Methane emission from a simulated rice field ecosystem as influenced by hydroquinone and dicyandiamide

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1 . The Science of the Totl Environment Methne emission from simulted rice field ecosystem s influenced by hydroquinone nd dicyndimide Xingki Xu,b,c,, Yuesi Wng b, Xunhu Zheng b, Mingxing Wng b, Zijin Wng, Liki Zhou c, Oswld Vn Cleemput d SKLEAC, Reserch Center for Eco-En ironmentl Sciences, Chinese Acdemy of Sciences nd Ntionl En ironmentl Protection Agency, P.O. Box 2871, Beijing, PR Chin b LAPC, Institute of Atmospheric Physics, Chinese Acdemy of Sciences, Beijing, PR Chin c Soil Ecology Deprtment, Institute of Applied Ecology, Chinese Acdemy of Sciences, P.O. Box 17, Shenyng, PR Chin d Lbortory of Applied Physicl Chemistry, Fculty of Agriculturl nd Applied Biologicl Sciences, Uni ersity of Ghent, B-9000, Gent, Belgium Received 19 Jnury 2000; ccepted 7 July 2000 Abstrct A simple pprtus for collecting methne emission from simulted rice field ecosystem ws formed. With no whet strw powder mended ll tretments with inhibitor. s hd so much lower methne emission during rice growth thn the tretment with ure lone control., which ws contrry to methne emission from the cut rice soil system. Especilly for tretments with dicyndimide DCD. nd with DCD plus hydroquinone HQ., the totl mount of methne emission from the soil system nd intct rice soil system ws % nd % of the control, respectively. Hence, DCD, especilly in combintion with HQ, not only incresed methne oxidtion in the floodwter soil interfce following ppliction of ure, but lso significntly enhnced methne oxidtion in rice root rhizosphere, prticulrly from its tillering to booting stge. Whet strw powder incorported into flooded surfce lyer soil significntly wekened the bove-mentioned simulting effects. Regression nlysis indicted tht methne emission from the rice field ecosystem ws relted to the turnover of mmonium-n in flooded surfce lyer soil. Diminishing methne emissions from the rice field ecosystem ws significntly beneficil to the growth of rice Elsevier Science B.V. All rights reserved. Keywords: Dicyndimide; Hydroquinone; Interfce; Methne; Rice field system; Ure Corresponding uthor. Present ddress: LAPC, Institute of Atmospheric Physics, Chinese Acdemy of Sciences, Beijing, PR Chin Fx: E-mil ddress: xingki xu@hotmil.com X. Xu $ - see front mtter 2000 Elsevier Science B.V. All rights reserved.. PII: S

2 2 ( ) X. Xu et l. The Science of the Totl En ironment Introduction Flooded rice soil hs been recognized s reltively importnt source of tmospheric methne Khlil nd Rsmussen, 1983; Brtlett nd Hrriss, 1993; Wng, becuse it provides the suitble conditions for methnogenesis nd lrge res of lnd re given over to rice production due to incresing food demnds Bchelet nd Neue, According to n estimtion mde by the IPCC 1992., flooded rice soil contributed pproximtely 12% 33.6 million metric tons of crbon equivlent, MMTCE. of the totl globl emission MMTCE. of methne in Although the increse of the totl globl emission of methne hs slowed in recent yers OECD, 1999., the expnsion nd intensifiction of rice cultivtion in the world will perhps induce n increse of methne emission to the tmosphere if current technologies re continued. Rice cultivtion in Chin is more importnt griculturl prctice reltive to other countries in the world, nd it hs been estimted tht methne emission from rice cultivtion in Chin is MMTCE yer with rnge of MMTCE yer.wng nd Shnggun, It is therefore necessry to explore much more effective mitigtion method following better understnding of the processes involved in methne emission from rice fields Neue nd Roger, In rice fields oxic noxic interfces re found t the floodwter soil