Decomposition, δ13C, and the "lignin paradox"

The natural abundance of C-13 (delta C-13) generally increases with decomposition of organic matter. This is contrary to the expected decrease, as lignin is hypothesized to accumulate relative to isotopically heavier cellulose. Our objective was to test the hypothesis that C-13 depletion should be observed for gymnosperm logs that typically develop advanced brown-rot decay with high lignin content. With increasing lignin concentration [previously determined by nuclear magnetic resonance (NMR)], delta C-13 tended to become more negative for samples of Pseudotsuga menziesii, Tsuga heterophylla, Thuja plicata, and unidentified species from Coastal Forest Chronosequence sites of southern Vancouver Island. For a larger sample set without NMR analysis, delta C-13 was significantly more depleted for the highest decay classes, and total C was negatively correlated with delta C-13, consistent with the higher total C of lignin than of cellulose. Relationships of total C and delta C-13 with density were much weaker. We discuss causes for the variability of delta C-13 in coarse woody debris from these sites, and how the apparent paradox in the predicted change of delta C-13 with decomposition is largely due to the confusion of lignin, the biopolymer produced by higher plants, with the acid-unhydrolyzable residue (AUR) of the proximate analysis procedure commonly used to assess litter quality in decomposition studies.