrition, 217, 17 ( 3),

3 Soil wtr ontnt ffts miroil iomss n nutrint vilility 74 WHC n wks in whih it ws not wtr (.g. DWDD n DDWD wr only wtr in th son n thir wk, rsptivly). Th pots wr pl in glsshous with nturl light. In th moist prios, wtr ontnt ws mintin t 5% WHC n monitor thr tims y y wight. During th ry prios, pots wr lft to ry until thy rh 2% (within 2 ys), ftr whih this wtr ontnt ws mintin. Aftr four wks, plnt roots n shoots wr ollt n ry wight msur. Aftr rfully rmoving ll visil roots, soils wr kpt t 4 C for th trmintion of vill N n P, wtr xtrtl orgni C (WEOC) n miroil iomss C (MBC). Figur 1. Shmti igrm of th wtring trtmnts in Exprimnt 1. Th son xprimnt ws rri out to ssss th fft of low soil wtr ontnt on miroil iomss, tivity n nutrint vilility in plnt n unplnt soil. Th soil ws just to 5% WHC. Thn, soil (4 g ry wight quivlnt) ws fill into 16 pots. To otin plnt soil, ight of th pots wr nsly plnt with pr-grmint wht ss (2 pr pot). Th high plnt nsity ws us to nsur high root nsity n thrfor ll soil in th plnt pots ws influn y roots. Th othr ight pots rmin unplnt. Th pots wr pl in glsshous with nturl light n wtr thr tims y to mintin onstnt soil wtr ontnt throughout plnt growth. Any ws grminting in th unplnt pots wr rmov. Four wks ftr plnting, whn ns plnt ovr ws stlish in th plnt pots, roots n shoots wr rmov. Th soil ws ri in fn-for ovn t 4 C within 1-3 h to fiv wtr ontnt ontnts (1, 2, 3, 4 n 5% of WHC, quivlnt to.37,.74,.11,.15,.19 g wtr g -1 soil n wtr potntils of -1.7, -.7, -.32, -.16, -.78 Mp). Ths wtr ontnts orrspon to volumtri wtr ontnts of.48,.97,.14,.19,.24 g m -3. Thn, 3 g soil (ry wight quivlnt) of h wtr ontnt trtmnt (h wtr ontnt with 12 rplits, oth plnt n unplnt soils) ws pl into PVC ors (hight 5 m n imtr 3.7 m with nylon msh s). Soil ulk nsity ws just to 1.3 g m -3 y pking th soil in th ors to th rquir hight. Thn th ors wr trnsfrr into glss jrs n kpt t 2-23 C in th rk. Th sir wtr ontnt ws mintin y wight vry two ys. Cors wr strutivly smpl on ys 5, 1 n 25 with four rplits t h hrvst for trmintion of WEOC, vill N n P, miroil iomss C (MBC), N (MBN). Journl of Soil Sin n Plnt Nutrition, 217, 17 ( 3),

4 Soil wtr ontnt ffts miroil iomss n nutrint vilility Anlyss Soil txtur ws trmin using th hyromtr mtho (Bowmn t l., 22). WHC ws msur using sintr glss funnl (Hins, 193). Soil ws pk into rings n just to fil ulk nsity, pl in th sintr glss funnl whih ws onnt to 1 mm wtr olumn (ψ m =1 kp) n thoroughly wtt. Th soil ws llow to rin for 48 h, ry wight of th soil ws trmin ftr ovn rying t 15 C till onstnt wight. Soil ph n EC wr msur in 1:5 (w/v) soil to rvrs osmosis (RO) wtr rtio ftr 1 h n-ovr-n shking. Soil totl orgni C ws trmin ftr Wlkly n Blk (1934). Soil totl N ws msur using th Kjlhl mtho (MKnzi n Wll, 1954). Soil rspirtion ws msur ily y quntifying th CO 2 -C onntrtion in th