Organic Nitrogen Cycling in Ectomycorrhizal Mat and Non-Mat Soils of An Old-Growth Douglas-Fir Forest.

In old-growth Douglas-fir forest soils, organic nitrogen dynamics represent an important but underinvestigated component of the nitrogen cycle.  Because fungal biomass is high in these soils, organic matter derived from fungal cell wall material could be an important energy and/or nitrogen source to soil microbiota.  Chitin, the polymer that comprises fungal cell walls, is nitrogen rich, and microbially produced enzymes including NAGase (which releases glucosamine monomers from complex chitinous molecules) facilitate its uptake.  We investigated the dynamics of microbial chitin degradation and N-acetylglucosamine utilization in organic soils dominated by rhizomorphic ectomycorrhizal mats (Piloderma sp.) and in non-mat organic soils.  These two soil patch types harbor distinct microbial communities with differing potentials for chitin breakdown.  In the laboratory, we amended both soil types with chitin and glucosamine (0.5%, 1%, 2%, 4% w/w), monitored respiration, and measured NAGase activity, soil amino sugars, nitrogen mineralization, and microbial biomass after one (N-acetylglucosamine) to three (chitin) weeks of incubation.  Mat soils displayed higher (approximately 2x) basal respiration rates, higher substrate-induced respiration rates, and higher (approximately 3x) NAGase activities than non-mat soils.  In both soil types, total respiration increased slightly with chitin addition (0–27%) and markedly with glucosamine addition (32–222%) indicating priming of soil organic matter.  NAGase activity decreased (mat) or remained low (non-mat) with low-level substrate amendments but increased (non-mat) or remained constant (mat) with high-level substrate amendments.  These results show that old-growth forest soil microbiota have a high affinity for microbial utilization of N-acetylglucosamine, particularly in ectomycorrhizal dominated patches, and suggest a rapid response in microbial function and possibly a shift in community composition with changing N-acetylglucosamine and chitin availability.  Continuing research includes investigation of the utilization of carbon and nitrogen from chitinous substrates and the identification of active chitin cycling microbial community members.