Direct molecular evidence for the degradation and mobility of black carbon in soils from ultrahigh-resolution mass spectral analysis of dissolved organic matter from a fire-impacted forest soil
Introduction
The ubiquity and abundance of black carbon (BC) in the environment have become increasingly urgent matters in geochemistry. Rapidly accumulating evidence suggests that BC, the refractory organic products of incomplete combustion, account for a substantial portion of molecularly uncharacterized natural organic matter (NOM) (Hedges et al., 2000). The contribution of BC as a percentage of NOM-C is 15–45% in soils (Schmidt et al., 1999) and 3–15% in ocean sediments (Gustafsson and Gschwend, 1998, Masiello and Druffel, 1998). Therefore, understanding the biogeochemical cycling of this carbon pool is essential to our efforts to model global-scale carbon cycling. A number of studies show that BC is an inherently refractory component of geomedia (Skjemstad et al., 1996, Glaser et al., 1998). Still other workers conclude that BC losses from soil and sediment over time are due to chemical degradation (Czimczik et al., 2003), and suggest that factors such as regional climatic conditions and soil properties mediate the residence time of BC (Schneour, 1966, Bird et al., 1999, Glaser and Amelung, 2003).
Despite the relative recalcitrance of BC in soils, it is clear that BC is not inert. For instance, charcoal plays important roles in soil formation (Glaser and Amelung, 2003), fertility (Glaser et al., 2000), humification (Shindo et al., 1986), carbon sequestration (Kuhlbusch and Crutzen, 1996, Glaser and Amelung, 2003), and pollutant availability (Gustafsson et al., 1997). Spectroscopic evidence that natural weathering or oxidative depolymerization of soil charcoal results in the formation of humic substances has been observed for several decades (e.g., Kumada, 1983, Schnitzer and Calderoni, 1985, Shindo et al., 1986, Kramer et al., 2004). The dominant structural features generated from pyrogen humification are condensed aromatics with carboxylic acid functionality. In controlled laboratory experiments, the nitric-acid-oxidation of furnace blacks and charcoals generated similar products: condensed aromatic structures substituted with carboxylic, phenolic, and carbonyl functional groups, with molecular weights in the range ca. 400–1200 Da (Haumaier and Zech, 1995, Kamegawa et al., 2002, Trompowsky et al., 2005). The high cation exchange capacity of these types of structures is thought to be responsible for correlations between soil fertility and BC content observed in tropical terra pretta soils and native grassland soils of North America (Glaser et al., 2000, Glaser and Amelung, 2003).
As discussed by Masiello (2004), natural degradation and mineralization of soil BC is certainly occurring, despite the paucity of evidence from environmental measurements explaining degradation pathways. For instance, Czimczik et al. (2003) showed that physical removal processes such as erosion and translocation through the soil profile can not account for the loss of BC from a boreal forest soil occurring in the 250 years following a biomass burning event. In situ chemical degradation/oxidation is also invoked as an explanation for BC depletion in sedimentary deposits that have become oxic after a period of anoxia (Middleburg et al., 1999). However, degradation mechanisms and byproducts remain marginally characterized at the molecular level and their biogeochemical fate remains unclear.
As the contribution of BC to the alkali-soluble fraction (HA) of soil organic matter is being recognized, Masiello and Druffel (1998) proposed that BC cycles as dissolved organic matter (DOM), and used the radioisotope signature with sedimentation rates to estimate that dissolved BC may comprise as much as 4–22% of marine DOM. Estimates from the thermal isolation of BC in ultrafiltered DOM from the Chesapeake Bay, suggest that BC contributes roughly 4–7% of the DOC in the coastal Atlantic Ocean (Mannino and Harvey, 2004). The first molecular-level evidence of BC-like structures in DOM was observed by Kim et al. (2003a). Specifically, ultrahigh-resolution Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry was used to analyze DOM from the Rio Negro, and a blackwater stream from the New Jersey Pine Barrens, revealing the presence of hydrogen-deficient molecules that are likely to be the soluble products of BC degradation (Kim et al., 2004).
We present evidence that directly links these hydrogen-deficient molecules to the degradation of charcoal BC in the soils of a fire-impacted watershed. We use ultrahigh-resolution electrospray ionization (ESI) FT-ICR to resolve BC degradation products within the soil pore water DOM at the site, and introduce a new structural criterion for identifying the condensed aromatic ring structures (CARS) that are directly linked to BC.
