12Microbialcontrolofsoilorganicmatterminera

MicrobialcontrolofsoilorganicmattermineralizationresponsestolabilecarboninsubarcticclimatechangetreatmentsKATHRINROUSK1,2,ANDERSMICHELSEN1,2andJOHANNESROUSK31TerrestrialEcologySection,DepartmentofBiology,UniversityofCopenhagen,Universitetsparken15,2100Copenhagen,Denmark,2CenterforPermafrost(CENPERM),UniversityofCopenhagen,ØsterVoldgade10,1350Copenhagen,Denmark,3SectionofMicrobialEcology,DepartmentofBiology,LundUniversity,22362Lund,SwedenAbstractHalftheglobalsoilcarbon(C)isheldinhigh-latitudesystems.ClimatechangewillexposethesetowarmingandashifttowardsplantcommunitieswithmorelabileCinput.LabileCcanalsoincreasetherateoflossofnativesoilorganicmatter(SOM);aphenomenontermed‘priming’.Weinvestigatedhowwarming(+1.1°Coverambientusingopentopchambers)andlitteraddition(90gm
2yr
1)treatmentsinthesubarcticinfluencedthesusceptibilityofSOMmineralizationtopriming,anditsmicrobialunderpinnings.LabileCappearedtoinhibitthemineralizationofCfromSOMbyupto60%withinhours.Incontrast,themineralizationofNfromSOMwasstimulatedbyupto300%.Theseresponsesoccurredrapidlyandwereunrelatedtomicrobialsuccessionaldynamics,suggestingcatabolicresponses.Consideredseparately,thelabileCinhibitedCmineralizationiscompatiblewithpreviouslyreportedfind-ingstermed‘preferentialsubstrateutilization’or‘negativeapparentpriming’,whilethestimulatedNmineralizationresponsesechorecentreportsof‘realpriming’ofSOMmineralization.However,CandNmineralizationresponsesderivedfromthesameSOMsourcemustbeinterpretedtogether:ThissuggestedthatthemicrobialSOM-usedecreasedinmagnitudeandshiftedtocomponentsricherinN.ThisfindinghighlightsthatonlyconsideringSOMintermsofCmaybesimplistic,andwillnotcaptureallchangesinSOMdecomposition.TheselectiveminingforNincreasedinclimatechangetreatmentswithhigherfungaldominance.Inconclusion,labileCappearedtotriggercatabolicresponsesoftheresidentmicrobialcommunitythatshiftedtheSOMminingtoN-richcomponents;aneffectthatincreasedwithhigherfungaldominance.Extrapolatingfromthesefindings,thepredictedshrubexpansioninthesubarcticcouldresultinanalteredmicrobialuseofSOM,selectivelyminingitforN-richcomponents,andlead-ingtoareducedtotalSOM-use.Keywords:bacterialgrowth,biogeochemistry,fungalgrowth,microbialecology,nitrogencycling,nitrogenlimitation,soilcarbonsequestration,theprimingeffectReceived18January2016;revisedversionreceived17February2016andaccepted11March2016IntroductionNearlyhalfoftheglobalsoilcarbon(C)stockisheldinhigh-latitudeecosystems(Schuuretal.,2009).TheCbalanceinthesesystemsisregulatedbythebalancebetweenCinput,viaprimaryproductivityandClosses,viasoilorganicmatter(SOM)decomposition(McKaneetal.,1997).Inadditiontothedirecteffectsofwarming,ongoingclimatechangeisalsoprojectedtofundamentallychangeArcticecosystems,includingplantcommunityshiftstowardswoodyshrubgrowth(Mynenietal.,1997;Sistlaetal.,2013)andsoilfoodwebrestructuring(Neutel&Thorne,2014;Crowtheretal.,2015).Theshrubexpansionwillaffectthetype,rateandspatialallocationofplantCinput(Sistlaetal.,2013),inducingashiftfromrecalcitrantmosstomorelabileshrubleavesatthesoilsurfaceandanincreaseinrootproductionwithanassociatedinputoflabilerhizo-sphereCinthesoilprofile(Chapinetal.,1996;Tapeetal.,2006;Hartleyetal.,2012).Thissystemicchangeinabove-groundcommunitieswillalsoaffectmicrobialdecomposers,bymodulatingtheirprocessratesandaffectingtheirbiomassproductionandsubsequentcon-tributiontoCsequestration(Clemmensenetal.,2013),andbyaffectingthestructureofthecommunity(Rin-nanetal.,2007a;Yergeauetal.,2012)includingthebal-ancebetweenfungiandbacteria(Deslippeetal.,2012;Rousk&Frey,2015).TherelativeimportanceofthesemechanismsalongwiththeirinterdependenceremainsCorrespondence:JohannesRousk,tel.[phone],fax[phone],e-mail:[e-mail]1©2016JohnWiley&SonsLtdGlobalChangeBiology(2016),doi:10.1111/gcb.13296uncertain,makingpredictionsforhowclimatechangewillaffectSOMdecompositionrates,andthesoilCbal-ance,inarcticsystemselusive.InadditiontothedirecteffectbychangesinCinputonsoilCstocks,theinputoflabileOMcanalsogreatlyaffecttherateofmineralizationofnativeSOM;aphe-nomenontermed‘priming’(Kuzyakovetal.,2000;Kuzyakov,2010).The‘primingeffect’isdefinedasanincreaseindecompositionrateofSOMinresponsetotheinputofexogenouslabileCsources(Kuzyakov,2010;Garcia-Pausas&Paterson,2011).AlthoughreportsofprimingeffectsonSOMmineralizationaregrowingincreasinglyabundantintheliterature(Kuzyakov,2010;Finzietal.,2015),theunderlyingmechanismsremainlargelyelusive,andaremostoftentreatedimplicitlyratherthanbeingexplicitlyaddressed(Hartleyetal.,2012;Hilletal.,2015).ThiscurrentlyhamperstheincorporationofmechanismsintosoilCmodels(Perveenetal.,2014;Finzietal.,2015).Explana-tionsforthemicrobialunderpinningsfortheprimingeffecthavebeguntoemergeinrecentyears,including,forexample,thenutrientmininghypothesis(Dijkstraetal.,2013;Chenetal.,2014).However,themostwidelyheldexplanationfortheprimingeffecttodateisthattheadditionoflabileCstimulatesthegrowthandmetaboliccontributionbymicrobialr-strategists,fol-lowedbythegradualincreaseinabundanceofK-strate-gistswhosemetaboli