Off-line pyrolysis and compound-specific stable carbon isotope analysis of lignin moieties: a new mthod 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 13C values of these derivatives were determined by gas chromatography-combustion-isotope ratio mass spectrometry. Initial 13C 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 (13Cbulk = -12.6%) and C3 (13Cbulk = -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 13C values, this may have resulted in an under-estimation of dung C incorporation based on bulk 13C values. Therefore, an investigation of the compound-specific 13C values of dung-derived lignin moieties extracted from soils sampled up to 372 days was undertaken. 13C 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