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 δ13C values of these derivatives were determined by gas chromatography-combustion-isotope ratio mass spectrometry. Initial δ13C 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 C4 (δ13Cbulk = -12.6%) and C3 (δ13Cbulk = -30.1‰) dung confirmed the robust and reproducible nature of the off-line preparation technique. C4 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‰ depleted in 13C compared with bulk dung δ13C values, this may have resulted in an underestimation of dung C incorporation based on bulk δ13C values. Therefore, an investigation of the compound-specific δ13C values of dung-derived lignin moieties extracted from soils sampled up to 372 days was undertaken. δ13C 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 © 2008 John Wiley & Sons, Ltd.