Isotopic and compositional evidence for carbon and nitrogen dynamics during wood decomposition by saprotrophic fungi

Sporocarps of wood decay fungi contain functional information about how different taxa partition carbon and nitrogen resources from wood. We combined carbon and nitrogen concentrations, isotopic ratios (13C:12C, 15N:14N, and 14C:12C, expressed as d13C, d15N, and ?14C values), and compositional patterns in wood, cellulose, and sporocarps to investigate functional and isotopic differences in six taxa of decay fungi during log decomposition. Radiocarbon (?14C) measurements separated fungi into heartwood colonizers (Fomitopsis and Hericium, ~30+-year-old carbon) and sapwood colonizers (Mycena, Hypholoma, and Trametes, 1-12-year-old carbon). Decay modes influenced d13C, with Hericium, a selective white-rot fungus, higher in d13C than nonselective white-rot fungi because Hericium preferentially assimilated 13C-enriched hemicellulose rather than cellulose. Fungal d15N was lower in heartwood colonizers than in sapwood colonizers, presumably reflecting greater N turnover and 15N enrichment in sapwood than in heartwood. Sporocarp d15N correlated with sporocarp %N and with the relative proportion of protein in N-containing pyrolysis products because fungal protein was 4–5 permille higher in d15N (and 3–4 permille higher in d13C) than non-protein. From these measurements, we improved the quantitative and conceptual understanding of how sources, composition and metabolic processing determined isotopic composition of fungi.