A transect of 68 acid grasslands across Great Britain, covering the lower range of ambient annual nitrogen deposition in the industrialized world (5 to 35 kg Nha-1 year-1), indicates that long-term, chronic nitrogen deposition has significantly reduced plant species richness. Species richness declines as a linear function of the rate of inorganic nitrogen deposition, with a reduction of one species per 4-m2 quadrat for every 2.5 kg Nha-1 year-1 of chronic nitrogen deposition. Species adapted to infertile conditions are systematically reduced at high nitrogen deposition. At the mean chronic nitrogen deposition rate of central Europe (17 kg Nha-1 year-1), there is a 23% species reduction compared with grasslands receiving the lowest levels of nitrogen deposition.

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45Evidence from an international survey in the Atlantic biogeographic region of Europe 46indicates that chronic nitrogen deposition is reducing plant species richness in acid 47 grasslands. Across the deposition gradient in this region (2 to 44 kg N ha -1 yr -1 ) species 48 richness showed a curvilinear response, with greatest reductions in species richness when 49 deposition increased from low levels. This has important implications for conservation 50 policies, suggesting that to protect the most sensitive grasslands resources should be 51 focussed where deposition is currently low. Soil pH is also an important driver of species 52 richness indicating that the acidifying effect of nitrogen deposition may be contributing to 53 species richness reductions. The results of this survey suggest that the impacts of nitrogen 54 deposition can be observed over a large geographical range.

1In this study we investigate the impact of nitrogen (N) deposition on the diversity of 2 three different vegetation functional groups -forbs, grasses and mosses -using a field 3 survey of acid grasslands across Great Britain. Our aim is to identify the vegetation 4 types that are most vulnerable to enhanced N deposition, and to shed light on the 5 mechanisms that may be driving N-initiated species changes in the UK. Sixty-eight 6 randomly selected grasslands belonging to the U.K. National Vegetation 7Classification group U4 (Festuca ovina-Agrostis capillaris-Galium saxatile grassland) 8were studied along a gradient of atmospheric N deposition ranging from 6 to 36 kg N 9 ha -1 yr -1 . At each site, vegetation was surveyed and samples were taken from the 10 topsoil and subsoil. Above-ground plant material was collected from 3 species: a 11 forb, grass and moss. 12Both the species richness and cover of forbs declined strongly with increasing 13 N deposition, from greater than 8 species/20% cover per m 2 quadrat at low levels of N 14 to fewer than 2 species/5% cover at the highest N deposition levels. Grasses showed 15 a weak but significant decline in species richness, and a trend toward increasing cover 16 with increasing N input. Mosses showed no trends in either species richness or cover. 17