• Changes in above-and below-ground ectomycorrhizal species composition are reported following nitrogen addition for 2 yr to a subalpine spruce (Picea abies) stand.
• The macrofungal sporocarp production was recorded before and during N addition. Belowground ectomycorrhizal diversity was measured by PCR-RFLP analysis of the internal transcribed spacer (ITS) region of rDNA extracted from single mycorrhizal root tips before, and after 1 and 2 yr of fertilization.
• Sporocarp surveys showed that diversity of the ectomycorrhizal community was drastically reduced following 1 yr of N addition, whereas the saprobic fungal community was not affected. The impact on belowground ectomycorrhizal diversity was less pronounced with no change either in the number of ectomycorrhizal taxa or in Simpson's index of diversity. However, a change in belowground species composition 2 years after N addition was observed with significant changes in abundances of single species.
• Species which produced large sporocarps accounted for 25% of all sampled root tips. At least 44% of all ectomycorrhizas were formed by species belonging to the Thelephoraceae and Corticiaceae, taxa which produce inconspicuous sporocarps.
• Addition of N caused a shift in ectomycorrhizal abundance from species forming large sporocarps to species with no or resupinate sporocarps.