Species-rich grassland can persist under nitrogen-rich but phosphorus-limited conditions

Aim: Deposition of nitrogen is assumed to cause loss of botanical diversity, probably through increased production and exclusion of less competitive species. However, if production is (co-)limited by phosphorus, acceleration of the phosphorus cycle may be responsible for the diversity loss and, where that is the case, nitrogen emission reduction may turn out to be an ineffective mitigation strategy. Here we study the feasibility of this mechanism through adding potassium and phosphorus to grassland where nitrogen limitation is absent. Methods: We made vegetation relevés in a long-term agricultural fertilisation experiment where potassium, phosphorus and nitrogen were being added to grassland on drained peat where nitrogen availability was high, even in unfertilised plots. We applied a multivariate analysis to investigate the effect of additions of K, K + P and K + P + N on the species composition. Results: Unfertilised plots had a very low biomass production and were rich in plant species despite their high nitrogen availability. Addition of potassium led to a strongly increased production but did not result in a reduction of species numbers. Phosphorus in addition to potassium increased production still further and decreased species numbers, most notably the number of endangered species. Conclusions: Even under nitrogen rich conditions species richness may be high in grasslands where phosphorous provides a limitation to plant growth. Phosphorus limitation and phosphorus enrichment are both common in grassland, at least in north-western Europe. Part of the general decrease in species numbers that is commonly ascribed to nitrogen enrichment may therefore be due to phosphorus enrichment. If phosphorus and nitrogen are co-limiting (which is often the case) the current nitrogen emission reduction policies may be effective, but not sufficient to restore grassland diversity to its pre-industrial level.