Effect of moisture, texture and aggregate size of paddy soil on production and consumption of CH4

Effectofmoisture,textureandaggregatesizeofpaddysoilonproduction
andconsumptionofCH
4

UdoJaÈckel,SylviaSchnell,RalfConrad
*

Max-Planck-InstitutfuÈrterrestrischeMikrobiologie,Karl-von-Frisch-Strasse,D-35043Marburg,Germany
Received8March2000;accepted4December2000

Abstract
Laboratoryexperimentswereconductedto®ndoutunderwhichconditionsthesoilfromItalianrice®eldscouldchangefromasourceinto
asinkofatmosphericCH
4.Moist(30%H2O󰂈68%ofthemaximumwaterholdingcapacity(whc))rice®eldsoiloxidizedCH4
withbiphasic

kinetics,exhibitingbothalow(145ppmvCH4)andahigh(20,200ppmvCH4)KmvalueandVmaxvaluesof16.8and839nmolgdw21h21,
respectively.TheactivitywiththelowK
mallowedtheoxidationofatmosphericCH4.UptakeratesofhighCH4
concentrations(16.5%v/v)

andofO
2linearlydecreasedwithaggregatesizeofsoilbetween2and10mm.AtmosphericCH4
(1.8ppmv)wasconsumedinsoilaggregates

,6mm,butsoilaggregates.6mmreleasedCH4intotheatmosphere.Similarly,netuptakeofatmosphericCH4turnedintonetreleaseof
CH
4whenthesoilmoisturewasdecreasedbelowawatercontentofabout20%whc.TheuptakerateofatmosphericCH4
increasedthreefold

whenthesoilwasamendedwithsterilequartzsand.Floodedmicrocosmswithnon-amendedandquartz-amendedsoilemittedCH4intothe
atmosphere.TheCH
4emissionrateincreasedwhenthe¯uxwasmeasuredunderanatmosphereofN2
insteadofair,indicatingthat30±99%

oftheproducedCH4wasoxidizedintheoxicsoilsurfacelayer.Removalofthe¯oodwaterresultedinincreaseofCH4emissionratesuntila
watercontentofabout75±82%whcwasreached,andsubsequentlyinarapiddecrease.However,thesoilmicrocosmsnevershowednet
uptakeofatmosphericCH
4.OurresultsshowthatthemicroorganismsconsumingatmosphericCH4
wereinactivatedatanearlierstageof

drainagethanthemicroorganismsproducingCH
4
,irrespectiveofthesoilporositywhichwasadjustedbyadditionofquartzsand.Hence,itis

unlikelythattheItalianrice®eldscanactasanetsinkforatmosphericCH
4
evenwhendrained.q2001ElsevierScienceLtd.Allrights

reserved.

Keywords:Methaneconsumption;Rice®eldsoil;Drainage;AtmosphericCH
4

1.Introduction
The¯uxofCH
4fromrice®eldsintotheatmosphereisdeterminedbythebalancebetweenCH4productionintheanoxicsoilandCH4oxidationintheoxicsoillayersatthesurfaceandaroundthericeroots.Whereas¯oodedrice®eldsareanimportantsourceofatmosphericCH4,animportantgreenhousegas(CiceroneandOremland,1988;Crutzen,1995;Watsonetal.,1992),rain-fedrice®eldshavebeenshowntoactasasinkexceptwhenthesoilissaturatedwithwaterafterrainfall(Singhetal.,1998a).ManystudieshaveaddressedthebalancebetweenproductionandoxidationofCH4in¯oodedrice®elds(Holzapfel-Pschornetal.,1985;SchuÈtzetal.,1989;DeniervanderGonandNeue,1996;Sigrenetal.,1997)andhavestudiedtheCH4oxidationprocessintheoxicsoilsurfacelayerandtherhizosphere(ConradandRothfuss,1991;BosseandFrenzel,1997;Kumaraswamyetal.,1997;Wata-nabeetal.,1997;GilbertandFrenzel,1998;BodelierandFrenzel,1999).Thesestudiesfoundthat20±90%oftheCH4

