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Trial by Fire: On the Terminology and Methods Used in Pyrogenic Organic Carbon Research

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Trial by Fire: On the Terminology and Methods Used in Pyrogenic Organic Carbon Research
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Frontiers in Earth Science
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Zimmerman, Andrew
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Frontiers
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Our understanding of the cycling of fire-derived, i.e., pyrogenic organic matter (pyOM), as well as the goals of the community of researchers who study it, may be inhibited by the many terms and methods currently used in its quantification and characterization. Terms currently used for pyOM have evolved by convention, but are often poorly defined. Further, each of the different methods now used to quantify solid and dissolved pyrogenic carbon (pyC) comes with its own biases and artifacts. That is, each detects only a fraction of the total pyrogenic products produced by fire, while, at the same time, include some fraction of non-pyrogenic OM. This may be evident in the commonly observed correlations between pyC and total organic C reported for both soils and dissolved OM in many different systems. We suggest that our research area can be placed on a stronger footing by: (1) agreement upon a common set of terms tied to the method used for detection (e.g., of the form pyCmethod), (2) implementation of another “ring trial” study with a wider set of natural soil and water samples that cross-compare more recently developed methods, and (3) further investigation of the processes which preserve/degrade/transport pyOM in the environment.
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Collected for University of Florida's Institutional Repository by the UFIR Self-Submittal tool. Submitted by Andrew Zimmerman.

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PERSPECTIVE published:17November2017 doi:10.3389/feart.2017.00095 FrontiersinEarthScience|www.frontiersin.org 1 November2017|Volume5|Article95 Editedby: CristinaSantin, SwanseaUniversity,UnitedKingdom Reviewedby: MichaelW.I.Schmidt, UniversityofZurich,Switzerland PhilippaLouiseAscough, ScottishUniversitiesEnvironmental ResearchCentre,UnitedKingdom MichaelIanBird, JamesCookUniversityCairns, Australia *Correspondence: AndrewR.Zimmerman azimmer@u.edu Specialtysection: Thisarticlewassubmittedto Biogeoscience, asectionofthejournal FrontiersinEarthScience Received: 10August2017 Accepted: 06November2017 Published: 17November2017 Citation: ZimmermanARandMitraS(2017) TrialbyFire:OntheTerminologyand MethodsUsedinPyrogenicOrganic CarbonResearch. Front.EarthSci.5:95. doi:10.3389/feart.2017.00095 TrialbyFire:OntheTerminologyandMethodsUsedinPyrogenicOrganicCarbonResearchAndrewR.Zimmerman 1 andSiddharthaMitra 2 1 DepartmentofGeologicalSciences,UniversityofFlorida, Gainesville,FL,UnitedStates, 2 DepartmentofGeological Sciences,EastCarolinaUniversity,Greenville,NC,Unite dStates Ourunderstandingofthecyclingofre-derived,i.e.,pyro genicorganicmatter(pyOM), aswellasthegoalsofthecommunityofresearcherswhostudy it,maybeinhibited bythemanytermsandmethodscurrentlyusedinitsquantica tionandcharacterization. TermscurrentlyusedforpyOMhaveevolvedbyconvention,bu tareoftenpoorlydened. Further,eachofthedifferentmethodsnowusedtoquantifys olidanddissolvedpyrogenic carbon(pyC)comeswithitsownbiasesandartifacts.Thatis ,eachdetectsonlya fractionofthetotalpyrogenicproductsproducedbyre,wh ile,atthesametime,include somefractionofnon-pyrogenicOM.Thismaybeevidentinthe commonlyobserved correlationsbetweenpyCandtotalorganicCreportedforbo thsoilsanddissolved OMinmanydifferentsystems.Wesuggestthatourresearchar eacanbeplacedon astrongerfootingby:(1)agreementuponacommonsetofterm stiedtothemethod usedfordetection(e.g.,oftheformpyC method ),(2)implementationofanother“ring trial”studywithawidersetofnaturalsoilandwatersample sthatcross-compare morerecentlydevelopedmethods,and(3)furtherinvestiga tionoftheprocesseswhich preserve/degrade/transportpyOMintheenvironment.Keywords:pyrogeniccarbon,blackcarbon,quantication, artifacts,biochar,ringtrial INTRODUCTIONTheunderstandingthatreandpyrogenicorganicmatter(pyOM) havecontributedtoshaping Earth'sbiosphereisonethathasevolvedwithinanumberofdi sparateeldsincludinggeology, ecology,atmospherescience,agriculture/soilscience,an danthropology.Ineacheld,this realization,alongwithassociatedterminologyandmethod ology,hastendedtoevolveseparately, withlimitedcross-disciplinarycommunication.Humanshav ebeenusingpyOMinindustry (charcoalusedinthesmeltingofcopperasearlyas5,000BCE)an dmedicineforthousandsof years( ScottandDamblon,2010 ).However,formalresearchintocharcoal'spropertiesbegan as earlythebeginningofthetwentiethcentury(e.g., Hedin,1907;Sweetser,1908;Homfray,1910 ). Charcoalwasnotedinsoilsevenearlier(e.g., Heer,1866;Fliche,1907;GodwinandTansley,1941 ), butwasnottreatedquantitativelyandusedasanindicatorof pastvegetationandhumansettlement bypaleoecologistsandarcheologists,respectively,untilmu chlater( Western,1963;Camps,1971 ). Detectionandquanticationofcharcoalinmarineandlakes edimentsledtotherstuseofpyOM asaproxyforpastre-frequencyandclimateinthe1970's( Smithetal.,1973;Herring,1976;Swain, 1978 ),thoughtherewassomedebateatthistimeastoitspyrogenic origin( Schopf,1975 ).

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ZimmermanandMitra PyrogenicOrganicCarbonResearch Considerationofthepyrogeniccomponentofaerosolsand thedevelopmentoftherstinstrumenttomeasureaerosolsoo t concentrations( Thomas,1952 )wassparkedbythe“London fog”ofDecember1952whichkilledatleast4,000( Wilkins, 1954 ).Thedarkcoloroftheltersusedtocollectthe“soot” derivedfromcoal-burningledtouseoftheterm“blackcarbo n” (BC)withintheeldofatmosphericchemistry(e.g., Novakov, 1981,1984;Gundeletal.,1984 ). Novakov(1981,1984) dened BCas“combustion-producedblackparticulatecarbonhavingagraphiticmicrostructure”andsoon,theterm“BC”wasappliedmorebroadlytopyOMinbothitsatmosphericandgeologicalforms( Goldberg,1985 ).However,theterm“elementalcarbon,” reectingitsC-richcharacter,wasandisstillalsowidely usedby theatmosphericcommunity,whichhaslargelybeenconcernedwithitslightabsorptionandassociateddirectradiativefo rcingof Earth'sclimate( Bondetal.,2004 ).Althoughtherehavebeenquite afewinter-laboratorycomparisonsofmethodsusedtoquanti fy aerosolBC(e.g., Countess,1990;Birch,1998;Hitzenbergeretal., 1999,2006;Schmidetal.,2001;tenBrinketal.,2004 ),thereis stillnouniversallyacceptedmethodforisolatingaerosolpy OM (discussedfurtherinnextsection). AwarenessofelevatedatmosphereCO 2 andotheratmosphere greenhousegasconcentrationsinthe1970'sledtoconsider ation oftheroleofreintheglobalCcycle.Therstestimateofgl obal biomassburningandcharcoalproduction,500–700Tgyr 1 ( SeilerandCrutzen,1980 )wasanattempttobalancethepoor matchbetweenCinputstotheatmospherewithknownremovalmechanisms.Thisestimatewassincereviseddownwardandisnowgenerallyagreedtobeintherangeof50–300Tgyr 1 ( KuhlbuschandCrutzen,1996;Forbesetal.,2006;Birdetal. 