The effects of long-term nitrogen fertilization on the ectomycorrhizal communities of a temperate deciduous ecosystem

In North America, temperate deciduous ecosystems experience high inorganic nitrogen deposition but the impact on ectomycorrhizal fungal communities in these ecosystems is not understood. In a temperate oak savanna, we examined two co-occurring consequences of elevated N deposition, increased N supply and altered root allocation, on the ectomycorrhizal fungal communities in a 16 yr N addition experiment in which replicate 1000 m2 plots received 0, 5.4 (moderate fertilization), or 17 g N m-2 yr-1 (high fertilization). High fertilization more than doubled inorganic N, nearly halved oak fine root length and biomass yet doubled fine root production and turnover. N fertilization also altered the carbohydrate profiles of fine roots. In the 14th, 15th and 16th year of fertilization, total sporocarp richness was reduced by >50% in both fertilization levels while Russula spp. produced approximately five times more sporocarps with high fertilization. In the 14 th and 15th year of fertilization, belowground treatment-scale species richness and species area curves were lower with high fertilization but smaller spatial scale richness, diversity indices and evenness were not. Dominant fungi colonizing roots collected in savanna and oak seedling bioassays included Cenococcum geophilum, common in all treatments, Cortinarius spp., dominant in unfertilized plots, and Russula spp., dominant with high fertilization. In contrast to Russula spp. in coniferous ecosystems, oak savanna Russula were stimulated by N increase suggesting species-specific impacts of N increase depend on particular host plant-fungi combinations. Belowground diversity at smaller spatial scales may be constant because diversity-promoting effects of increased fine root productivity counteract adverse effects of increased N.