Long‐term increase in nitrogen supply alters above‐ and below‐ground ectomycorrhizal communities and increases the dominance ofRussulaspp. in a temperate oak savanna

• Here we examine the effects of increased nitrogen (N) supply on the ectomycorrhizal fungal communities of a temperate oak savanna. • In a 16-yr N-addition experiment in which replicate 1000 m2 plots received 0, 5.4 or 17 g N m−2 yr−1, ectomycorrhizal sporocarp production was measured in the 14th, 15th and 16th year of fertilization. Ectomycorrhizal fungi (EMF) colonizing roots were examined by morphotyping-PCR-RFLP and sequence analysis in the 14th and 15th year of fertilization. • Total sporocarp richness was reduced by > 50% in both fertilization treatments in all 3 yrs, whereas Russula spp. produced approx. five times more sporocarps with 17 g N m−2 yr−1. Below-ground, treatment-scale species richness and species area curves were lower with 17 g N m−2 yr−1 but richness, diversity indices and evenness at smaller spatial scales were not. Dominant fungi colonizing roots included Cenococcum geophilum, common in all treatments, Cortinarius spp., dominant in unfertilized plots, and Russula spp., dominant with 17 g N m−2 yr−1. • Communities of EMF in this temperate deciduous ecosystem responded to N addition similarly to those of coniferous ecosystems in that increased N supply altered EMF diversity and community composition but differently in that dominance of Russula spp. increased.