Section snippets
Experimental
Soil and water samples were collected during summer 2002 at the University of Michigan Biological Station (UMBS), in Cheboygan County, Michigan, where intense logging and biomass burning during the years 1890–1920 deposited a visible layer of BC to the soils. Soils at the station are excessively-drained, medium sand under a poorly developed, acidic O horizon (∼5 cm thick, pH 4.4). Soils are spodsols, specifically, Emmet medium, frigid, Typic Orthods. Development of the hard ornstein layer of
Results and discussion
ESI-FTICR mass spectrometry with dual electrospray ionization provided sufficient mass accuracy (<0.5 ppm) and resolving power (600,000 at 300 m/z) to assign unique empirical formulas to 95% of the 4000 signals in the mass spectrum of each sample, without a priori physical separation. Fig. 1 shows the ESI-FTICR mass spectrum of C18-extractable charcoal leachates, and several micrographs of soil charcoal surfaces. The 100-year-old charcoal particles from UMBS soils are inhabited by microorganisms
Conclusions
We report on the molecular composition of water-soluble products extracted directly from charcoal particles after 100 years of natural degradation in a forest soil. ESI FT-ICR mass spectrometry reveals condensed aromatic ring structures extensively substituted with oxygen-containing functional groups. The calculation of carbon normalized double bond equivalents from mass spectral data serves as a means to recognize BC degradation products by identifying species with condensed aromatic ring
Acknowledgements
We thank Dr. H. Knicker and an anonymous reviewer for their insightful comments that led to significant improvements of this manuscript. This work was supported by the National Science Foundation, Ohio State Environmental Molecular Science Institute (CHE-0089147 and CHE-0089172). We thank Dr. Alan Marshall and Dr. Ryan Rodgers at the National High Magnetic Field laboratory, Florida State University, for making available their 9.4 T FTICR-MS (funded through their NSF Grant CHE-9903528).
References (46)
- et al.
Water soluble organic compounds formed by the oxidation of soot
Atmospheric Environment
(2002) - et al.
Etude du mechanisme d’oxydation des noirs de carbone par l’ozone en milieu aqueux
Carbon
(1972) - et al.
Black carbon in soils: the use of benzene carboxylic acids as specific markers
Organic Geochemistry
(1998) - et al.
Black carbon in density fractions of the Brazilian Amazon region
Organic Geochemistry
(2000) - et al.
The flux of black carbon to surface sediments on the New England continental shelf
Geochimica et Cosmochimica Acta
(1998) - et al.
A dual electrospray ionization source combined with hexapole accumulation to achieve high mass accuracy of biopolymers in Fourier transform ion cyclotron resonance mass spectrometry
Journal of the American Society of Mass Spectrometry
(2000) - et al.
Black carbon – possible source of highly aromatic components of soil humic acids
Organic Geochemistry
(1995) - et al.
The molecularly uncharacterised component of nonliving organic matter in natural environments
Organic Geochemistry
(2000) - et al.
Oxidative degradation of carbon blacks with nitric acid. II. Formation of water-soluble polynuclear aromatic compounds
Carbon
(2002) - et al.
On the mechanisms by which the charged droplets produced by electrospray lead to gas phase ions
Analytica Chimica Acta
(2000)
High resolution electrospray ionization mass spectrometry and 2D NMR for the analysis of DOM extracted by C18 solid phase disk
Organic Geochemistry
Hydrogen-deficient molecules in natural riverine water samples-evidence for the existence of black carbon in DOM
Marine Chemistry
Probing molecular-level transformations of dissolved organic matter: insights on photochemical degradation and protozoan modification of DOM from electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry
Marine Chemistry
New directions in black carbon organic geochemistry
Marine Chemistry
Some chemical characteristics of paleosol humic acids
Chemical Geology
Characterization of humic-like substances obtained by chemical oxidation to eucalyptus charcoal
Organic Geochemisty
The structure of hexane soot: I. Spectroscopic studies
Applied Spectroscopy
Bacterial film growth in adsorbent surfaces
American Institute of Chemical Engineers Journal
Stability of elemental carbon in savanna soil
Global Biogeochemical Cycles
Biological processing of coal
Applied Microbiology and Biotechnology
How surface fire in Siberian Scots pine forests affects soil organic carbon in the forest floor: stocks, molecular structure, and conversion to black carbon (charcoal)
Global Biogeochemical Cycles
Reburial of fossil organic carbon in marine sediments
Nature
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