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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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