1. Atmospheric deposition of nitrogen (N) is a global problem resulting in negative consequences for biodiversity due to direct toxicity, increases in invasive species, increased susceptibility to environmental stresses and soil-mediated effects of acidification and eutrophication.
2. Reductions in plant species richness related to N deposition have been observed in a number of habitats including calcifuge (acid) grasslands but the mechanisms of this decline have not been fully investigated. We test the hypotheses that along a large-scale gradient of N deposition there is (i) an increase in species tolerant of low pH conditions as a result of soil acidification and (ii) an increase in competitive and nitrophilic species as a result of soil eutrophication. As competitive species can occur in low pH habitats, both of these hypotheses could be true.
3. Using plant characteristics, we examined changes in vegetation species composition along the gradient of N deposition in the UK. Mean C–S–R signatures were used to identify the competitive response of plant communities together with Ellenberg N (nitrogen) scores to identify increases in nitrophilic species. Ellenberg R (reaction, pH) scores were used to identify change in response to soil pH together with an index of soil acidity preference developed using regional survey data.
4. Mean C–S–R signatures showed no significant correlation with N deposition, nor did mean Ellenberg N scores. Ellenberg R scores and the index of soil acidity preference showed significant relationships with N deposition indicating an increased dominance of acid-tolerant species.
5. The results suggest that soil acidification as opposed to eutrophication and consequent competition between species is contributing to shifts in species composition and diversity linked to N deposition in calcifuge grasslands. Soil acidification may be leading to reduced nutrient availability preventing the effects of N addition from being apparent.