producedisoxidizedbeforeitreachestheatmosphere.The

CH
4
emissionrateisdrasticallyreducedwhen¯oodedrice

®eldsaredrainedduetothedecreasedproductionofCH
4

(Sigrenetal.,1997;RateringandConrad,1998).However,

itisunclearwhetherthese®eldscanactasasinkforatmo-
sphericCH4upondrainage(Thurlowetal.,1995).
Themethanotrophicbacteriain¯oodedricesoilare
exposedtoratherhighCH
4
concentrationsinthemillimolar

range.Thesemethanotrophsarecharacterizedbyarela-
tivelylowaf®nityforCH4,theKmvaluesforCH4typically
being.1mM.Ontheotherhand,methanotrophicbacteria
exhibitingmuchlowerK
m
values,typically20±200nM

(Conrad,1996),consumeCH
4
atlowatmosphericconcen-

trations.Thesemethanotrophstypicallyoccurinupland
soilsthatconsumeatmosphericCH4(BenderandConrad,
1993),butcanalsobeactivatedinpaddysoil(Benderand
Conrad,1992).However,theoxidationofatmosphericCH
4

SoilBiology&Biochemistry33(2001)965±971
0038-0717/01/$-seefrontmatterq2001ElsevierScienceLtd.Allrightsreserved.
PII:S0038-0717(00)00248-0

www.elsevier.com/locate/soilbio
*Correspondingauthor.Tel.:149-6421-178801;fax:149-6421-78809.
E-mailaddress:conrad@mailer.uni-marburg.de(R.Conrad).
hashardlybeenstudiedinirrigatedrice®eldsoils.Thus,itwasfoundthatmoisturizedair-driedpaddysoildoesnotoxidizeCH4atatmosphericconcentrationsunlessithasbeenpreincubatedunderelevatedCH4concentrationsbeingnecessarytoinduceCH4oxidationactivity(HenckelandConrad,1998;Henckeletal.,1999).Similarly,theabil-itytooxidizeatmosphericCH4isinducedin¯oodedricesoilmicrocosmspreparedfromair-driedsoilandincubatedunderambientconditions(Henckeletal.,2000a),indicatingthattheCH4producedinthedeeperanoxicsoillayersissuf®cienttoinduceCH4oxidationactivityintheupperoxicsoillayers.However,itisunclearunderwhichconditionsCH4oxidationmaybecomesopronouncedthattherice®eldsoilturnsfromasourceintoasinkforatmosphericCH4.Therefore,westudiedtheproductionandoxidationofCH4indrainedItalianrice®eldsoilsunderde®nedlabora-toryconditions.2.MaterialsandmethodsThesoiloriginatedfromrice®eldsoftheRiceResearchInstituteinVercelli,Italy,andwasofloamytexturewith19%clay,56%siltand25%sand(RateringandConrad,1998).AftersamplinginSpring1997whenthe®eldsweredrainedandfallowthesoilwasair-driedandstoredasdrylumps(1.5%H2O)atroomtemperature.Fortheexperi-ments,thesoilwasmechanicallycrushedandsprayedwithdeionizedwatertoobtainagravimetricwatercontentof30%,equivalentto68%ofthemaximumwaterholdingcapacity(whc).Subsequentlythemoistsoilwaspassedthroughastainless-steelsieveof2mmmeshsize.Thispreparationprotocolmimicsthe®eldsituationinItaly,wherethesoilstaysmoreorlessdryduringthefallowperiod,followedbyplowing,harrowingand¯oodinginthespring.Thesoilsampleswerethenprocessedintwodifferentwaystoinducemethanotrophicactivity,i.e.preparationof¯oodedsoilmicrocosmsorpreincubationunderanelevatedCH4atmosphere.Soilmicrocosmsconsistedofplexiglasstubes(length󰂈10cm;i.d.󰂈3.4cm)whichwere®lledwithsievedsoil.Insomemicrocosms50gsoilweremixedwith16.6gsterilequartzsand(Merck,Darmstadt,Germany)ofeither100±200mmor200±400mmdiameterbysieving.Thesoilwasthen¯oodedwithdeionizedwaterandincubatedfor6weeksat258Cindarkness.Then,thesurfacewaterwasremovedandincubationcontinued.Thedecreaseofsoilmoisturewasfollowedgravimetrically.The¯uxofCH4betweenthesoilandtheatmospherewasmeasuredbyclosingtheheadspaceofthemicrocosmswitharubberstopperandfollowingthetemporalchangeoftheCH4concentration.ThecontributionofCH4oxidationtothenetCH4¯uxwasdeterminedafterexchangeoftheairheadspacewithN2andrepeatingthe¯uxmeasurement.TherateofCH4oxidationwascalculatedfromtheCH4¯uxunderN2minustheCH4¯uxunderair.ThepotentialforoxidationofatmosphericCH4wasdeterminedbytransferof
asectionofthesoilmicrocosm(0±2mmdepth)intoaglass
bottle(seebelow),aeratingwithambientairandmeasuring
thedecreaseofCH
4
withtime.Allrateswerecalculatedper

