As a consequence of changes in the traditional agricultural system, lowproductive, species rich grasslands have become increasingly rare. The remaining relics are suffering from increased nutrient availability due to nitrogen deposition and phosphorus inputs. While both research and environmental policy have mainly targeted nitrogen enrichment in this context, scientific evidence is emerging that phosphorus, rather than nitrogen, is the key nutrient controlling productivity and species richness in these systems. The main goal of this study is to elucidate whether nitrogen or phosphorus availability controls species richness of low-productive species-rich grasslands and to seek critical values of nutrient availability applicable in restoration projects.
The results are consistent with the recent hypothesis that increased phosphorus availability has a detrimental effect on species richness. The critical value for phosphorus availability appears 3mg P/kg. No plot above this value has been observed containing more than 20 species. The occurrence of typical species was also correlated with low phosphorus availability. Although nitrogen availability did not have a clear impact, the results indicate that 20mg ammonium/kg can also be used as a critical value. We therefore conclude that further research and conservation management should also focus on the reduction phosphorus-availability to enable successful restoration and preservation of these species-rich grassland communities.