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

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Summary

  • 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 15N tracer) and 2010 (tracer) and δ15N 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). 15N 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 δ15N of taxa reflecting substrate differences. 15N enrichment between sources and sporocarps varied across taxa and contributed to δ15N patterns.
  • Natural abundance and 15N 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 δ15N patterns.

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