Overestimates of black carbon in soils and sediments
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Several recent reports suggest that black carbon (BC), which broadly encompasses charcoal, soot, and other forms of pyrogenic carbon, may constitute a significant proportion of the refractory carbon in soil and sedimentary organic matter. BC is a sink for biospheric and atmospheric carbon dioxide, and is intimately tied to the biogeochemical cycling of both carbon and oxygen through its role in organic matter cycling. Additionally, BC may represent a large fraction of the “missing carbon sink” in global carbon accounting. Here, we demonstrate that documented measurements of BC may be the result of methodological artifacts, which inadvertently overestimate the amount of BC. We found that a widely used thermal oxidative method can create a residue that falls under the operational definition of BC in samples that are relatively BC-free. Moreover, during this procedure, labile organic matter constituents are condensed into pyrogenic carbon, implying that the labile components are present in lesser quantities. These methodological deficiencies are promoting overestimates in the amount of refractory carbon in soil and sedimentary organic matter and may endorse inaccuracies in the rates of carbon fluxes, the mean residence times of terrestrial carbon, and organic matter burial rates in oceanic environments.
KeywordsLignin Nuclear Magnetic Resonance Black Carbon Thermal Oxidation Natural Organic Matter
We thank Dr. J. Skjemstad for providing the Australian and German soil samples and the BC values determined by UV photo-oxidation and solid-state NMR. This research was supported by the National Science Foundation, Environmental Molecular Science Institute (CHE-0089147) and a postdoctoral fellowship to M.J.S. from the Natural Science and Engineering Research Council (NSERC) of Canada.
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