hsp of th jrs using Srvomx 145 infr-r nlyzr (Srvomx Group, Croworough, UK) s sri in (Sti t l., 211). Aftr h msurmnt, th jrs wr vnt using fn to rfrsh th hsp n thn rsl for msurmnt on th following y. Th CO 2 volv uring givn intrvl ws lult s th iffrn in CO 2 onntrtion twn msur n mint CO 2 onntrtion. Linr rgrssion s on injtion of known mounts of CO 2 into mpty jrs of similr siz ws us to fin th rltionship twn CO 2 onntrtion n ttor ring. Miroil iomss C (MBC) ws trmin y hloroform fumigtion-xtrtion with.5 M K 2 SO 4 t 1:4 soil to xtrtnt rtio (Vn t l., 1987). Th orgni C onntrtion in th filtr xtrt ws msur y titrtion with.33 M iifi (NH 4 ) 2 F(SO 4 ) 2.6H 2 O ftr ihromt oxition (Anrson n Ingrm, 1993). Th hloroform-lil C onntrtion is th iffrn twn fumigt n non-fumigt soil whih ws multipli y 2.64 to lult MBC (Vn t l., 1987). For miroil iomss N (MBN), th mmonium onntrtion in th K 2 SO 4 xtrt ws trmin (Willis t l., 1996; Moor t l., 2). Th iffrn twn fumigt n non-fumigt soil ws multipli y 1.75 to lult MBN (Moor t l., 2). For trmining soil vill N (mmonium + nitrt), soil ws xtrt with 2 M KCl t 1:5 soil to xtrtnt rtio in horizontl shkr t 8 rpm for on hour. Ammonium-N in th filtr xtrts ws msur s sri for MBN ftr Willis t l. (1996). Nitrt-N ws trmin s sri in Mirn t l. (21). Avill P ws trmin s sri in Kouno t l. (1995). Th P onntrtion in th xtrts ws trmin olorimtrilly oring to Murphy n Rily (1962). Wtr xtrtl orgni C (WEOC) ws xtrt t 1:5 soil:wtr rtio. Aftr 1h n ovr n shking, orgni C ws msur s sri ov for MBC Sttistil nlysis Shoot n root iomss, MBC, WEOC, N n P in xprimnt 1 wr nlyz y on-wy nlysis of vrin. Th son xprimnt ws rrng in omplt rnomiz lok sign with 2 soil trtmnts (plnt n unplnt soil) 5 wtr ontnts 3 smpling tims n 4 rplits for h smpl tim. Rpt msurs ANOVA ws prform to tst th fft of trtmnt n soil wtr ontnt ovr tim using tim s rpt msur. Th intrtion twn smpling tim n xprimntl trtmnts ws signifint. Thrfor, t of umultiv rspirtion, vill N, P, WEOC, MBC n MBN pr smpling tim wr sujt to two-wy ANOVA (trtmnt wtr ontnt) for h smpling tim sprtly. Avrg vlus wr ompr using post-ho Tuky tst. All nlyss wr rri out with Gnstt (GnStt for Winows, 18th ition, 215; VSN Intrntionl Lt, Hml Hmpst, UK). Only signifint iffrns r mntion in th txt (p<.5). Journl of Soil Sin n Plnt Nutrition, 217, 17 ( 3),

5 76 Xu t l. 3. Rsults 3.1. Exprimnt 1 Shoot n root iomss wr highst in th onstntly wt (CW) trtmnt n lowst in th onstntly ry (CD) trtmnt (Figur 2 n ). Shoot iomss ws mor snsitiv to th onstntly ry soil thn root iomss. For xmpl, shoot iomss ws thr tims lowr in CD thn CW whrs root iomss ws only 5% lowr in CD. Compr to CW, shoot iomss ws out 2% lowr in th trtmnt with on ry wk (DWWW). In trtmnts with two ry wks, shoot iomss ws highst in th trtmnt whih ws ry in th first n th lst wk (DWWD). Shoot iomss i not iffr twn trtmnts