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Isotopic evidence for condensed aromatics from non‐pyrogenic sources in soils – implications for current methods for quantifying soil black carbon
Author(s) -
Glaser Bruno,
Knorr KlausHolger
Publication year - 2008
Publication title -
rapid communications in mass spectrometry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.528
H-Index - 136
eISSN - 1097-0231
pISSN - 0951-4198
DOI - 10.1002/rcm.3448
Subject(s) - chemistry , soil water , isotope ratio mass spectrometry , environmental chemistry , carbon black , combustion , carbon fibers , fraction (chemistry) , isotope analysis , organic matter , isotopes of carbon , charring , isotopic signature , soil test , total organic carbon , mass spectrometry , stable isotope ratio , organic chemistry , chromatography , soil science , ecology , natural rubber , environmental science , materials science , physics , quantum mechanics , composite number , composite material , biology
Black carbon (BC) is a complex continuum of partly charred organic matter predominantly consisting of condensed aromatic and graphitic moieties and it has high potential for long‐term carbon sequestration in soils and sediments. There has been common agreement that BC is exclusively formed by incomplete combustion of organic matter, while non‐pyrogenic sources are negligible. In this study, we investigated the stable carbon isotope signature of benzenepolycarboxylic acids (BPCAs) as molecular markers for BC to test if there is also a significant contribution of non‐pyrogenic carbon to this fraction in soils. BPCAs were formed by hot nitric acid oxidation of different soils and analyzed by three different procedures: (i) elemental analysis – isotope ratio mass spectrometry (EA‐IRMS) of bulk BPCAs and gas chromatography – combustion – isotope ratio mass spectrometry (GC‐C‐IRMS) of (ii) BPCA trimethylsilyl (TMS) derivatives, and (iii) BPCA methyl derivatives. Best accuracy and precision of isotope measurements were obtained by EA‐IRMS of bulk BPCAs although this method has a risk of contamination by non‐BC‐derived compounds. The accuracy and precision of GC‐C‐IRMS measurements were superior for methyl derivatives (±0.1‰ and 0.5‰, respectively) to those for TMS derivatives (+3.5‰ and 2.2‰, respectively). Comparison of BPCA δ 13 C values of soil samples prior to and after laboratory and field incubations with both positive and negative 13 C labels at natural and artificial abundances revealed that up to 25% of the isolated BC fraction in soils had been produced in situ , without fire or charring. Commonly applied methods to quantify BC exclusively formed by pyrogenic processes may thus be biased by a significant non‐pyrogenic fraction. Further research is encouraged to better define isolated BC fractions and/or understand mechanisms for non‐pyrogenic BC production in soils. Copyright © 2008 John Wiley & Sons, Ltd.