The mechanisms causing shifts in ectomycorrhizal fungal (EMF) communities exposed to increased nitrogen (N) by atmospheric deposition or fertilization are not understood. The effects of N increase could be direct, or could be due to ancillary effects such as altered pH or a change in limiting nutrients. This study examines the direct effects of increased N by examining the response of EMF to a unique 16-year fertilization experiment in an oak savanna. The experiment consists of two levels of nitrogen fertilization (5 and 17 g N m-2 yr-1) and unfertilized plots. Each fertilization treatment also receives equal background levels of P, K, Ca, Mg, S added to offset ancillary effects of N addition. Fertilization has increased soil N, held pH constant and not caused limitation by other nutrients. Meanwhile, fertilization 1) decreased the abundance of aboveground EMF sporocarps for all species except Russula amoenolens (which increased 7 fold with fertilization); 2) held belowground EMF richness constant (at ca. 4 types per sample from a total site type pool of over 20) as measured with morphotype and PCR-RFLP methods; but 3) altered the dominant fungi in the EMF communities: Unfertilized EMF communities are dominated by species with extensive external hyphae notably Cortinarius species while heavily fertilized communities are dominated by those that lack external hyphae primarily R. amoenolens. These data suggest that the direct effect of N addition may be the driving mechanism behind the dramatic shifts in EMF communities in ecosystems that experience excessive N deposition.

Key words: communities, ectomycorrhiza, fungi, mycorrhiza, nitrogen