Black carbon in soils and sediments_ Analysis, distribution, implications, and current challenges
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field thals.
1. Introduction
producehighly aromatichumic acidsthat can interferewith extracts from soil humic substances [Haurnaierand Zech, 1995; Skjernstad et al., 1997]. In additionto bumingvegetation, fossil fuel combustion presentlyalso contributes similar amountsof aerosol BC to the global biogeochemical cycle of carbon [Kuhlbusch, 1998a]andup to 80% of thetotalsoilorganic carbon [Schmidtet al., 1996]. Charredparticlesfrom both biomass burningand fossilfuel combustion share a relativelack of (bio) chemical reactivity and thereby stronglyresist decomposition lNowGeographisches Institut, Universit•it zuK61n, Cologne, Germany. over a geological timescale. For example, BC residues are conAlso at Max-Pianck-Institut ftir Biogeochemie, Jena,Germany. stituent of manycoalsdatingbackto theDevonian [Tayloret al., 1998]. Copyright 2000 by theAmerican Geophysical Union. The purpose of this review is to introduce differentscientific communities, includingsoil, sediment, and marinegeochemists Papernumber1999GB9001208. 0886-6236/00/1999GB001208512.00 andbiologists, to the many differentaspects of the globalimpor777
Briimmer,1997]. The presence of BC may also have led to an
overestimation of marineorganic productivity [Verado,1997] andsoilhumus content [Skjemstad etal., 1996]. In soils, Bce [Crutzen andAndreae,1990],be a useful tracerfor Earth'sfire history [Bird and Cali, 1998],buildup a significant fraction of carbon buriedin soils[Skjemstad et al., 1996;Schmidt et al., 1999]andsediments [Masiello andDruffel, 1998], and act as an important carrierof organicpollutants [GustaJ•son and Gschwend, 1997]or heavymetals[Hiller and
Michael W. I. Schmidt •
Lehrstuhi ftir Bodenkunde, Technische Universit//t Miinchen,Munich,Germany
AngelaG. Noack
Facultyof Agricultural andNaturalResource Sciences, Universityof Adelaide, Glen Osmond, South Australia, Australia
Black carbon(BC) is produced by incompletecombustion of fossil fuels and vegetation [Goldberg,1985]. BC can be understoodas a continuum from partly charredplant materialthrough charandcharcoal to graphite andsootparticles recondensed from the gasphase, with no general agreement on clear-cut boundaries [Seiler and Crutzen,1980]. BC is purely terrestrialin origin and occurs ubiquitously in soilsand terrestrial sediments and is coupled to a commonmarinefate via fluvial and atmospheric transport [Goldberg,1985]. In recentyears,geochemical and biological studiesof different forms of BC, suchas plant chars,charcoals,and soots, havereceived increasing attention owingto their potential importance in a wide range of biogeochemical processes. As examples, BC may represent a significantsink in the globalcarbon cycle [Kuhlbusch, 1998a],affectthe Earth'sra-
Abstract. This reviewhighlights the ubiquityof blackcarbon (BC) produced by incomplete combustion of plantmatehalandfossilfuelsin peats, soils,andlacusthne andmarinesediments. We examine vahous definitions andanalytical approaches andseek to provide a common language. BC represents a continuum frompartlycharred matehalto graphite andsootparticles, with no general agreement on clear-cut boundares. Formation of BC canoccurin two fundamentally differentways.Volatilesrecondense to highlygraphitized soot-BC, whereas the solidresidues form char-BC.Both formsof BC arerelativelyinert and are disthbuted globallyby waterand wind via fluvial andatmosphehc transport. We summarize, chronologically, the ubiquityof BC in soilsand sediments sinceDevonian times,differentiating betweenBC from vegetation firesandfrom fossil fuel combustion. BC hasimportant implications for various biological, geochemical andenvironmentalprocesses. As examples, BC may represent a significant sinkin the globalcarbon cycle, affectthe Earth'sradiativeheatbalance, be a usefultracerfor Earth'sfire history,build up a significant fractionof carbon buhedin soilsand sediments, andcarryorganicpollutants. On land, BC seems to be abundant in dark-colored soils,affected by frequent vegetation burningandfossil fuel combustion, thusprobably contributing to the highlystable aromatic components of soil organicmatter.We discuss challenges for futureresearch. Despite the greatimportance of BC, only limitedprogress hasbeenmadein calibrating analytical techniques. Progress in thequantification of BC is likely to comefrom systematic intercomparson usingBCs from differentsources andin differentnaturalmathces. BC identification couldbenefitfrom isotopic andspectroscopic techniques applied at thebulk andmolecular levels.The key to estimating BC stocks in soilsand sediments is an understanding of the processes involvedin BC degradation on a molecular level. A promising approach wouldbe the combination of short-term laboratory expehments andlong-term