interfce nd in the rice root rhizosphere, nd greter potentil for methne oxidtion lwys ppers t the two interfces Conrd nd Rothfuss, 1991; Denier Vn der Gon nd Neue, 1996; Xu nd Zhou, Approximtely 80% of methne borne in rice fields is consumed by soil methnotrophs before its emission to the tmosphere Conrd nd Rothfuss, 1991; Wssmn et l., Hence, improving methne oxidtion potentil in rice fields will be more effective method for controlling its emission to the tmosphere. Ure min source. nd mmonium sulfte ccount for 80 90% of totl N fertilizer consump- tion by rice cultivtion FAO, 1989., nd ure is widely used in Chinese griculturl prctice nd the mount used will further increse in the coming century. At higher levels of pplied N, methne emissions from rice fields mended with ure were pprently much higher thn those mended with other nitrogen fertilizers Kimur, 1992; Lindu, Ammoni formtion following the hydrolysis of pplied ure cn more comprehensively destroy the growth of plnt thn the ccumultion of ure in the urese inhibitor. s -treted soil Krogmeier et l., 1989., thereby influencing the formtion nd oxidtion of methne in rice root rhizosphere. Keerthisinghe et l found nitrifiction inhibitors could significntly reduce methne emission from rice fields. However, Ci et l observed tht methne emission from rice fields in Chin ws promoted in the presence of nitrifiction inhibitors. These phenomen were prtly relted to different kinds of inhibitors nd the methods of their ppliction. Even so, little ttention hs been pid to the synergistic effect of urese inhibitors nd nitrifiction inhibitors on the formtion nd oxidtion of methne in rice root rhizosphere so fr. This pper studied methne emission from ure- nd whet strw mended rice field ecosystem s influenced by the urese inhibitor hydroquinone HQ. nd nitrifiction inhibitor dicyndimide DCD.. The reserch im ws to find n effective mitigtion method for controlling methne emissions from flooded rice fields to the tmosphere. 2. Mterils nd methods 2.1. Soil. A surfce smple 0 20 cm of sndy-lom soil collected from the Agriculturl Experimentl Sttion of Ghent University, Belgium, ws ir-dried, crushed to 2 mm nd thoroughly mixed for use.

3 ( ) X. Xu et l. The Science of the Totl En ironment Its min physicl nd chemicl properties were cly 8.9%, silt 1.9%, snd 76.2%, totl N 0.15%, vilble N 16.7 mg kg, orgnic C 1.75% nd ph H O Experiment 1: with no whet strw powder incorported into surfce lyer soil Two-kilogrm ir-dried nd 2-mm-seived soil smples were put in ech experimentl pot, nd 1.0 g K 2HPO nd 2.0 g ure were pplied s bsl fertilizers before flooding. Seven 20-dy-old helthy rice seedlings were trnsplnted into the pot fter 1 dy of flooding. The pot experiment ws divided into plnted nd non-plnted groups. These two groups ll contined four urese nitrifiction inhibitors tretments: ure U., ure hydroquinone U HQ., ure dicyndimide U DCD. nd ure hydroquinone dicyndimide U HQ DCD.. Ech tretment for plnted group hd 12 replictions while tht for the non-plnted group ws three replictions. The ppliction rte of HQ nd DCD ws 0.3% nd 5% of pplied ure w w., respectively Zho et l., 1993; Xu et l., Throughout the period of the experiment the floodwter lyer on the surfce of soil ws 2 cm. At indefinite intervls, self-mde PVC cover covered ech experimentl pot nd ws seled with wter for h until the gs ws tken using 10-ml vcuum tube Fig. 1.. Methne concentrtion in the gs ws determined by gs chromtogrph GC. with FID indictor under the following conditions: injector temperture 65 C, oven temperture 55 C nd detector temperture 200 C Boeckx et l., Ammonium-N nd nitrite plus nitrte. -N concentrtions in the floodwter were determined by the MgO-Devrd lloy procedure Keeney nd Nelson, Every 20 dys