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 δ 13 C values of these derivatives were determined by gas chromatography‐combustion‐isotope ratio mass spectrometry. Initial δ 13 C 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 ( δ 13 C bulk  = −12.6%) and C 3 ( δ 13 C bulk  = −30.1‰) 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‰ depleted in 13 C compared with bulk dung δ 13 C values, this may have resulted in an under‐estimation of dung C incorporation based on bulk δ 13 C values. Therefore, an investigation of the compound‐specific δ 13 C values of dung‐derived lignin moieties extracted from soils sampled up to 372 days was undertaken. Δ 13 C 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.