with two or thr ry wks, xpt in DDDW whr it ws lowr thn in DWWD n DWDW. Trtmnt iffrns wr similr for root iomss, ut with fwr signifint iffrns thn in shoot iomss. MBC h similr trtmnt iffrns s shoot n root iomss (Figur 2 ); it ws out two-fol highr in CW thn CD n mong trtmnts with two ry wks highst in DWWD. WEOC ws out 3% highr in CW thn CD ut th othr trtmnts iffr littl in WEOC (Figur 2 ). Biomss g pot -1 1,8 1,5 1,2,9,6 () Shoot f () Root,3, MBC mg kg () WEOC mg kg -1,25,2,15,1,5 (), N mg kg f f () f P mg kg (f) Figur 2. Shoot () n root iomss () of wht, miroil iomss C (), wtr xtrtl orgni C (), vill N () n vill P (f) onntrtion ftr four wks in iffrnt wtring trtmnts. Vrtil lins t th top of th rs init stnr rror. Diffrnt lttrs init signifint iffrns (P<.5, n=4). For trtmnt xplntion s Figur 1. Journl of Soil Sin n Plnt Nutrition, 217, 17 ( 3),

6 Soil wtr ontnt ffts miroil iomss n nutrint vilility 77 Avill N n P (Figur 2 n f) wr lowst in CW n highst in CD with grtr iffrns in vill P (CW two-fol highr thn CD) thn in vill N (CW 3% highr). Among trtmnts with vrying wtr ontnt, vill N ws lowst in th trtmnt with on wk ry (DWWW) n highst with th first thr wks ry (DDDW).Avill P ws lso lowst with on wk ry (DWWW), ut it ws highst in th trtmnt with th son wk wt (DWDD). In trtmnts with thr ry wks, vill P inrs in th following orr: DDDW< DDWD < DWDD Exprimnt Cumultiv rspirtion Cumultiv rspirtion ws out two-fol highr in plnt thn unplnt soil (Figur 3). It rs with rsing soil wtr ontnt for oth plnt n unplnt soil. In plnt soil, umultiv rspirtion rs from 5 to 2% WHC. In unplnt soil, it rs from 5 to 4% n from 2 to 1% WHC. Th rltiv rs in umultiv rspirtion ompr to 5% WHC ws smllr in plnt thn in unplnt soil. For xmpl, t 3% WHC, umultiv rspirtion ws 21% lowr in plnt soil, ut 3% lowr in unplnt soil. Cumultiv rspirtion mg CO 2 -C g -1 soil 1,4 1,2 1,,8,6,4,2, Unplnt f Plnt g Soil wtr ontnt (%WHC) Figur 3. Cumultiv rspirtion ftr 25 ys in prviously plnt n unplnt soil with 1-5% of wtr holing pity. Vrtil lins t th top of th rs init stnr rror. Diffrnt lttrs init signifint iffrns (P<.5, n=4) Miroil iomss MBC ws up to two-fol highr in plnt soil thn unplnt soil on y 5, ltr th iffrns m smllr or isppr (Figur 4). Gnrlly, MBC ws highr on y 5 thn ys 1 n 25. Th fft of soil wtr ontnt on MBC iffr twn unplnt n plnt soil n mong smpling tims. In plnt soil, MBC ws lowr t 5% thn t 1% WHC. But th rvrs ws tru for unplnt soil on ys 5 n 25 (Figur 4, ). Th rtio of umultiv rspirtion to MBC on y 25 ws out two-fol highr in plnt thn unplnt soil n two-fol lowr t 1 ompr to 5% WHC (plnt 2.1 n 5.1 mg kg -1 ; unplnt 1.1 n 2.1 mg kg -1 for 1 n 5% WHC, rsptivly). h Journl of Soil Sin n Plnt Nutrition, 217, 17 ( 3),