778
SCHMIDT
AND NOACK:
BLACK CARBON IN SOILS AND SEDIMENTS
tanceof BC. BC hasbeenstudied in a varietyof widely separated scientific fields, with the result that essentially no generally accepted analytical protocols, terminologies, and conceptual approaches exist. Independentliteratureson BC continue to developdepending on the medium(soil,sediment, atmosphere, or water), timescale(pre-Quaternary versusQuaternary),analytical approach (qualitative versusquantitative) and operational definitions(opticalproperties, chemicalandthermalresistance, or indirect evidence). A combination of knowledge on BC coupled with a carefulcalibrationof analyticaltechniques holdspromisefor a better understanding of data obtainedin biogeochemical, environmental, and agriculturalstudies. Sincethe 1980s,increasing interest in fire, and thusthe formationof BC in the globalcarbon cycle, resultedin a seriesof interdisciplinary conferences and monographs which attempted to integrate approaches and findings of traditionallyseparate scientificdisciplines [Clark et al., 1997; Crutzenand Goldammer,1993;Zeppand Sonntag,1995]. With this review we want to bridge the gap between disciplines andenhance familiaritywith information contained in literaturefrom the fields of coal petrology, palynofacies, palynobotany,fire ecology, and marine, environmental, and soil chemistry. We point out the variabilityof analytical techniques and the ubiquitous presence of BC originating from vegetation burning andfossilfuel combustion in thegeological record andin contemporary sediments and soils.To facilitate access to literature of the differentscientific disciplines, we focuson recentand generallyavailablepublications providing access to furtherliterature,ratherthana comprehensive literature review.
GLOBAL BIOGEOCHEMICAL
CYCLES, VOL. 14, NO. 3, PAGES 777-793, SEPTEMBER 2000
Black
carbon
in soils and sediments'
Analysis, distribution, implications, and current challenges
1. Introduction
producehighly aromatichumic acidsthat can interferewith extracts from soil humic substances [Haurnaierand Zech, 1995; Skjernstad et al., 1997]. In additionto bumingvegetation, fossil fuel combustion presentlyalso contributes similar amountsof aerosol BC to the global biogeochemical cycle of carbon [Kuhlbusch, 1998a]andup to 80% of thetotalsoilorganic carbon [Schmidtet al., 1996]. Charredparticlesfrom both biomass burningand fossilfuel combustion share a relativelack of (bio) chemical reactivity and thereby stronglyresist decomposition lNowGeographisches Institut, Universit•it zuK61n, Cologne, Germany. over a geological timescale. For example, BC residues are conAlso at Max-Pianck-Institut ftir Biogeochemie, Jena,Germany. stituent of manycoalsdatingbackto theDevonian [Tayloret al., 1998]. Copyright 2000 by theAmerican Geophysical Union. The purpose of this review is to introduce differentscientific communities, includingsoil, sediment, and marinegeochemists Papernumber1999GB9001208. 0886-6236/00/1999GB001208512.00 andbiologists, to the many differentaspects of the globalimpor777
Briimmer,1997]. The presence of BC may also have led to an
overestimation of marineorganic productivity [Verado,1997] andsoilhumus content [Skjemstad etal., 1996]. In soils, Bce [Crutzen andAndreae,1990],be a useful tracerfor Earth'sfire history [Bird and Cali, 1998],buildup a significant fraction of carbon buriedin soils[Skjemstad et al., 1996;Schmidt et al., 1999]andsediments [Masiello andDruffel, 1998], and act as an important carrierof organicpollutants [GustaJ•son and Gschwend, 1997]or heavymetals[Hiller and
Michael W. I. Schmidt •
Lehrstuhi ftir Bodenkunde, Technische Universit//t Miinchen,Munich,Germany
AngelaG. Noack
Facultyof Agricultural andNaturalResource Sciences, Universityof Adelaide, Glen Osmond, South Australia, Australia