the bove-ground prts of rice were cut, nd methne emissions from the cut rice soil system ws determined with the close PVC method s described bove. The fresh weight of boveground prts of rice ws observed until the end of the experiment. Fig. 1. Simple pprtus for collecting gs smples Experiment 2: with whet strw powder incorported into surfce lyer soil To understnd the influences of relted inhibitors on methne emission from rice field ecosystem mended with orgnic mtter, whet strw powder 10 g. ws incorported into the surfce lyer soil in ech experimentl pot before flooding. The pot experiment ws lso divided into plnted nd non-plnted groups. These two groups only contined three tretments: no ure CK., ure U. nd ure HQ DCD U HQ DCD., nd ech tretment ws replicted three times. Methne emissions from the non-plnted soil system nd intct rice soil system, mmonium-n nd nitrite plus nitrte. -N concentrtions in the floodwter were determined during the whole period of cultivtion s described in Experiment 1. The fresh weight of bove-ground prts of rice ws lso observed t the end of the experiment. 2.. Clcultion nd sttisticl nlysis Methne emissions from the non-plnted soil system, intct rice soil system, nd cut rice soil system were clculted s follows: V0. F W t where F represents methne emission g CH kg ir-dried soil h.; W is the weight of irdried soil in ech pot kg.; V is the vilble

4 26 ( ) X. Xu et l. The Science of the Totl En ironment procedures using the SPSS for windows softwre pckge. Multiple liner or curve regressions were used to fit independent vribles in the models, which indicted the reltionship between methne emission from rice field ecosystems nd N turnover in flooded surfce soil nd showed the beneficil function of diminishing methne emission from rice field ecosystems to rice growth. 3. Results nd discussions 3.1. Dynmic nd totl mount of methne emission from simulted rice field ecosystem during rice growth Fig. 2.. Dynmic of CH emission from intct rice soil system without pplying whet strw powder error br in this figure indictes stndrd devition of three replictions for ech tretment, the sme below.. b. Dynmic of CH emission from non-plnted soil system without pplying whet strw powder. volume in the self-mde PVC ml.; is methne density in 10-ml vcuum tube determined by GC g CH ml. nd t is the time closed prior to smpling h.. All dt received were subjected to nlysis of vrince nd to Duncn s multiple rnge tests In the bsence of whet strw powder, the dynmics of methne emissions from non-plnted soil systems nd intct rice soil systems during the whole period of cultivtion re shown in Fig. 2,b. All tretments with inhibitors, especilly with DCD nd with DCD HQ pprently could reduce methne emissions from both systems within 80 dys fter fertiliztion, nd hence significntly eliminted totl mount of methne emission, prticulrly tht from intct rice soil system T- ble 1.. In the presence of DCD nd of DCD HQ, the fresh weight of bove-ground prts of rice lwys remined much higher during the whole period of cultivtion Fig. 3., thereby indicting the formtion of more highly oxidized rhizosphere. The presence of more highly oxidized rhizosphere would result in the reduction of methne emission from rice fields to the tmosphere Denier Vn der Gon nd Neue, Keerthisinghe et l lso reported tht nitrifiction inhibitors could significntly eliminte methne emission from rice fields. Methne emissions from cut rice soil systems in ny tretment reched the highest vlue t dy 60 fter fertiliztion, when the rice plnt entered its vigorous vegettion growth stge Tble 2.. Photosyntheticlly produced crbohydrtes by rice plnts possibly benefited from methnogensis growth in the rhizosphere. Kimur et l , Whiting nd Chnton nd Thoms et l.