2015;Santinetal.,2016 ).Anotherturningpointingeochemists' understandingofpyOMwasthe“BCcombustioncontinuum”rstproposedby Hedgesetal.(2000) and SchmidtandNoack (2000) andlaterelaborateduponbyothers( Masiello,2004; Elmquistetal.,2006;PrestonandSchmidt,2006 ).Itmaintained thatpyOMiscomposedofawiderangeofmaterialsfromslightlycharredbiomasstohighlycondensedgraphiteandsoot.Witht his “continuumperspective”camethewiderrecognitionthatpyOMcannotbewhollyrefractory,butisdegradedtodierentexte nts throughavarietyofabioticandmicrobially-enhancedproce sses. Acalltocomparemethodsusedtoquantifypyrogenic carbon(pyC)insoilsandsediments,whichrequirestheuseoftechniquesdierentfromthoseusedinatmosphericsciencesduetothepresenceofinterferingmatrices,cameatmuchthesametime.Therstmethodcomparisoneortsexaminedalimitedrangeofsampletypes( Currieetal.,2002 )or resultedinwidelyrangingvaluesforindividualsamples( Schmidt etal.,2001 ).Thus,a“SteeringCommitteeforBlackCarbon ReferenceMaterials”wasestablishedandrecommendedalarg er setofpyrogenicandnon-pyrogenictestmaterials( Schmidt etal.,2003 ).TheresultingcomprehensiveevaluationofpyC quanticationmethods( Hammesetal.,2007 ),thesocalled“BC ringtrial,”madeitclearthateachanalyticalmethodissel ective foradierentpartofthepyOMcontinuum. Atthesametimethatsoilscientistsandagronomistswere realizingthepotentialofsoilamendmentsofpyCtoenhancesoilfertilityandmitigateclimatechangethroughCseques tration ( Lehmannetal.,2002;Lehmann,2007a,b ),environmental scientistsidentiedsubstantialamountsofpyCinanever-wideningrangeofsettingsincludingriverwater( Dingetal., 2013;Jaeetal.,2013 ),marshes( Dittmaretal.,2012b )and theocean( Dittmar,2008;Stubbinsetal.,2010;Ziolkowskiand Druel,2010 ).However,allofthesepursuitsrequiretheability toaccuratelyquantifypyCandtotrackthechemicalevolutio n andtransportofpyOMintheenvironment.PYROGENICSUBSTANCESQUANTIFICATIONMETHODSThemethodsusedtoquantifypyCvarywidelyintheircost,easeofapplication,andunfortunately,inthepyOMfractionthattheytarget.ThesemethodscanbecategorizedasthosethatrelyonpyOMdetectionof:(1)morphology,(2)lightabsorption,(3)thermalorchemicalstability,and(4)chemi cal composition.Archeologistsandsomegeoscientistscommonlyuselightmicroscopyandparticlemorphologytocountthenumberandsizeofcharcoalparticles(e.g., Smithetal.,1975; FigueiralandMosbrugger,2000;ScottandDamblon,2010 ). However,thesemethodscannotquantifypyCandarebiasedtowarddetectionofparticlesoflargersizes( Masiello,2004; CrawfordandBelcher,2016 ).Atmosphericscientistsquantify pyCusingopticaltechniques( RosenandNovakov,1977 ), thermalheatingcombinedwithopticalmethods,orlaser-inducedincandescence( Watsonetal.,2005 ;forahistorical perspective,see NovakovandRosen,2013 ).Whilemanyof thesemethodsaresubjecttointerferencesintroducedduri ng mixingofcombustion-derivedaerosolswithnon-pyrogenicO M ( Bondetal.,2013 ),quanticationofpyCinsoilorsedimentary matricesisevenmoredicultduetotheevengreaterpresence of mineralsandcomplexnon-pyrogenicgeopolymers.Tocopewiththis,manyapproachesassumepyOMtobethemostrefractoryOMfraction.Thus,dierentthermaland/orchemicaloxidati on techniques(e.g.,CTO-375,dichromateoxidation,UVoxidat ion, catalytichydrogenpyrolysis(hy-py)havebeenusedtoremovemorelabileOMandassumetheresidualtobepyOM(e.g., Lim andCachier,1996;Gustafssonetal.,1997,2001;Thevenone tal., 2010 ). AnalgroupofmethodsidentifypyOMusingsomeaspectof itschemicalstructure.Theabundanceofbothlevoglucosan ,an