Ectomycorrhizal fungal taxa differing in response to nitrogen deposition also differ in pure culture organic nitrogen use and natural abundance of nitrogen isotopes

• Ectomycorrhizal fungal species vary in their response to nitrogen (N) availability and ability to use organic N. We hypothesized that taxa dominant at sites with high soil inorganic N would be less likely to use organic N than taxa dominant at low soil inorganic N. We also asked whether these taxa differed in natural abundance of N isotopes. • Pure culture N use for taxa from an N deposition gradient in Alaska was examined and N isotopes of sporocarps, soils and foliage collected over this gradient were quantified. • Taxa common in low inorganic N soils grew on protein, glutamine and serine, whereas dominant taxa in high inorganic N soils grew on glutamine, but poorly on protein and serine. Sporocarp δ15N was highest in protein users, and lowest in nonprotein users. With increasing soil inorganic N, sporocarps became more isotopically enriched relative to foliage. • The importance of organic N use might decline with increasing N availability, although field tests are required. The relationship between organic N use and N isotopes also merits further study. However, sporocarp isotopic enrichment may be a useful indicator of soil N availability.