5 ( ) X. Xu et l. The Science of the Totl En ironment Tble 1 Totl mount of CH emissions from non-plnted soil system nd intct rice soil system without pplying whet strw powder mg.. CH pot nd their reltive vlues % Tretments Intct rice soil system Non-plnted soil system Totl CH emission Reltive vlues Totl CH emission Reltive vlues mg CH pot. %. mg CH pot. %. c U U HQ b c U DCD b U HQ DCD The vlues in the tble nd in the prentheses represent the men nd stndrd devition of three replictes for ech tretment, respectively. Within ech column, mens followed by different superscript letters re significntly different t P The sme below. c ll showed tht methne ws produced from substnces relesed by rice root. In the presence of DCD nd of DCD HQ, methne emissions from the cut rice soil system within 80 dys fter fertiliztion were much higher thn in the bsence of inhibitor. s Tble 2.. This ws the opposite to methne emissions from the intct rice soil system Fig. 2.. Accordingly, the reduction of methne emission from flooded rice soil ws relted to the incresed methne oxidtion in rice root rhizosphere. DCD, especilly in combintion with HQ, significntly enhnced methne oxidtion in the rice root rhizosphere, prticulrly between its tillering nd booting stges t dys fter fertiliztion.. With whet strw powder mended ll tretments hd much higher totl mount of methne emissions from the non-plnted soil system nd Fig. 3. Dynmic of fresh weight of bove-ground prts of rice. intct rice soil systems Tble 3., s compred to those without whet strw powder Tble 1.. Methne emissions from both systems ll incresed with time, nd reched their mximum vlues t dy 30 fter fertiliztion Tble.. This is obviously relted to the nerobic decomposition of pplied whet strw powder entering its pek time Ygi nd Minmi, 1990; Chidthisong nd Wtnbe, In comprison with the control no ure., the tretments with ure lone nd with HQ DCD could significntly reduce methne emission from intct rice soil systems but n inhibitory influence of HQ DCD on methne emissions ws not observed Tbles 3 nd.. Although significntly higher methne emissions from the ure-treted soil system t dys fter fertiliztion ws found Tble., its totl mount in ny tretment during the whole period of cultivtion hd no difference P Tble 3.. If nitrogen ws pplied it would improve rice growth in the presence of whet strw powder dt not shown., nd more highly oxidized rhizosphere ws promoted. This resulted in much lower methne emissions from intct rice soil ecosystems during rice growth Tbles 3 nd.. Anerobic decomposition of pplied whet strw powder offered some C sources for soil microbes, nd would ccelerte the bio-degrdtion of pplied inhibitor. s in the ure-treted soil to some degree Huser nd Hselwndter, 1990; Zho et l., Therefore, no pprent inhibi-

6 28 ( ) X. Xu et l. The Science of the Totl En ironment Tble 2 Dynmic of CH emission from cut rice soil system without pplying whet strw powder g CH kg ir-dried soil h. Tretments Dys fter fertiliztion U U HQ U DCD U HQ DCD b b b c b b b b b c b tory effects of HQ DCD on methne emissions from non-plnted soil systems nd intct rice soil systems were observed. The totl mount of methne emission from the intct rice soil system in the control hd no pprent vrition but significntly decresed in the other two tretments P 0.05., s compred to tht from the non-plnted soil system Tble 3.. Hence, when dissolved methne in the whet strw-treted soil shrply incresed Nouchi et l., 199; Wssmn et l., 1993., pplied N could pprently promote methne oxidtion in the rhizosphere of rice through improving rice growth Methne emission from rice field ecosystem s. mmonium-n concentrtion in