Nitrogen addition changed macromycete sporocarp production and below-ground ectomycorrhizal species composition in a Norway spruce stand

The effects of simulated N deposition on the ectomycorrhizal and total macrofungal community in a subalpine Picea abies stand were investigated by adding 150 kg N ha-1year-1 as ammonium nitrate as a long-term fertilizer. Ectomycorrhizal diversity was studied 1) above-ground in terms of sporocarp production, including three years of recording before N addition, and 2) below-ground by PCR-RFLP analysis of the ITS region of rDNA extracted from single mycorrhizal root tips before and after one and two years of fertilization in treated and control plots. Sporocarp surveys revealed a strong impact of increased N input on the ectomycorrhizal community by drastically decreasing species diversity already one year after the start of N addition, whereas the saprobic fungal community was not affected. The impact of N addition on below-ground ectomycorrhizal diversity was less pronounced. No change in the number of ectomycorrhizal taxa and Simpson's index of diversity was found. However, ordination analysis as well as comparison of similarities among control and treated plots indicated a change in below-ground species composition two years after N addition. Significant changes in abundances of single species were observed in fertilized plots. Eleven out of 25 species which produced conspicuous sporocarps in the six years of survey were identified on the root system. These species accounted for approx. 25% of all sampled root tips. At least 44% of all ectomycorrhizas, comprising nine different RFLP-types, were formed by species belonging to the Thelephoraceae and Corticiaceae, taxa which produce inconspicuous sporocarps. Our data indicate that the abundance of these species on the root system increases at higher N concentrations, whereas the abundance of species which produce fewer sporocarps after N input decreases. (Resume d'auteur)