Fungal functioning in a pine forest: evidence from a 15N-labeled global change experiment

We used natural and tracer nitrogen (N) isotopes in a Pinus taeda free air CO2 enrichment (FACE) experiment to investigate functioning of ectomycorrhizal and saprotrophic fungi in N cycling. Fungal sporocarps were sampled in 2004 (natural abundance and N-15 tracer) and 2010 (tracer) and delta N-15 patterns were compared against litter and soil pools. Ectomycorrhizal fungi with hydrophobic ectomycorrhizas (e. g. Cortinarius and Tricholoma) acquired N from the Oea horizon or deeper. Taxa with hydrophilic ectomycorrhizas acquired N from the Oi horizon (Russula and Lactarius) or deeper (Laccaria, Inocybe, and Amanita). N-15 enrichment patterns for Cortinarius and Amanita in 2010 did not correspond to any measured bulk pool, suggesting that a persistent pool of active organic N supplied these two taxa. Saprotrophic fungi could be separated into those colonizing pine cones (Baeospora), wood, litter (Oi), and soil (Ramariopsis), with delta(1)5N of taxa reflecting substrate differences. N-15 enrichment between sources and sporocarps varied across taxa and contributed to delta N-15 patterns. Natural abundance and N-15 tracers proved useful for tracking N from different depths into fungal taxa, generally corresponded to literature estimates of fungal activity within soil profiles, and provided new insights into interpreting natural abundance delta N-15 patterns.