7 78 Xu t l. MBC mg kg Dy 5() Dy 1() 8 MBC mg kg Dy 25() Unplnt Plnt f f f Soil wtr ontnt (%WHC) Figur 4. Miroil iomss C onntrtions on ys 5 (), 1 () n 25 () in prviously plnt n unplnt soil with 1-5% wtr holing pity. Vrtil lins t th top of th rs init stnr rror. Diffrnt lttrs on th sm y init signifint iffrns (P<.5, n=4). MBN ws lwys out two-fol highr in plnt thn unplnt soil n hng littl ovr tim (Figur 5). In plnt n unplnt soil, MBN ws lowst t 1% WHC t ll smpling tims n it ws out two-fol highr t wtr ontnts 2% WHC thn t 1%. In unplnt soil, MBN ws lowst on y 25. In plnt soil, it ws lso lowst on y 25 t 3-5% WHC, ut not t 1 n 2% WHC. 1 Dy 5 () Dy 1 () MBN mg kg MBN mg kg Unplnt Plnt Dy 25 () Soil wtr ontnt (%WHC) Figur 5. Miroil iomss N onntrtions on ys 5 (), 1 () n 25 () in prviously plnt n unplnt soil with 1-5% wtr holing pity. Vrtil lins t th top of th rs init stnr rror. Diffrnt lttrs on th sm y init signifint iffrns (P<.5, n=4). Journl of Soil Sin n Plnt Nutrition, 217, 17 ( 3),

8 Soil wtr ontnt ffts miroil iomss n nutrint vilility WEOC, vill N n P WEOC onntrtion ws highr in plnt thn unplnt soil, with grtr iffrns t highr wtr ontnts (Figur 6). At 5% WHC, WEOC ws t lst two-fol highr in plnt soil thn unplnt soil, ut t 1% it ws only 2-3% highr. WEOC hng littl ovr tim. In plnt soil, WEOC rs with wtr ontnt t ll smpling tims. It ws 5-6% lowr t 1% thn t 5% WHC. Soil wtr ontnt h littl fft on WEOC in unplnt soil. On ys 1 n 25, WEOC ws lowr t 1% thn t 5% WHC, ut th iffrn ws quit smll (out 2% lowr t 1% WHC). WEOC mg kg -1,6,5,4,3,2,1 Dy 5() Dy 1() f WEOC mg kg -1,6,5,4,3,2 Dy 25() Unplnt Plnt f, Soil wtr ontnt (%WHC) Figur 6. Wtr xtrtl orgni C onntrtions on ys 5 (), 1 () n 25 () in prviously plnt n unplnt soil with 1-5% wtr holing pity. Vrtil lins t th top of th rs init stnr rror. Diffrnt lttrs on th sm y init signifint iffrns (P<.5, n=4). Avill N ws two-fol highr in unplnt thn in plnt soil (Tl 1). It hng littl ovr tim. Soil wtr ontnt h no lr fft on vill N in plnt soil. In unplnt soil on ys 5 n 1, vill N ws 1-2% highr t 1% WHC thn t highr wtr ontnts. Journl of Soil Sin n Plnt Nutrition, 217, 17 ( 3),

9 71 Xu t l. Tl 1. Avill N onntrtions on y ys 5, 1 n 25 in plnt n unplnt soil t 1-5% of wtr holing pity. Diffrnt lttrs on th sm y init signifint iffrns (P<.5, n=4). Trtmnt Unplnt Plnt Soil wtr ontnt (%WHC) N mg kg -1 Dy 5 Dy 1 Dy Avill P ws two to thr-fol lowr in plnt thn in unplnt soil with grtr iffrns on y 5 thn ltr (Tl 2). In unplnt soil, vill P rs with tim, whil it ws gnrlly lowst on y 5 in plnt soil. Tl 2. Avill P onntrtions on ys 5, 1 n 25 in plnt n unplnt soil t 1-5% of wtr holing pity. Diffrnt lttrs on th sm y init signifint iffrns (P<.5, n=4). Trtmnt Unplnt Plnt P mg kg -1 Soil wtr ontnt Dy 5 Dy 1 Dy 25 (%WHC) Journl of Soil Sin n Plnt Nutrition, 217, 17 ( 3),