anhydroussugarformedduringcellulosecombustion( Eliasetal., 2001;Kuoetal.,2008 ),andbenzenepolycarboxylicacids(BPCAs) whichareformedvianitricacidoxidation( Glaseretal.,1998; Brodowskietal.,2005;Dittmar,2008 )havebeenusedaschemical markersforlowertemperaturecharredOMandcondensedaromaticC,respectively.AnothertechniqueinfersthepyCcontentofasampleusingthesorptivecharacteristicsofpyreneontothesample( Flores-Cervantesetal.,2009a,b ).Spectroscopic toolssuchas 13 C-nuclearmagneticresonance(NMR)and mid-infrared(MIR)spectroscopyprovideinformationonthestructureandchemicalbondsofOMpresentinamaterial.InthecaseofNMR,itisgenerallyaromaticCthatisquantiedandassociatedwithpyC( SimpsonandHatcher,2004;Nelson andBaldock,2005 ),thoughspecializedtechniqueshavebeen FrontiersinEarthScience|www.frontiersin.org 2 November2017|Volume5|Article95

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ZimmermanandMitra PyrogenicOrganicCarbonResearch developedtoestimatethecondensedaromaticfraction( Knicker etal.,2005;McBeathetal.,2011 ).ThoughMIRalsoexaminesa sample'sspectralcharacteristics,itrequirescalibration usingone oftheotherpyCmethodsinordertobequantitative( Janiketal., 2007;Bornemannetal.,2008;Cotrufoetal.,2016 ).Moredetailed reviewsofmethodsusedtoquantifypyCcanbefoundelsewhere( Meredithetal.,2012;Birdetal.,2015 ). PROBLEMSANDPERPLEXITIESWhilethemethodsusedtostudypyOMproductionandcyclinghaveadvancedgreatlyinthelastdecade,webelievethatthe re arestillterminology,methodology/conceptualimpediments to furtherprogressthaturgentlyneedtobeaddressed.Thisinh ibits bothresearchfundingopportunitiesandimplementationofthere-sciencecommunityndingsandrecommendations,bethe y insettingsofagriculture,policy,orclimate/geochemical model incorporation.TerminologyIssuesFromthediscussionabove,itshouldbeclearthatdierentr e researchsub-communitieshaveadoptedvarioustermsforpyOMorpyOMfractions,oftenforhistoricalreasonsonly.Confus ion arisesbecausethesetermsareoftennotassociatedwithspec ic denitions,orbecausetermshavebeenadoptedacrossresear ch communityboundarieswithoutregardfortheiroriginalnar row denitions.Forexample,theatmosphericcommunityreferstoBCascombustion-generatedaerosolsthatabsorbvisiblelig ht, areinsolubleinwater,andexistasaggregatesofcarbonsphe rules ( Novakov,1984;Bondetal.,2013 ).Ontheotherhand,BC hascometobeusedbymanyasshorthandforpyrogenicCofallforms.Theterm“dissolvedblackcarbon”conictswit h theatmosphericcommunity'sdenitionthatBCshouldnotbesolubleinwater.Organiccompoundsdetectedintheaqueousfractionofcombustion-derivedaerosolsaretypicallyrefe rredto bytheatmosphericcommunityaswatersolubleorganiccarbon( Decesarietal.,2000;Mayol-Braceroetal.,2002 ).Inthepast, sootwasdenedasthetotalcarbonaceousmaterialproducedbycombustion( Novakov,1984 ).However,forothers,sootis thecondensateofcombustiongasesandhasgraphiticstructu re ( Hammesetal.,2007 ).Thematerialsreferredtoascharcoal, biochar,andagricharmayallbethesame,orsomewhatdieren t ( LehmannandJoseph,2015 ).Whileitisunderstoodthat dierentanalyticalmethodstargetdierentpyCfractions,th ere isasyetnoconsistentmethodorproperty-basedterminologyappliedacrossdierentresearchcommunities.Thislackofconsistencymayinhibitcross-disciplinarycommunication and fertilizationofnewconcepts.Methodology/ConceptualIssuesSeveralmethodologicalissuesandrecentobservationssho uld causeustoquestionourabilitytoquantifytheamountofpyOM(orpyC)innaturalsamples.Therstissueisthatmanyofthesemethodsrequirea“ conversionfactor ”ofsometypeto transformmeasuredparameterssuchasapost-treatmentresid ue