Soil phosphorus – a multidimensional resource that plays an important role for grassland plant species richness

Sammanfattning: The semi-natural grassland habitats contain an important part of the plant diversity in Europe. Changes in farming practices have lead to drastic decreases in plant species richness. The aim of the present thesis is to contribute to the knowledge about mechanisms governing the plant species richness in grasslands with emphasis on soil nutrients. Most species rich grasslands are found on soils with low nutrient availability where nutrients limit growth creating a higher potential for niche differences. The results of my thesis show differences in niches or strategies to acquire phosphorus from soil among grassland plant species in cultivation. One herb without mycorrhiza had the highest phosphorus uptake and biomass production, but a higher available phosphorus concentration remained in soil after growth of that species, indicating an efficient phosphorus mobilisation. The results of this thesis show a strong inverse relationship between soil phosphorus availability and species richness in grazed grasslands of southern Sweden. Species richness was also inversely related to biomass phosphorus concentration, as a measure of soil phosphorus status, in Matorral forests of northern Argentina. We also observed increasing phosphorus uptake efficiency with increasing species richness of the plant communities. Despite less available soil phosphorus the plants were able to use the phosphorus resources more completely. Soil phosphorus and nitrogen appear to relate to species richness in different ways. The strong linear relationships observed for soil phosphorus were not observed for soil nitrogen. A CCA relating species richness of sites in different types of grassland habitats to environmental variables showed that phosphorus availability and vegetation biomass influence species richness negatively, while soil nitrogen was less important. A 3-D projection of the same data reveals that relatively high species richness is observed on soils with high availability of one of the nutrients, nitrogen or phosphorus, and low availability of the other nutrient. This finding suggests that high species richness is not only found at balanced nutrient ratios and that low availability of both nitrogen and phosphorus is not crucial for diverse vegetation. Processes governing plant diversity in grasslands act on different spatial scales. In the grasslands studied, mainly differences in land use were responsible for the level of species richness. There were more species in the pastures than in the other grassland types. Nevertheless the landscape properties were important for what parameters that were influencing species richness of the grasslands on a local scale. The experience from our studies suggests that soil phosphorus availability should be considered in plant diversity studies and in management of grasslands. Different nutrients can not be considered as one plant resource acquired in the same way by plant species. The differences in the ways nitrogen and phosphorus are related to species richness and how they interact is why both nutrients should be considered.