10 Soil wtr ontnt ffts miroil iomss n nutrint vilility Disussion 4.2. Exprimnt 2 This stuy show tht th fft of low soil wtr ontnt on miroil iomss n nutrint vilility pns on whthr it is impos uring or ftr plnt growth. Furthr, in th son xprimnt thr wr lr iffrns in th msur prmtrs twn plnt n unplnt soil in th fft of wtr ontnt on rspirtion, miroil iomss n WEOC Exprimnt 1 Th hypothsis (plnt growth n miroil iomss will rs with lngth of th ry prio with grtr fft if th ry prio is in th rly stgs of plnt growth) nnot unquivolly pt or lin us th ssss plnt n soil prmtrs iffr littl twn th onstntly moist trtmnt n tht with on ry wk follow y thr wt wks. But ompr to CW, two or mor ry wks rs shoot n root iomss n MBC, whrs vill N ws inrs. In grmnt with prvious stuis (Mtsui n Singh, 23; Ash t l., 25, Wng t l., 214, Xu t l., 216), th fft of low wtr ontnt ws grtr for shoots thn roots us plnts invst rltivly mor ron into roots thn shoots whn wtr vilility is low to ss th rmining wtr. Th orgni C input into th soil of th smllr plnts in trtmnts with two or mor ry wks will lowr whih xplins th smllr MBC in ths trtmnts. Th highr N vilility in trtmnts with two or mor ry wks ompr to CW n xplin y lowr nutrint uptk y th smllr plnts. Th lk of iffrn twn trtmnts with iffrnt istriution of two or mor ry wks suggsts tht plnt growth i not rovr suffiintly in two moist wks to ompnst for th two ry wks. As xpt from prvious stuis, plnt soil h highr onntrtions of WEOC, MBC/N n lso highr umultiv rspirtion thn unplnt soil (Hyns n Frnis, 1993; Liu t l., 212). This n xplin y th grtr sustrt supply in plnt soils vi root xuts n root frgmnts tht woul sour of nutrints for miros (Wilung t l., 1975; Mrshnr, 212) Howvr, rls of C n N y hloroform from root frgmnts oul lso l to n ovrstimtion of miroil iomss (Mullr t l., 1992). Diffrns in MBC/N twn plnt n unplnt soil m smllr ovr tim us MBC/N rs in plnt soil, initing pltion of rltivly sily omposl nutrints ftr sprtion from th roots. Avill N n P s wll s EC wr lowr in plnt soil thn unplnt soil s rsult of nutrint uptk y th plnts (Mrshnr, 212). Th highr ph suggsts prominnt nitrt uptk (Ny, 1981) whih is orroort y th low nitrt onntrtions in plnt soil (t not shown). Th rution in MBC t 2% WHC ompr to 5% in Exprimnt 2 ws smllr thn in Exprimnt 1 us, in Exprimnt 1, low wtr ontnt fft oth plnt growth n soil miros. In Exprimnt 2 on th othr hn, plnts grw in moist soil n iffrnt wtr ontnts wr only impos ftr rmovl of shoots n roots. Th hypothsis tht th fft of rsing wtr ontnt on soil rspirtion n miroil iomss will smllr in plnt soil thn unplnt soil nnot unquivolly pt or lin us th wtr ontnt fft pn on th ssss prmtrs. In plnt ut not in unplnt soil, WEOC onntrtion rs with wtr ontnt. Th plnts wr grown unr wll-wtr onitions, thrfor it n ssum tht th WEOC onntrtion ws similr in ll wtr trtmnts for rying. Th f- Journl of Soil Sin n Plnt Nutrition, 217, 17 ( 3),

