Balance matters : N:P stoichiometry and plant diversity in grassland ecosystems

Eutrophication of Nitrogen (N) and Phosphorus (P) is threatening the functioning and biodiversity of grassland ecosystems. A well known effect of eutrophication on grasslands is an increase of above-ground productivity, which intensifies light competition and allows only a few competitive species to dominate, resulting in the loss of species diversity. Additionally, eutrophication changes the balance between the availability of multiple nutrients, such as N and P, which may cause extra impacts on grassland ecosystems. This thesis aims at unraveling the effects of N:P stoichiometry (i.e. the balance between N and P) on the functioning of grassland plant communities. How do different plant species respond to a changing N:P stoichiometry and which functional traits influence the responses? Do inter-specific differences in these traits play a role in shaping the species distribution patterns along a gradient from N- to P-limited grasslands? How does N:P stoichiometry influence community- and ecosystem-level properties? On individual growth level, contrasting responses of plants to changing N:P stoichiometry were observed in greenhouse experiments. Root-surface phosphatase activity, an important trait for plant P uptake from bound-P, is influenced by N:P ratio rather than by single availability of N or P. Species typically from P-limited grassland have a higher efficiency of phosphatase activity, indicating that phosphatase activity plays a role in shaping species occurrence patterns from N- to P-limited grasslands (Chapter 2). The effects of N:P stoichiometry on plants were different between the first and second year and among species (Chapter 3). A high N:P supply ratio was found to cause more negative effects on plant growth than a low N:P supply ratio, because of increased nutrient loss from dead roots in the second year. The severe death rate was caused by a high N:P supply ratio rather than by the overall nutrient supply level. The effects of N:P stoichiometry are also evident on community level (chapter 4). A meta-analysis of Eurasian herbaceous ecosystems reveals that species richness is highest at intermediate N:P ratios, whereas threatened species occur more frequently at higher N:P ratios. Furthermore, species occurrence along a N:P gradient was associated with their reproduction strategies: i.e. species with less dependency on seed reproduction occurred more often in P-limited grasslands. Threatened species tend to have such a strategy, which explains their frequent occurrence in P-limited conditions. A modeling study linked the individual- and community-level responses of plants to N:P stoichiometry (chapter 5). Species with an adaptive trait to P-limitation (i.e. high phosphatase activity) win in P-limited conditions unless under very eutrophic conditions. Moreover, if there is a trade-off between their phosphatase activity and growth rate, increased N input may increase ecosystem P retention via altering species composition. In conclusion: balance matters. When P availability is relatively low, N eutrophication may have an extra impact on grassland ecosystems by increasing nutrient loss from the rhizosphere in the short term. In the long term, N eutrophication mayalter species composition of plant communities, but the direction of the species shift may depend on the relative availability of P