2Ectomycorrhizalfungislowsoilcarboncycling

LETTEREctomycorrhizalfungislowsoilcarboncyclingColinAverill*andChristineV.HawkesDepartmentofIntegrativeBiologyUniversityofTexasatAustin,Austin,TX78712,USA*Correspondence:E-mail:[e-mail]soilorganiccarbonisoneofthelargestfluxesofCO2onearth.Understandingtheprocessesthatregulatesoilrespirationiscriticalforpredictingfutureclimate.Recentworkhassuggestedthatsoilcarbonrespirationmaybereducedbycompetitionfornitrogenbetweensymbioticectomycorrhizalfungithatassociatewithplantrootsandfree-livingmicrobialdecom-posers,whichisconsistentwithincreasedsoilcarbonstorageinectomycorrhizalecosystemsglob-ally.However,experimentaltestsofthemycorrhizalcompetitionhypothesisarelacking.Hereweshowthatectomycorrhizalrootsandhyphaedecreasesoilcarbonrespirationratesbyupto67%underfieldconditionsintwoseparatefieldexclusionexperiments,andthislikelyoccursviacom-petitionforsoilnitrogen,aneffectlargerthan2°Csoilwarming.Thesefindingssupportmycor-rhizalcompetitionfornitrogenasanindependentdriverofsoilcarbonbalanceanddemonstratetheneedtounderstandmicrobialcommunityinteractionstopredictecosystemfeedbackstoglobalclimate.KeywordsBiogeochemistry,ecosystemecology,mycorrhizalfungi,soilcarbon,soilecology,soilnitrogen.EcologyLetters(2016)INTRODUCTIONCarbon(C)storageinterrestrialecosystemsisregulatedbyCinputsfromnetprimaryproductionandCoutputsduetodecompositionandrespirationbymicrobialdecomposers(Schlesinger&Bernhardt2012).Nitrogen(N)limitationispervasiveinterrestrialecosystems,andcanlimitfluxesofCthroughbothprimaryproducersandfree-livingmicrobialdecomposers(Schimel&Weintraub2003;LeBauer&Trese-der2008).Plantswithroot-associatedectomycorrhizal(EM)fungalsymbiontsdominateboreal,temperate,montaneandsometropicalecosystems(Read1991;Tortietal.2001).ThesefungiproduceenzymesthatdegradeorganicN,whichcanunlockNtrappedinsoilorganicmattertofuelplantprimaryproduction(Rineauetal.2012;Shahetal.2015).However,bydoingsoEMfungimayinduceorexacerbateNlimitationoffree-livingmicrobialdecomposers.Theoreticalmodelspre-dictthatcompetitionbetweenEMfungiandfree-livingdecomposersforNwillslowsoilCcyclingandincreasesoilCstorage(Orwinetal.2011),whichissupportedbyaglobalpatternofincreasedsoilCstorageinEMecosystems(Averilletal.2014).Yet,despitethepotentialimportanceforpredict-ingsoilandecosystemCstorage,thereisstillnodirecttestofthemechanismofaction(Fernandez&Kennedy2015).Previousworkonthistopichasfocusedonlaboratorystud-ieswhichhaveshownevidencethatEMfungiselectivelymineorganicN(Abuzinadahetal.1986;Bending&Read1995;Rineauetal.2012)andslowdecompositionofplantlitter(Gadgil&Gadgil1975).Althoughtheselaboratorystudiesarecompellingmechanistically,theycannotevaluatetherele-vanceofectomycorrhizal-decomposercompetitionforbiogeo-chemicalcyclinginreal-worldconditions.TherearenotablecorrelationsbetweenfungalcommunitycompositionandsoilCsequestration,howevertheseremaintobeexperimentallyandmechanisticallyvetted(Clemmensenetal.2013,2015).Furthermore,whilesomefieldexperimentsreportchangesindecomposeractivityandsoilCturnoverwhenrootsandEMfungiareexcluded(Gadgil&Gadgil1971;Lindahletal.2010;Ekbladetal.2013;Phillipsetal.2013;Brzosteketal.2015),theycannotaccountfordisturbanceeffectsoftreat-mentsthatmaygenerateatemporaryincreaseinlabileCinputstosoil,increasingfree-livingmicrobialactivity.ForestgirdlingexperimentshaveshownpositiveeffectsofEMfungiandplantrootsonsoilCcycling,howeveragaincommunityeffectsareconfoundedwithdisturbanceofthetreatments(H€ogbergetal.2001;Subkeetal.2004,2011).Finally,ithasbeensuggestedthatinhibitionofsoilCcyclingbythepres-enceofEMrootsmaybedrivenbyasoildryingeffectratherthancompetitionbetweenEMfungiandsoilsaprotrophsperse(Bending2003;Koide&Wu2003).Becauseoftheseshortcomings,thereisstillnoclearbiogeochemicaltestofEM-decomposercompetitionalteringsoilCandNcycling.WeconductedtwocomplementaryfieldexperimentstotestforandquantifyEM-rootinhibitoryeffectsonsoilCandNcycling.BothexplicitlyincludedcontrolsforthedisturbanceinvolvedinexperimentallyexcludingEMfungi.First,wetestedtheeffectofEMexclusiononsoilCandNcyclingoverthecourseofonegrowingseasoninanoldgrowthtemperateforestusingmeshexclusionbags(‘EM-exclusionexperiment’).Second,wedeterminediftheeffectofEMfungalinhibitionwasquantitativebyleveragingagradientofEMfungalabun-danceoverthecourseofanentireyear,whichincludedEMexclusionsacrossthegradientcreatedbytrenching(‘EM-gra-dientexperiment’).Thegradientstudyallowedustocontrolfortheroleofsoildrying,asdryingeffectsshouldbepresentinboththelow-andhigh-EMforestsites.Wemeasuredchangesinsoilrespirationpergrammicrobialbiomass(here-after,‘biomass-specificrespiration’)andpergramsoil,aswellasindicatorsofmicrobialNavailability.Biomass-specificres-pirationrateshavebeenwidelyusedtobetterunderstandsoil©2016JohnWiley&SonsLtd/CNRSEcologyLetters,(2016)doi:10.1111/ele.12631respirationresponsestotemperature(Bradfordetal.2008;Hartleyetal.2008;Karhuetal.2014)andsoilmoisture(Waring&Hawkes2014),andallowustodetectshiftsinmicrobialactivityduetoreleasefromNlimitation.METHODSSitedescriptionandexperimentaldesignTheex