weightorcompoundabundancetoanamountofpyOMorpyCpresentinasample.Forexample,aconversionfactorhasbeencalculatedbasedontheoreticalBPCAyieldsoforganic structuresofmarineDOMobservedviaultrahigh-resolutio n massspectrometry(about3, Dittmar,2008;Stubbinsetal., 2012 )orfromtheBPCAyieldsofvariousaromaticsubstances includingactivatedC(2.27, Glaseretal.,1998 )PAHs,soot andCnanotubes(about4, Ziolkowskietal.,2011 ).However, applicationofanysingleBPCAconversionfactortoavarietyofpyOMtypeshasbeencalledintoquestion( Brodowskietal., 2005 ).Forthedichromateoxidationprocedure,acorrection factorderivedfromtheresidualyieldofplantcharoxidatio nwas usedtoaccountforpyClossesduringthechemicaltreatment( Knickeretal.,2008 ).Adegreeofuncertaintyhasbeengenerated intheresciencecommunitybecauseofthevarietyandrangeofconversionandcorrectionfactorsused,evenwithinspeci c analyticalmethods. ThesecondissueisthatpyCcanbefalselyidentiedas non-pyCandviceversa.UnderestimatesinpyCquanticationcertainlyoccurbecausenotallpyCisrecalcitrantorpurelycondensedaromatic,asassumedbymostanalyticalmethods(e.g.,Bosticketal.,thisissue, NguyenandLehmann,2009; Zimmerman,2010;Singhetal.,2012 ).SomeportionofpyCis likelydestroyedintheharshthermalandchemicaloxidatio n stepsused.Overestimatescanoccurinanumberofways.Notallnon-pyCisremovedbythermal/chemicaloxidationsteps(e.g., Knickeretal.,2007 ).Oxidativetreatmentscan evengenerateapparently“pyrogenic”OM(e.g., Derenneand Largeau,2001;Hammesetal.,2007;NovakovandRosen,2013 ).Thishasbeenshowntooccur,forexample,during theoxidationrequiredtoformBPCAcompounds( Brodowski etal.,2005;Kappenbergetal.,2016 )aswellasinstepsused toremovenon-pyrogenicaromaticCpriortoNMRanalysis( SimpsonandHatcher,2004 ).OtherNMRquantication approachesusespectraleditingormolecularmixingmodelstoseparatecondensedaromaticfromnon-condensedaromaticbiomolecules,bothofwhichrequireassumptionsorcorrectio ns thathavenotbeenfullyvalidated( Cusacketal.,2012;Paetsch etal.,2017 ).Lightabsorption-basedmethodsusedmainlyby atmospherechemistssuerfromuncertaintiesrelatedtonon-linearitiesinthelightattenuationcoecientwhichvaryw ith lterloadingandparticletype,aswellastointerferencesfromnon-pyrogenicOM(reviewedin KirchstetterandNovakov, 2007 ). Lightabsorption,BPCAandsomeNMRquantication techniquesusetheassumptionthatonlypyOM,takestheformofcondensedaromaticsstructures.Butincreasingly,this has beenshownnottobethecase.Forinstance,melanoidins,severalplants,fungiandpigmentsyieldedquantitiesofBPCA s (evenhighly-carboxylatedBPCAswhichareindicativeofver y condensedaromaticOM)likethatofcharredplantmaterial( Brodowskietal.,2005;GlaserandKnorr,2008 ).Thesemaybe derivedfromligninortannin,whichincludeawidevarietyo f polycondensedaromatics( HernesandHedges,2000;Waggoner etal.,2015 ).Othernon-pyrogenicOMsourcesofcondensed aromaticOMareabundantinthegeosphere,includingwoodypeat,coal,kerogen,andoil( YoshiokaandIshiwatari,2005; Hammesetal.,2007;Wangetal.,2012;Hartmanetal.,2015;Lietal.,2017 ).StillotherstudieshavefoundthatnonpyrogenicOMcanbereadilytransformedtocondensedaromaticOM,whichwouldappeartobepyrogenic,throughphotolytic, FrontiersinEarthScience|www.frontiersin.org 3 November2017|Volume5|Article95