the floodwter The highest vlue of whet strw mended mmonium-n concentrtion in the floodwter in ny tretment ppered t dy 30 fter fertiliztion Tble 5., nd methne emissions from the soil system nd intct rice soil system lso reched the highest vlues, respectively Tble.. Regression nlysis showed tht methne emission from rice field ecosystems mended with whet strw hd significntly positive exponentil reltionship with mmonium-n concentrtion in the floodwter Fig... Nmely, mmonium-n concentrtions in the floodwter less thn mg N l could result in much lower methne emissions from rice field ecosystems while higher concentrtions were more conducive to incresing methne emissions. Hence, under these conditions, the oxidtion cpcities of methne in flooded rice fields were vrible with mmonium- N level in the floodwter. Mrtin nd Rlf confirmed tht mmonium concentrtions higher thn 5 22 mmol l in the soil queous phse inhibited methne oxidtion, but lso showed tht lower concentrtions were not inhibitory nd even resulted in stimultion. In the bsence of whet strw, mmonium-n concentrtion in the floodwter in ny tretment decresed by degrees with time Tble 6., pprently different from tht in the presence of whet strw Tble 5.. Methne emissions from rice field ecosystems without whet strw hd significntly negtive logrithmic reltionship with mmonium concentrtion in the floodwter Fig. 5.. Accordingly, under this condition methne emissions from rice field ecosystem would decrese with the increse of mmonium concentr- Tble 3 Totl mount of CH emissions from non-plnted soil system nd intct rice soil system mended with whet strw powder g CH pot. nd their reltive vlues %. Tretments Intct rice soil system Non-plnted soil system Totl CH emission Reltive vlues Totl CH emission Reltive vlues g CH pot. %. g CH pot. %. b CK U U HQ DCD

7 ( ) X. Xu et l. The Science of the Totl En ironment Tble Dynmics of CH emissions from non-plnted soil system nd intct rice soil system mended with whet strw powder g CH kg ir-dried soil h. Tretments Dys fter fertiliztion With rice CK U b U HQ DCD b b b b No rice CK U U HQ DCD b b b b b b tion in the floodwter. The tretments with inhibitor. s, especilly with HQ DCD could significntly enhnce mmonium concentrtions in the floodwter Tble 6.. Hence, methne emission from rice field ecosystems mended with inhibitor. s significntly decresed Tble 1.. Step regression nlysis showed tht mmonium-n nd nitrite nitrte-n concentrtions in the floodwter ll significntly influence methne emissions from rice field ecosystems. With whet strw powder incorported into the flooded surfce lyer soil: Y b, 2 R 0.622, P 0.05 Without pplying whet strw powder:. 1 Y b, 2 R 0.633, P where Y is the methne emission from the rice field ecosystem g CH kg ir-dried soil h. ; nd b represent mmonium-n nd nitrite nitrte-n concentrtions in the floodwter mg N l., respectively. From Eqs.. 1 nd. 2, the reltive importnce of mmonium-n concentrtion in the floodwter to methne emission from rice field ecosystems mended with whet strw ws higher thn tht of nitrte nitrite-n concentrtion, but this phenomenon in the bsence of whet strw ws not found. Hence, methne emission from rice field ecosystem ws relted to the turnover of mmonium-n in the flooded surfce lyer soil. Tble 5 Dynmic of NH -N concentrtion in the floodwter on the surfce of soil mended with whet strw powder mg N l. Tretments Dys fter fertiliztion With rice CK U U HQ DCD b b b b b b b b No rice CK U U HQ DCD b b b b b b b b

