Off-line pyrolysis and compound-specific stable carbon isotope analysis of lignin moieties: a new method for determining the fate of lignin residues in soil

Off-line pyrolysis was used to liberate lignin moieties from dung and soil and, after trimethylsilylation, the delta C-13 values of these derivatives were determined by gas chromatography-combustion-isotope ratio mass spectrometry. Initial delta C-13 values determined for 4-vinylphenol, syringol, 4-vinylguaiacol, 4-acetylsyringol, 4-vinylsyringol, 4-(2-Z-propenyl)syringol, 4-(2-E-propenyl)syringol and 4-(2-propenone)syringol pyrolysis products of the lignin polyphenol structure from C-4 (delta C-13(bulk) = -12.6%) and C-3 (delta C-13(bulk) = -30.1 parts per thousand) dung confirmed the robust and reproducible nature of the off-line preparation technique. C-4 dung was used as a treatment in a randomised field experiment to assess the short-term sequestration of dung carbon in managed grasslands. Since lignin was on average 3.5 parts per thousand depleted in C-13 compared with bulk dung delta C-13 values, this may have resulted in an underestimation of dung C incorporation based on bulk delta C-13 values. Therefore, an investigation of the compound-specific delta C-13 values of dung-derived lignin moieties extracted from soils sampled up to 372 days was undertaken. Delta C-13 values between lignin moieties extracted from treated and untreated soils showed that dung-derived lignin was not especially resistant to degradation and suggested that individual moieties of the lignin macromolecule must: (i) move into soil, (ii) be degraded, or (iii) be transformed diagenetically at different rates. This adds to a gathering body of evidence that lignin is not particularly stable in soils, which has considerable significance for the perceived role of different biochemical components in the cycling of C in soils. Copyright (C) 2008 John Wiley & Sons, Ltd.