Black carbon(BC) is produced by incompletecombustion of fossil fuels and vegetation [Goldberg,1985]. BC can be understoodas a continuum from partly charredplant materialthrough charandcharcoal to graphite andsootparticles recondensed from the gasphase, with no general agreement on clear-cut boundaries [Seiler and Crutzen,1980]. BC is purely terrestrialin origin and occurs ubiquitously in soilsand terrestrial sediments and is coupled to a commonmarinefate via fluvial and atmospheric transport [Goldberg,1985]. In recentyears,geochemical and biological studiesof different forms of BC, suchas plant chars,charcoals,and soots, havereceived increasing attention owingto their potential importance in a wide range of biogeochemical processes. As examples, BC may represent a significantsink in the globalcarbon cycle [Kuhlbusch, 1998a],affectthe Earth'sra-
Abstract. This reviewhighlights the ubiquityof blackcarbon (BC) produced by incomplete combustion of plantmatehalandfossilfuelsin peats, soils,andlacusthne andmarinesediments. We examine vahous definitions andanalytical approaches andseek to provide a common language. BC represents a continuum frompartlycharred matehalto graphite andsootparticles, with no general agreement on clear-cut boundares. Formation of BC canoccurin two fundamentally differentways.Volatilesrecondense to highlygraphitized soot-BC, whereas the solidresidues form char-BC.Both formsof BC arerelativelyinert and are disthbuted globallyby waterand wind via fluvial andatmosphehc transport. We summarize, chronologically, the ubiquityof BC in soilsand sediments sinceDevonian times,differentiating betweenBC from vegetation firesandfrom fossil fuel combustion. BC hasimportant implications for various biological, geochemical andenvironmentalprocesses. As examples, BC may represent a significant sinkin the globalcarbon cycle, affectthe Earth'sradiativeheatbalance, be a usefultracerfor Earth'sfire history,build up a significant fractionof carbon buhedin soilsand sediments, andcarryorganicpollutants. On land, BC seems to be abundant in dark-colored soils,affected by frequent vegetation burningandfossil fuel combustion, thusprobably contributing to the highlystable aromatic components of soil organicmatter.We discuss challenges for futureresearch. Despite the greatimportance of BC, only limitedprogress hasbeenmadein calibrating analytical techniques. Progress in thequantification of BC is likely to comefrom systematic intercomparson usingBCs from differentsources andin differentnaturalmathces. BC identification couldbenefitfrom isotopic andspectroscopic techniques applied at thebulk andmolecular levels.The key to estimating BC stocks in soilsand sediments is an understanding of the processes involvedin BC degradation on a molecular level. A promising approach wouldbe the combination of short-term laboratory expehments andlong-term
778
SCHMIDT
AND NOACK:
BLACK CARBON IN SOILS AND SEDIMENTS
tanceof BC. BC hasbeenstudied in a varietyof widely separated scientific fields, with the result that essentially no generally accepted analytical protocols, terminologies, and conceptual approaches exist. Independentliteratureson BC continue to developdepending on the medium(soil,sediment, atmosphere, or water), timescale(pre-Quaternary versusQuaternary),analytical approach (qualitative versusquantitative) and operational definitions(opticalproperties, chemicalandthermalresistance, or indirect evidence). A combination of knowledge on BC coupled with a carefulcalibrationof analyticaltechniques holdspromisefor a better understanding of data obtainedin biogeochemical, environmental, and agriculturalstudies. Sincethe 1980s,increasing interest in fire, and thusthe formationof BC in the globalcarbon cycle, resultedin a seriesof interdisciplinary conferences and monographs which attempted to integrate approaches and findings of traditionallyseparate scientificdisciplines [Clark et al., 1997; Crutzenand Goldammer,1993;Zeppand Sonntag,1995]. With this review we want to bridge the gap between disciplines andenhance familiaritywith information contained in literaturefrom the fields of coal petrology, palynofacies, palynobotany,fire ecology, and marine, environmental, and soil chemistry. We point out the variabilityof analytical techniques and the ubiquitous presence of BC originating from vegetation burning andfossilfuel combustion in thegeological record andin contemporary sediments and soils.To facilitate access to literature of the differentscientific disciplines, we focuson recentand generallyavailablepublications providing access to furtherliterature,ratherthana comprehensive literature review.
GLOBAL BIOGEOCHEMICAL
CYCLES, VOL. 14, NO. 3, PAGES 777-793, SEPTEMBER 2000
Black
carbon
in soils and sediments'
Analysis, distribution, implications, and current challenges