Plant species richness response to atmospheric nitrogen deposition across bedrock types in the United States and Czechia

Abstract Atmospheric nitrogen (N) deposition leads to many changes in terrestrial and aquatic ecosystems, affecting ecosystem processes and species composition. Terrestrial vegetation often shifts from being N‐limited to light‐limited and becomes dominated by a few fast‐growing strong competitors and generalist species. However, once vegetation is not N‐limited, other nutrients such as phosphorus or base cations might limit growth, constraining the expansion of strong competitors. Using extensive data from the United States (US) and Czech Republic (CZ) for open‐canopy (herbaceous vegetation and shrublands) and closed‐canopy (forest) vegetation, we examined differences in response to N deposition arising from either base cation‐rich, moderate, or poor bedrock. We hypothesized that increased N deposition on cation‐poor bedrock could have a weaker effect on plant species richness changes than on cation‐rich bedrock, because under the multiple element limitation paradigm, the expansion of strong competitors and competitive exclusion could be limited by other nutrients in a cation‐poor environment. Our results, after controlling for other environmental factors, including S deposition, show that the effect of N deposition on species richness differs by bedrock type and plant species richness declines the most on cation‐rich bedrock as hypothesized, except for open‐canopy vegetation in the United States. We found a canonical unimodal relationship to N deposition for both vegetation types in both countries. Although species richness generally shows an initial increase at low nitrogen deposition loads before declining, it decreases steadily across the full nitrogen deposition gradient in both vegetation types occurring on cation‐rich bedrock in the Czech dataset. Although the species richness change is driven by the interplay among several factors, the same response of species richness to N deposition and the same derived critical N deposition values, ranging from 8 to 14 kg N ha −1  year −1 , across such a broad gradient in both countries, suggest the robustness of the results. Overall, the results also agree with the empirical critical loads defined for the US ecosystems and European ecosystems, but suggest that limitation by cations or other nutrients may play a role as well in the response to N deposition.