8 250 ( ) X. Xu et l. The Science of the Totl En ironment Fig.. Reltionship between CH floodwter mg N l.. emission from rice field ecosystem mended with whet strw nd NH -N concentrtion in the The ppliction of orgnic mtter could influence this reltion Methne emission from rice field ecosystem s. fresh weight of bo e-ground prts of rice Without pplying whet strw powder: Y , 2 R 0.99, P With whet strw powder incorported into the flooded surfce lyer soil: Y , 2 R 0.88, P where Y is the methne emission from the rice field ecosystem g CH kg ir-dried soil h. ; is the fresh weight of bove-ground prts of rice g pot.. Eqs.. 3 nd. ll showed tht methne emissions from rice field ecosystems hd significntly Tble 6 Dynmic of NH -N concentrtion in the floodwter on the surfce of soil without pplying whet strw powder mg N l. Tretments Dys fter fertiliztion With rice U U HQ U DCD U HQ DCD b b b b b c b No rice U U HQ U DCD U HQ DCD b b b b b b b b b b c b b b b

9 ( ) X. Xu et l. The Science of the Totl En ironment Fig. 5. Reltionship between CH the floodwter. emission from rice field ecosystem without pplying whet strw nd NH -N concentrtion in negtive liner reltionship with the fresh weight of bove-ground prts of rice. Hence, diminishing methne emissions from this wetlnd rice cultivtion ws significntly beneficil to the growth of rice. This ws in ccordnce with the results observed by Sss et l nd by Nouchi et l Conclusions In the bsence of whet strw, methne emissions from inhibitor. s -treted intct rice soil systems nd soil systems ppered to hve much lower vlues during rice growth, which ws contrry to methne emissions from the cut rice soil system. The ppliction of dicyndimide DCD. nd especilly of DCD hydroquinone HQ. induced tht the totl mount of methne emissions from the soil system nd the intct rice soil system ws % nd % of the control, respectively. Hence, DCD, especilly in combintion with HQ, could eliminte methne emissions from rice cultivtion fields following ppliction of ure. Whet strw incorported into flooded surfce lyer soil significntly wekened the bove-mentioned inhibitory effects. Regression nlysis indicted tht methne emissions from rice field ecosystems were relted to the turnover of mmonium-n in the flooded surfce lyer soil. Diminishing methne emissions from the wetlnd rice cultivtion ws significntly beneficil to the growth of rice. In recent yers, some investigtions hve showed the right choice of rice vriety nd irrigtion mngement my result in mitigting or reducing methne emissions from rice cultivtion Singh et l., 1999; Zheng et l., 1999; Xu nd Zhou, Even so, lot of work regrding the cultivtion of rice vriety with high root-oxidizing cpcity nd with high yield need to be done, nd some resonble irrigtion mngement for diminishing methne emissions from rice fields cn usully induce n increse in nitrous oxide emission. Thus, combintion of both inhibitors with ure my be n effective mitigtion strtegy for controlling methne emissions from flooded rice fields to the tmosphere. Wht s more, this strtegy is lso beneficil to diminishing the gseous N losses out of ure-treted soil whet

10 252 ( ) X. Xu et l. The Science of the Totl En ironment systems Xu et l., nd to the reduction of nitrous oxide emissions from rice fields dte not shown.. Acknowledgements Funding for this reserch ws prtilly provided by ABOS Ministry of Foreign Affirs, Belgium. nd by the Ntionl Nturl Sciences Foundtion of Chin Grnt No This work ws lso finncilly supported by LAPC-KF from the Institute of Atmospheric Physics, Chinese Acdemy of Sciences, by IAP progrm nd by RCEES-KIP References Bchelet D, Neue HU. Methne emission from wetlnd rice res of Asi. Chemosphere 1993;26: Brtlett KB, Hrriss RC. Review nd ssessment of CH emissions from wetlnds. Chemosphere 1993;26: Boeckx P, Vn Cleemput O, Villrlvo IDA. Methne emission from lndfill nd the methne oxidising cpcity of its covering soil. Soil Biol Biochem 1996; : Ci ZC, Xu H, Zhng HH. Estimte of methne emission from rice pddy field in Tihu Region, Chin. Pedosphere 199;. : Chidthisong A, Wtnbe I. Methne formtion nd emission from flooded rice soil incorported with 13 C-lbeled rice strw. Soil Biol Biochem 1997;29. 