11 712 Xu t l. ft of wtr ontnt on WEOC ws pprnt on y 5. Drying my hv us WEOC to in to soil prtils s wtr films m thinnr. Ltr, WEOC onntrtion my lso influn y miroil utiliztion n turnovr s isuss low. Wtr ontnt influn WEOC only in plnt soil whr onntrtions wr up to two-fol grtr thn in unplnt soil. Cumultiv rspirtion rs with wtr ontnt in plnt n unplnt soil whih n xplin y thinning of wtr films roun soil prtils s soil ris whih will ru sustrt iffusion to lls. At vry low wtr ontnt, wtr my lso rwn out of th lls (Ilstt t l., 2). As rsult, th rtio of umultiv rspirtion to MBC on y 25 ws twofol highr t 5% thn t 1% WHC. Th rltiv rution of umultiv rspirtion with wtr ontnt ws slightly lss in plnt thn unplnt soil (t 1% WHC ompr to 5% WHC, umultiv rspirtion ws 49 n 42% in plnt n unplnt soil). This suggsts tht in plnt soil, miros r ttr l to rmin tiv in ry soil, likly u to grtr sustrt vilility. At highr sustrt onntrtion in th soil solution, miros my hv ss to sustrts vn whn th wtr film is thin. This is orroort y th two-fol highr rtio of umultiv rspirtion to MBC on y 25 t 1% WHC in plnt thn unplnt soil. Wtr ontnt only h onsistnt fft on MBC n MBN in plnt soil whr onntrtions wr up to two-fol highr thn in unplnt soil. In plnt soil, MBC on ys 5 n 1 ws lowst t 5% n highst t 1% WHC. This is likly u to iffrns in miroil turnovr. At 5% WHC, miros will rpily ompos th root posits lft ftr th rmovl of th plnts n prt of th miroil iomss i ftr pltion of rily vill sustrts. Th smll iomss t 5% WHC h high rspirtion rts s lso init y th high rtio of umultiv rspirtion to MBC. Th tiv iomss is likly to hv high turnovr rts whih n xplin th highr WEOC onntrtions t this wtr ontnt. At 1% WHC on th othr hn, miroil tivity ws low (two-fol lowr rtio of umultiv rspirtion to MBC thn t 5% WHC) n thus root posits only slowly ompos. Thrfor, mor sustrt rmin ftr 5 n 1 ys in ry soil whih mintin grtr miroil iomss. MBC rs ovr tim only in plnt soil whih os not sm to u to orgni C vilility us WEOC onntrtions rmin unhng throughout th xprimnt. Howvr, WEOC omposition my hv hng. It is known tht issolv orgni C (DOC) in soil n inlu simpl s wll s omplx ompouns (Amon n Bnnr, 1996). In this stuy, it is likly tht t th strt of th inution, WEOC in plnt soil ws minly sily omposl ompouns from root xuts n root ll lysts. Ltr s MBC rs, grtr proportion of WEOC ws miroil-riv whih my mor omplx n thus lss rily omposl thn root xuts. In ontrst to MBC, MBN in plnt soil ws lowst t 1% n highst t 5% WHC. Msur vill N onntrtions wr not influn y wtr ontnt. Howvr, N vilility is trmin t 1:5 soil to KCl rtio, thus t vry high wtr ontnt. It is likly tht t low wtr ontnt, iffusion of N to lls n thus N uptk y miros ws ru. Th ontrsting fft of wtr ontnt on MBC n MBN in plnt soil oul u to iffrnt lvls of miroil tivity. Th MBC/MBN rtio ws Journl of Soil Sin n Plnt Nutrition, 217, 17 ( 3),

