Decomposition, δ13C, and the “lignin paradox”

Abstract

The natural abundance of 13C (δ13C) 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 13C 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)], δ13C 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, δ13C was significantly more depleted for the highest decay classes, and total C was negatively correlated with δ13C, consistent with the higher total C of lignin than of cellulose. Relationships of total C and δ13C with density were much weaker. We discuss causes for the variability of δ13C in coarse woody debrisfrom these sites, and how the apparent paradox in the predicted change of δ13C 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. Key words: Coarse woody debris, decomposition, lignin, 13C NMR, δ13C, proximate analysis