The scheme of nutrient addition affects vegetation composition and plant species richness in different ways: Results from a long-term grasslands experiment

The deposition of pollutants and agricultural fertilizers substantially increases nutrient supply to the environment. Nutrient addition may shift the nutrient limitation type of plants, e.g. from nitrogen (N)-limited to phosphorus (P)-limited. As the nutrient supply is expected to continue to grow, it is important to understand the patterns of nutrient limitation and their effects on species composition. The aim of this study was therefore to evaluate the long-term (39 years, 1968-2007) effects of varied fertilization schemes, i.e. N, P, PK, PK + 90 kg ha(-1) N, and PK + 180 kg ha(-1) N, on the species composition of a seminatural mountain grassland. In the first period of the experiment (13 years), NPK fertilization resulted in high yield and a considerably impoverished species composition, dominated by a single grass species (Holcus mollis). Plots fertilized with a single nutrient (N or P) developed a similar species composition to that of the control grassland (no fertilization) dominated by species typical of low-fertility habitats of the Nardo-Callunetea class. In the 1980s, the plots fertilized with NPK exhibited a considerable reduction in yield. Therefore, in the years 1993 and 1994 mowing was replaced by sheep grazing, and supplementary fertilization with micronutrients was applied. In 2007 the yield in the treatment fertilized with PK + 180 kg ha(-1) N was not restored, but the number of species increased. The species composition in the PK treatment became more similar to that of the plots treated with NPK. In contrast, changes in grasslands fertilized with single nutrients (N or P) were insignificant. To conclude, the factor that most strongly affected species composition was the increase in productivity caused by the addition of basic macronutrients. The addition of N or P alone does not significantly affect species composition if other elements limit the productivity of the grassland.