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

•Nitrogen deposition negatively affected plant diversity in mountain grasslands of Switzerland.•Nitrogen deposition was related to the homogenization of plant and bryophyte species communities.•Airborne N deposition may defeat conservation efforts in mountain hay meadows.•The critical load of N deposition for mountain hay meadows is currently set too large.

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 richness and community uniqueness of vascular plants and bryophytes in mountain grassland. The total species diversity, as well as the diversity of three subsets of species (i.e. oligotrophic species, eutrophic species and targeted grassland species 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 load of 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 richness and community uniqueness in mountain grassland were 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 eutrophic plant species. While for bryophytes, the decrease of community uniqueness is related to changes in both oligotrophic and eutrophic species. Furthermore, because plant species richness of target species for conservation was negatively related to N deposition, airborne N deposition is likely to defeat conservation efforts in mountain grassland.