The greater bioavailability of nitrogen (N), phosphorus (P) and potassium (K) in the Anthropocene has strongly impacted terrestrial plant communities. In northwest Europe, high N deposition is considered the main driver of plant diversity loss, so EU legislation to reduce N deposition is expected to promote plant species recovery. But this expectation is simplistic: it ignores the role of other macronutrients. To better understand the potential consequences of changes in the bioavailability of N, P and K for terrestrial plant community diversity, we analysed a dataset of 673 plots in herbaceous ecosystems from eight countries in northern Eurasia. In these plots, species composition of vascular plants was recorded and N, P and K concentrations in aboveground vegetation were measured. The dataset includes 574 vascular plant species, 216 of which are threatened species on European Red Lists. We found that both absolute and relative P availability are more critical than N or K availability. This result is consistent with stoichiometric niche theory and with findings from studies of hyper-diverse forests and shrublands at lower latitudes. We show that ecosystems with low absolute and relative P availability harbor a unique set of threatened species that have narrower nutrient-based niche widths than non-threatened species. Furthermore, we also investigated whether changing the stoichiometric balance between N and P to reflect current EU policies to reduce atmospheric N deposition would affect species numbers. We found that the largest number of species niches would occur at intermediate levels of N, suggesting that if N deposition in high deposition areas were reduced, plant species diversity could indeed recover. However, if N availability were to fall from intermediate to low levels, the effect would be reversed because of a decrease in the number of stoichiometric niches. Lowering of P availability to almost 1 mg g-1 would benefit both non-threatened and threatened species and this effect would be greatest at intermediate N availability. Hence, our study adds to the growing body of evidence showing that nutrient ratios are an important driver of biodiversity impacts and that the balance between N and P needs to considered in conservation efforts. Threatened species may be driven to extinction by latent effects of relative P enrichment arising from reducing N availability without simultaneously reducing P. The narrow focus of EU legislation on reducing N but not P may therefore inadvertently increase the threat to many of Europe's already threatened plant species. An EU Phosphate Directive is needed. Contact: Martin Wassen - M.J.Wassen@uu.nl