12 Soil wtr ontnt ffts miroil iomss n nutrint vilility 713 wir t 1% thn t 5% WHC (13 n 35). Growing miros hv nrrowr MBC/MBN rtio us thy ontin mor N-ompouns (.g. protins, nzyms) thn lls in sttionry phs (Frnknrgr n Dik, 1983). Thus, th nrrow MBC/MBN rtio t 5% WHC is in grmnt with th high rtio of umultiv rspirtion to MBC, oth initing highly tiv miros. Th lss tiv miroil iomss t 1% WHC h wir MBC/MBN rtio n lowr rtio of umultiv rspirtion to MBC. Soil wtr ontnt h no lr fft on vill N n P n ny iffrns mong wtr ontnts wr smll. This suggsts tht soil wtr ontnt i not influn th rtio of minrliztion to immoiliztion whih lso i not hng ovr tim. 5. Conlusions Th first xprimnt show tht th ngtiv fft of low wtr ontnt on miroil iomss C is xrt if th soil is ry uring plnt growth likly u to th lowr orgni C input into th soil y th smllr rought-strss plnts. Distriution of two or thr ry wks h littl fft on th msur prmtrs suggsting tht plnts oul not rovr in on or two moist wks. From th son xprimnt w n onlu tht miros in plnt soil n mintin highr rspirtion in ry soil spit low iomss us tivity pr unit iomss is high. Nvrthlss, rspirtion ws lowr t low wtr ontnt ompr to optiml wtr ontnt (5% WHC) initing tht sustrt vilility ws ru. Furthr, MBC in prviously plnt soil rs uring th 25-y inution whih inits tht high tivity my not mintin muh longr. Aknowlgmnts Rn Xu thnks th Chins Sholrship Counil for proviing th sholrship. Rfrns Amon, R.M.W., Bnnr, R Btril utiliztion of iffrnt siz lsss of issolv orgni mttr. Limnol. Onogr. 41, Anrson, J., Ingrm, J Colorimtri trmintion of mmonium. Tropil Soil Biology n Frtility, A Hnook of Mthos, son. CAB Intrntionl, Wllingfor, UK, pp: Ash, F., Dingkuhn, M., Sow, A., Aurt, A. 25. Drought-inu hngs in rooting pttrns n ssimilt prtitioning twn root n shoot in upln ri. Fil Crops Rs. 93, Brg, G., Smll, K. 29. Plnt spis n soil typ ooprtivly shp th strutur n funtion of miroil ommunitis in th rhizosphr. Fms Miroiol. Eol. 68, Bowmn, G., Hutk, J., MKnzi, N., Coughln, K., Crsswll, H. 22. Prtil siz nlysis. Soil physil msurmnt n intrprttion for ln vlution. CSIRO pulishing, pp: Clrk, J.S., Cmpll, J.H., Grizzl, H., Aost- Mrtinz, V., Zk, J.C. 29. Soil miroil ommunity rspons to rought n pripittion vriility in th Chihuhun Dsrt. Miro. ol. 57, Frnknrgr, W., Dik, W Rltionships twn nzym tivitis n miroil growth n tivity inis in soil. Soil Si. So. Am. J. 47, Guntins, M.E., Gil-Sotrs, F., Liros, M.C., Trsr- Cp, C Snsitivity of soil rspirtion to moistur n tmprtur. J. Soil Si. Plnt Nutr. 13, Journl of Soil Sin n Plnt Nutrition, 217, 17 ( 3),