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ZimmermanandMitra PyrogenicOrganicCarbonResearch microbial,orchemicaldegradativeprocesses( GlaserandKnorr, 2008;Chenetal.,2014;Waggoneretal.,2015;DiDonatoetal ., 2016 ). GiventhatwearendingevermorewaysinwhichpyOMcan crosstheanalyticalwindowintonon-pyOMandviceversa,itisperhapsnotsurprisingthatmanyhaveobservedcorrelationsbetweenpyCandtotalCconcentrationsinsoilsregionally( GlaserandAmelung,2003;Jaussetal.,2015;Ahmedetal.,20 17; Qietal.,2017 )andglobally( Reisseretal.,2016 ),anddissolvedin naturalwatersregionally( Dittmaretal.,2012a;Dingetal.,2013, 2014,2015;Guerenaetal.,2015 )andglobally( Jaeetal.,2013; Wagneretal.,2015 )andeveninaqueousleachatesofmarine aerosols( Baoetal.,2017 ).Thesecorrelationsindicate,asdo othermoredetailedstatisticalexaminations(inthesames tudies), alackofdependenceofpyCconcentrationsonrehistoryorclimate.Aplotofdatacompiledbyarecentliteraturereview( Reisseretal.,2016 )showsasignicantrelationshipbetweenpyC andtotalCinglobalsoilsacrossallquanticationmethods and withineachmethod(exceptCTO-375, Figure1 ).Thestrongest correlationsarefoundfordataderivedfromBPCAandNMR,suggestingthatthesemethodsmayhavethegreatestlikelih ood forartifactsthatmisidentifypyC.Alternatively,thendi ngof correlationbetweenpyCandtotalCregardlessoftheanalytica l methodused,mightsuggesttherelationshipispresentinnat ure. Thatis,production,degradation/preservationormobilizati on processesmayactonpyCandnon-pyCinwaysthatcausethemtoco-vary.Forexample,regionsofhigherproductivity,thushighersoilC,alsohavemorebiomasstoburnandaretherefor e likelytohavegreaterpyCinsoilsanddrainagewaters( Alexis etal.,2007;vanLeeuwenetal.,2014 ).Soilswithgreateramounts ofclayormetaloxidemineralorevencharcoals,arelikelyt o sorbandthereforeprotectbothpyOMandnon-pyOMfrommicrobialmineralizationthroughsorptiveprotection( Kasozi etal.,2010;Zimmermanetal.,2011 )oraggregatestabilization ( Wangetal.,2017 ).Andsoiltranslocation,erosion,leaching,and otherhydrologic/climatic-relatedprocessesofaregionar elikely toacttomobilizebothpyOMandnon-pyOMinsimilarways( HilscherandKnicker,2011;JienandWang,2013 ),thoughnot necessarilytoequalextents( Rumpeletal.,2009 ).Finally,ithas beensuggestedthatpyCmobilizationmayoccurviaassociati on withotherOMindissolved( Jaeetal.,2013 )orperhapscolloidal ( ZandandGrathwohl,2016;Kumarietal.,2017 )form,butthe controllingmechanismsarestillunknown( Wagneretal.,2017 ). RECOMMENDATIONSDespite,orpossiblyevenbecauseofthemanyissuesfacingthepyCcyclingresearchcommunity,wemaybeonthebrinkofmakinggreatadvancesinthiseld,butonlyiftheseissuesa re acknowledgedanddealtwith.First,wesuggestmorestringe nt useofterminology.Theterms“pyOM”or“pyC”shouldbeusedwhenreferringtothetotalityofre-derivedcarbonac eous substances.Thesetermsrepresentthebroadestshortformsforthetotalsubstanceofpyrogenicoriginandthecarboninthesesubstances,respectively.Whenreportinganalyticalre sults, wesuggestthattermstiedtothemethodorpropertiesused FIGURE1| Relationshipsbetweentotalsoilorganiccarbon(SOC)and pyrogeniccarbon(pyC)asdeterminedbydifferentmethods. Pearsonlinear correlationcoefcients( r )andlevelsofsignicance( p )aregiveninthelegend. Dataaretakenfrom Reisseretal.(2016) ,acompilationofresultsofglobalsoil datafrom55studies. fordetectionshouldbeused.Forgreatestclarity,wethesetermscouldtaketheform“pyC method ”( Figure2 ).Forexample, substancesquantiedvialightmicroscopymightbereferredtoaspyC mic .Substancesisolatedbasedontheirchemical orthermalresistancecouldbedesignatedpyC CTR .Theterm “pyC LE ”shouldbeusedforsubstancesdetectedbasedupontheir nearcompletelightextinctionproperties,butfailingtoconvi nce themainlyatmosphericcommunityofthis,wesuggestthatterrestrialandaquaticscientistsleavetheterm“blackcar bon” tothem.UseofthesetermswillserveascontinuousreminderthataquantityofCrefersonlytoaportionofthesubstancesproducedbyresandmayevencontainanon-pyrogenicportion. Regardingthepotentialforanalyticalartifactsthatplague