8 : Conrd R, Rothfuss E. Methne oxidtion in the soil surfce lyer of flooded rice field nd the effect of mmonium. Biol Fert Soils 1991;12: Denier Vn der Gon HAC, Neue HU. Oxidtion of methne in the rhizosphere of rice plnts. Biol Fert Soils 1996;22: FAO. Fertilizer yerbook. FAO sttisticl series No. 83. Fertilizer yerbook. vol. 39. Rome, Itly:FAO Huser M, Hselwndter K. Degrdtion of dicyndimide by soil bcteri. Soil Biol Biochem 1990;22. 1 :1131. IPCCHoughton JT, Cllender BA, Vrney SK, editors. Climte chnge. The supplementry report to the IPCC scientific ssessment. Cmbridge: Cmbridge University Press, 1992:225p. Keeney DR, Nelson DW. Nitrogen inorgnic forms. In: Pge AL, Miller RH, Keeney DR, editors. Methods of soil nlysis, Prt 2. Chemicl nd microbiologicl properties. 2nd Mdison, Wisconsin, USA: Americ Society of Agronomy, Inc, 1982: Keerthisinghe DG, Freney JR, Mosier AR. Effect of wxcoted clcium crbide nd nitrpyrin on nitrogen loss nd methne emission from dry-seeded flooded rice. Soil Biol Biochem 1993;16. 1 : Khlil MAK, Rsmussen RJ. Sources, sinks nd sesonl cycles of tmospheric methne. J Geophys Res 1983;88: Kimur M, Murkmi H, Wd H. CO 2, H 2 nd CH production in rice rhizosphere. Soil Sci Plnt Nutr 1991;37: Kimur M. Methne emission from pddy soils in Jpn nd Thilnd. In: Btjies NH, Bridges EM, editors. World inventory of soil emission potentils. WISE report 2. Wgeningen: ISRIC, 1992:215. Krogmeier MJ, Mccrty GW, Bremner JM. Potentil phytotoxicity ssocited with the use of soil urese inhibitors phenylphosphorodimidte N- n-butyl. thiophosphoric trimide.. Proc Ntl Acd Sci USA 1989;86.: Lindu CW. Methne emission from Louisin rice fields mended with nitrogen fertilizers. Soil Biol Biochem 199;26. 3 : Mrtin B, Rlf C. Effect of methne concentrtion nd soil conditions on the induction of methne oxidtion ctivity. Soil Biol Biochem 1995;27: Neue HU, Roger PA. Rice griculture: fctors controlling emissions. In: Khlil MAK, Sherer M, editors. Globl tmospheric methne. NATO ASI ARW series. 1993: Nouchi I, Hosono T, Aoki K. Sesonl vrition in methne flux from rice pddies ssocited with methne concentrtion in soil wter, rice biomss nd temperture, nd its modelling. Plnt Soil 199;161: OECD. Environmentl Dt Compendium. Emissions of greenhouse gses., lte 1990s, 1999:53 pp. Sss RL, Fisher FM, Hrcombe DA. Methne production nd emission in Texs rice field. Globl Biogeochem Cycles 1990;:7 68. Singh S, Singh JS, Kshyp AK. Methne flux from irrigted rice fields in reltion to crop growth nd N-fertiliztion. Soil Biol Biochem 1999;31: Thoms KL, Price D, Lloyd D. A comprison of different methods for the mesurement of dissolved gs grdients in wterlogged pet cores. J Microbiol Meth 1995;2: Wng MX, Shnggun XJ. CH emission from vrious rice fields in P.R. Chin. Theor Appl Climtol 1996;55: Wng MX. Atmospheric chemistry. Beijing: Meteorologicl Press, 1999: Wssmn R, Ppen H, Rennenberg H. Methne emission from rice pddies nd possible mitigtion strtegies. Chemosphere 1993;26: Whiting GJ, Chnton J. Primry production control of methne emission from wetlnds. Nture 1993;36: Xu XK, Zhou LK, Vn Cleemput O, Wng ZJ. Fte of ure- 15 N in soil-whet system s influenced by urese inhibitor hydroquinone nd nitrifiction inhibitor dicyndimide. Plnt Soil 2000; :

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