13 714 Xu t l. Hins, W.B Stuis in th physil proprtis of soil. V. Th hystrsis fft in pillry proprtis, n th mos of moistur istriution ssoit thrwith. J. Agr. Si. 2, Hrris, R Efft of wtr potntil on miroil growth n tivity. Wtr potntil rltions in soil miroiology. Soil Sin Soity of Amri, Mison, pp: Hyns, R., Frnis, G Chngs in miroil iomss C, soil rohyrt omposition n ggrgt stility inu y growth of slt rop n forg spis unr fil onitions. J. Soil Si. 44, Ilstt, U., Norgrn, A., Mlmr, A. 2. Optimum soil wtr for soil rspirtion for n ftr mnmnt with gluos in humi tropil risols n orl mor lyr. Soil Biol. Biohm. 32, Kouno, K., Tuhiy, Y., Ano, T Msurmnt of soil miroil iomss phosphorus y n nion xhng mmrn mtho. Soil Biol. Biohm. 27, Liu, D., Fng, S., Tin, Y., Dun, X Vrition in rhizosphr soil miroil inx of tr spis on ssonl flooing ln: An in situ rhizoox pproh. Appl. Soil Eol. 59, Mrshnr, P Minrl Nutrition of Highr Plnts. Elsvir, pp: Mtsui, T., Singh, B. 23. Root hrtristis in owp rlt to rought tolrn t th sling stg. Exp. Agr. 39, MKnzi, H., Wll, H.S Th Kjlhl trmintion of nitrogn: ritil stuy of igstion onitions-tmprtur, tlyst, n oxiizing gnt. Aust. J. Chm. 7, Mrino, C., Nnnipiri, P., Mtus, F Soil ron ontroll y plnt, miroorgnism n minrlogy intrtions. J. Soil Si. Plnt Nut. 15, Mirn, K.M., Espy, M.G., Wink, D.A. 21. A rpi, simpl sptrophotomtri mtho for simultnous ttion of nitrt n nitrit. Nitri Oxi. 5, Moor, J., Klos, S., Tti, M. 2. Soil miroil iomss ron n nitrogn s fft y ropping systms. Biol. Frt. Soils. 31, Mullr, T., Jorgnsn, R., Myr, B Estimtion of soil miroil iomss C in th prsn of living roots y fumigtionxtrtion. Soil Biol. Biohm. 24, Murphy, J., Rily, J.P A moifi singl solution mtho for th trmintion of phospht in nturl wtrs. Anl. Chim. At. 27, Ny, P Chngs of ph ross th rhizosphr inu y roots. Plnt Soil. 61, Ppnik, R., Cmprll, G Thory n msurmnt of wtr potntil. Wtr potntil rltions in soil miroiology. Soil Sin Soity of Amri, Mison Sti, R., Mrshnr, P., Blok, J., Chittlorough, D., Vrm, V Rltionships twn ron ioxi mission n soil proprtis in slt-fft lnsps. Soil Biol. Biohm. 43, Vn, E., Brooks, P., Jnkinson, D An xtrtion mtho for msuring soil miroil iomss C. Soil iol. Biohm. 19, Vits, F Wtr fiits n nutrint vilility. Wtr fiits n plnt growth. Ami Prss, Nw York, pp: Wlkly, A., Blk, I.A An xmintion of th Dgtjrff mtho for trmining soil orgni mttr, n propos moifition of th hromi i titrtion mtho. Soil Si. 37, Journl of Soil Sin n Plnt Nutrition, 217, 17 ( 3),

14 Soil wtr ontnt ffts miroil iomss n nutrint vilility 715 Wng, X., Mohm, I., Xi, Y., Chn, F Effts of wtr n potssium strsss on potssium utiliztion ffiiny of two otton gnotyps. J. Soil Si. Plnt Nut. 14, Wilung, R., Grln, T., Bushom, R Th intrpnnt ffts of soil tmprtur n wtr ontnt on soil rspirtion rt n plnt root omposition in ri grssln soils. Soil Biol. Biohm. 7, Willis, R.B., Montgomry, M.E., Alln, P.R Improv mtho for mnul, olorimtri trmintion of totl Kjlhl nitrogn using sliylt. J Agr Foo Chm. 44, Xu, R., Shn, Y., Mrshnr, P Low soil wtr ontnt uring plnt growth influns soil rspirtion n miroil iomss ftr plnt rmovl n rwtting. J. Soil Si. Plnt Nut. 16, Journl of Soil Sin n Plnt Nutrition, 217, 17 ( 3),