pyCcyclingresearch,werecommend,rst,thatanother“ringtrial”studybeconductedsothattechniquesthathavebeendevelopedsincethelastringtrial(suchasMIRspectroscopyandcatalytichydrogenpyrolysisandimprovementsinNMRandBPCAanalyticalmethods)canbecomparedandtheirrelativestrengthsandweaknessesre-evaluated.Thisring study shouldincludenotjustgeochemists,butalsothosethatstudypyOMfromtheanthropology,atmosphereandagriculturecommunities.Moreover,thenewringtrialshouldalsomakeuseofmodernanalyticaltechniquesthatcandeconvolvecompositionofpyOMatunprecedentedlevels(e.g.,aerosolmassspectrometer,Fouriertransformioncyclotronresonan ce massspectrometer).Inadditiontothesetofpyrogenicand“potentiallyinterfering”materialsincludedintherstri ngtrial ( Hammesetal.,2007 ),thisnewringtrialshouldincludea biocharthermalseries,whichwouldbeexpectedtohavearegularlyincreasingdegreeofaromaticcondensation( Caoetal., 2012 ).Furthermore,werecommendabroadersetofatmospherederivedsamplessuchasthedieselsootstandard(NIST FrontiersinEarthScience|www.frontiersin.org 4 November2017|Volume5|Article95

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ZimmermanandMitra PyrogenicOrganicCarbonResearch FIGURE2| Proposedpyrogeniccarbonterminologybaseduponmethodsu sedforidentication/quanticationandestimatedplacement ofthesematerialsonthe “combustion/stabilitycontinuum.”Dottedlinesrepresent estimateduncertainlyrange. SRM2975)andaqueoussamples(preferablynotfreeze-dried)isolatedfromriverandoceanwater.Additionalnon-pyrogen ic materialssuchaswoodbiomass,leachatefromthisbiomass,andphotodegradedbiomassleachateshouldalsobeconsidere d foranalyses.Toevaluatematrixeectsassociatedwitheachmethod,astandardadditionexperimentshouldbeaddedtothemethodintercomparison,wherebydierentamountsofapyOM,suchaswoodchar,areaddedtoasoil-likemixturecontainingnopyOM.Thisringtrialshouldbefollowedupnotonlywithareportofresults,butwithabest-practicespaperthatincludesconsensusrecommendationsforuseofterminology. Untilnow,thefocusofmanyresciencestudieshasbeento establishpropertiesofpyrogenicsubstancesandtheirinvent ories indierentsystems.Giventhemajorquestionofthecauseoft he often-observedpyC/TOCcorrelation,agreaterfocusshould be placedonstudiesthatcomparetransformationandmovementofpyrogenicrelativetodierenttypesofnon-pyrogenicsubstanc es. MechanisticpyOMinvestigationsareneededtounderstandbot h preservationprocessessuchasadsorption,metal-complexatio n andaggregateformation,andtransformationprocessessuch as solubilization,volatilization,andmicrobial,chemical andphotodegradation.Inaddition,pyCmobilizationstudiesshouldf ocus notjustonparticlemovementinsoilviatranslocationanderosion,buttransportinaerosol,colloid,anddissolvedfo rms viaatmosphereandaqueousprocesses.Wehopethiscommentstimulatesgreaterdialogbetweenresearchcommunitiestha t studyvariousaspectsofpyrogenicsubstances.Thedesiredre sult wouldbenotonlyamorecompleteunderstandingoftheproductionandcyclingofpyC,butalsoagreaterapplicationoftheseinsightsinsuchareasasagricultureandclimatemodeling.AUTHORCONTRIBUTIONSAllauthorslistedhavemadeasubstantial,directandintel lectual contributiontothework,andapproveditforpublication.FUNDINGThisworkwasfundedbytheU.S.NationalScienceFoundation—GeobiologyandLow-TemperatureGeochemistryProgram(EAR-1451367).ACKNOWLEDGMENTSThisworkwassubstantiallyimprovedthroughhelpfulconversationswithDrs.GerardCornelissenandMichaelBird. FrontiersinEarthScience|www.frontiersin.org 5 November2017|Volume5|Article95

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