Nitrogen deposition is negatively related to species richness and species composition of vascular plants and bryophytes in Swiss mountain grassland

Nitrogen (N) deposition is a major threat to biodiversity of many habitats in the lowlands. In mountain habitats, however, the effect of N deposition on biodiversity is not well understood. Here, data from the biodiversity monitoring of Switzerland were used to investigate whether high N deposition is negatively related to species richnessand community uniqueness of vascular plantsand bryophytesin mountain grassland. The total species diversity, as well as the diversity of three subsets of species (i.e. oligotrophic species, eutrophicspecies and targeted grasslandspecies according to conservation objectives of the Swiss authorities) were analyzed. Overall, the empirical data from the present study indicate that the currently expert-based range of the critical loadof N deposition below which harmful effects on sensitive ecosystems should not occur (upper bound is currently at 20 kg N ha−1 yr−1) is set too large for mountain hay meadows. Negative relations between N deposition and species richnessand community uniqueness in mountain grasslandwere found already at 10–15 kg N ha−1 yr−1. The results suggest that the negative effect of N deposition on plant diversity is mainly due to a decrease of oligotrophic plant species and to a lesser extent to an increase in eutrophicplant species. While for bryophytes, the decrease of community uniqueness is related to changes in both oligotrophic and eutrophicspecies. Furthermore, because plant species richnessof target species for conservation was negatively related to N deposition, airborne N deposition is likely to defeat conservation efforts in mountain grassland.