57Ecosystemfluxesofhydrogeninamidlatitude

AcceptedArticleThisarticlehasbeenacceptedforpublicationandundergonefullpeerreviewbuthasnotbeenthroughthecopyediting,typesetting,paginationandproofreadingprocess,whichmayleadtodifferencesbetweenthisversionandtheVersionofRecord.Pleasecitethisarticleasdoi:10.1111/gcb.13463Thisarticleisprotectedbycopyright.Allrightsreserved.ReceivedDate:29-Apr-2016RevisedDate:22-Jul-2016AcceptedDate:29-Jul-2016Articletype:PrimaryResearchArticlesTitlepageTitle:Ecosystemfluxesofhydrogeninamid-latitudeforestdrivenbysoilmicroorganismsandplantsRunninghead(45characters):H2ecosystemfluxesinamid-latitudeforestListofauthors:LauraK.Meredith1,2,RóisínCommane3,TrevorF.Keenan4,StephenT.Klosterman5,J.WilliamMunger3,Pamela.H.Templer6,JianwuTang7,StevenC.Wofsy3,RonaldG.Prinn2Instituteorlaboratoryoforigin:(1)UniversityofArizona,DepartmentofEcologyandEvolutionaryBiology,Tucson,Arizona,USA(2)MassachusettsInstituteofTechnology,DepartmentofEarth,AtmosphericandPlanetaryScience,Cambridge,Massachusetts,USA(3)HarvardUniversity,DepartmentofEarthandPlanetaryScienceandSchoolofEngineeringandAppliedSciences,Cambridge,Massachusetts,USA(4)LawrenceBerkeleyNationalLaboratory,EarthSciencesDivision,Berkeley,California,USA(5)HarvardUniversity,DepartmentofOrganismicandEvolutionaryBiology,Cambridge,Massachusetts,USA(6)BostonUniversity,DepartmentofBiology,Boston,Massachusetts,USA(7)EcosystemsCenter,MarineBiologicalLaboratory,WoodsHole,Massachusetts,USAAcceptedArticleThisarticleisprotectedbycopyright.Allrightsreserved.Correspondingauthor:LauraK.Meredith,[phone],[e-mail](8):hydrogen,flux,soil,microbe,atmosphere,phenology,snow,carboncycleAbstractMolecularhydrogen(H2)isanatmospherictracegaswithalargemicrobe-mediatedsoilsink,yetcyclingofthiscompoundthroughoutecosystemsispoorlyunderstood.MeasurementsofthesourcesandsinksofH2invariousecosystemsaresparse,resultinginlargeuncertaintiesintheglobalH2budget.ConstrainingtheH2cycleiscriticaltounderstandingitsroleinatmosphericchemistryandclimate.WemeasuredH2fluxesathighfrequencyinatemperatemixeddeciduousforestfor15monthsusingatower-basedflux-gradientapproachtodetermineboththesoil-atmosphereandthenetecosystemfluxofH2.WefoundthatHarvardForestisanetH2sink(-1.4±1.1kgH2ha-1)withsoilsasthedominantH2sink(-2.0±1.0kgH2ha-1)andabovegroundcanopyemissionsasthedominantH2source(+0.6±0.8kgH2ha-1).AbovegroundemissionsofH2wereanunexpectedandsubstantialcomponentoftheecosystemH2flux,reducingnetecosystemuptakeby30%ofthatcalculatedfromsoiluptakealone.Soiluptakewashighlyseasonal(Julymaximum,Februaryminimum),positivelycorrelatedwithsoiltemperature,andnegativelycorrelatedwithenvironmentalvariablesrelevanttodiffusionintosoils(i.e.,soilmoisture,snowdepth,snowdensity).SoilmicrobialH2uptakewascorrelatedwithrhizosphererespirationrates(r=0.8,p0.001),andH2metabolismyieldedupto2%oftheenergygleanedbymicrobesfromcarbonsubstraterespiration.Here,weelucidatekeyprocessescontrollingthebiosphere-atmosphereexchangeofH2andraisenewquestionsregardingtheroleofabovegroundbiomassasasourceofatmosphericH2andmechanismslinkingsoilH2andcarboncycling.ResultsfromthisstudyshouldbeincorporatedintomodelingeffortstopredicttheresponseoftheH2soilsinktochangesinanthropogenicH2emissionsandshiftingsoilconditionswithclimateandlandusechange.IntroductionSoilmicroorganismsderiveenergyfromthetracelevels(500ppb;nmolmol-1or1e-9molmol-1)ofH2intheatmosphereanddrivethemajorityofturnoveroftheatmosphericburdenofH2(1.4to2.0yearatmosphericlifetime;Novellietal.,1999;Xiaoetal.,2007;EhhaltandRohrer,2009).MicrobialscavengingofatmosphericH2mayhelpsupplythebasalenergyrequirementsofsoilmicrobes,enablingnon-growinganddormantmicrobialcellstopersistthroughconditionsunfavorableforgrowth(Greeningetal.,2015c).WhilesoiluptakeiswidelyrecognizedasthelargestsinkintheH2budget(EhhaltandRohrer,2009),itisstillpoorlycharacterizedattheecosystemlevel.FurtherconstraintsonthesourcesandsinksofatmosphericH2areneededtounderstandandpredicttheroleofH2asanindirectgreenhousegas(Derwentetal.,2001)andasourceofwatervaportothestratosphere(LeTexieretal.,1998;Trompetal.,2003;Warwicketal.,2004).AcceptedArticleThisarticleisprotectedbycopyright.Allrightsreserved.SoiluptakeofatmosphericH2isoneoftheleastconstrainedcomponentsoftheH2budget,despitebeingthelargestoftheH2sourceandsinkfluxes.TheglobalatmosphericH2budgetincludestwomajoratmosphericH2sources,photodissociationofformaldehyde(HCHO)derivedfrommethaneandnon-methanehydrocarbons,andcombustionoffossilfuelsandbiomass,whicharenearlybalancedbyitstwomajorsinks,soilconsumptionandoxidationbyOH(EhhaltandRohrer,2009;Pieterseetal.,2013).Thelargestbudgetterms,soilconsumption(~75%oftotalsinks)andphotochemicalproduction(~50%oftotalsources),aretheleastwellconstrainedwithestimatesrangingfrom53to85TgH2yr-1and37to77TgH2yr-1,respectively,acrossstudiesinrecentyears(Xiaoetal.,2007;Bousquetetal.,2011;Yashiroetal.,2011;Yveretal.,2011a;Pieterseetal.,2013).AtmosphericH2molefractionsexhibitlargeyear-to-yearandmulti-yearvariability,primarilydrivenbyvariabilityinbiomassburningemissions,andh