cm
2
soilsurface,gramdryweight(gdw)orpergramfresh

weight(gfw)soil.
TheconsumptionandproductionofCH
4
insievedsoil

samplesat30%moisturewasmeasuredafterpre-incubation
for8±14daysunderanairatmospherecontaining20%CH
4

at258C.Afterpre-incubation,thesoilwasagainsieved

through2mmmeshsizeand®lled(10-galiquots)into
glassbottles(150ml;MuÈller&Krempel,BuÈlach,Switzer-
land)whichwereaeratedwithairandclosedwithlatex
stoppers.TheconcentrationofCH
4
wasadjustedbyinjec-

tionofthedesiredamountofCH
4,andtheinitialrateofCH4

oxidationwasdeterminedat258Cbymeasuringthe

decreaseoftheCH4intheheadspacewithtime.Vmaxand
K
m
valuesweredeterminedbynon-linearcurve®ttingofthe

initialCH
4oxidationratesagainsttheinitialCH4
concentra-

tionsusingtheMichaelis-MentenequationandOrigine5.0
(Microcal,Northampton,MA).
Differentaggregatesizesofthesoilwerepreparedby
pressingthepre-incubatedsoilthroughholesof2,4,6,8
and10mmdiameterwhichweredrilledthroughastainless
steeldisk.Theaggregates(8±12-galiquots)werethen®lled
intoglassbottles(seeabove),andincubatedunderahead-
spaceofambientaircontaining1.8ppmvCH
4
.Thechange

oftheCH
4
concentrationwithtimewasmeasuredandused

todetermineeitherthepseudo®rst-orderrateconstantof
CH4oxidation(ifCH4wastakenup)ortherateofCH
4

production(ifCH4wasreleased).CH4oxidationrates

werecalculatedfromtheuptakerateconstantsusinga
CH
4concentrationof1.8ppmv.OxidationofCH4
was

alsomeasuredathighCH4concentrationsusingagashead-
spacecontaining16.5%(v/v)CH
4
.

Forexperimentswithdifferentsoilmoistures,theprein-
cubatedsoilwassievedthrough2mmmeshanddistributed
(5-galiquots)intoserumbottles(80ml).Soilmoisturewas
adjustedtogiveaninitialwatercontentbetween7and97%
whcbyeitherslowlydryingwithaventilatororsprayingthe
soilwithdeionizedwater.Thebottleswhichcontained
ambientairwerethenclosed,andthechangeoftheCH
4

wasfollowedwithtime.RatesofCH4oxidationandproduc-

tionweredeterminedasdescribedabove.Similarexperi-
mentsweredonebyincubationofthesoilunderN
2

atmosphereresultinginthereleaseofCH4.

Soiltexturewasadjustedbymixing200gofmoist
soilwith200or400gofquartzsand(originalsize
mixturesuppliedbyMerck,Darmstadt,Germany).The
mixturewasthenpre-incubatedunder20%(v/v)CH
4

andusedforthemeasurementofCH4oxidationat

ambientCH
4
concentrationsasdescribedabove.Rates

aregivenpergfwsoil.
Alltheexperimentsdescribedwereconductedintripli-
cate.MethaneandO
2
wereanalyzedin0.5±1.0-mlgas

samplesinagas-chromatographwith¯ameionization

U.JaÈckeletal./SoilBiology&Biochemistry33(2001)965±971966