Soil and plant 15N values reveal ecosystem nitrogen dynamics and indicate niche partitioning for soil nitrogen among plant species

The study presented here addresses the effects of N dynamics on the structure and function of European semi-natural grassland ecosystems. We used the natural abundance of stable nitrogen (N) isotopes in soils of 19 different grasslands as a simple tool to link soil N cycling with productivity, plant species composition and diversity. In addition, we used stable N isotopes in plants from the 19 sites as an indicator for differential NO3 and NH4 uptake among co-occurring plant species, seeking for empirical evidence of niche partitioning for soil N among co-occurring plant species. Bulk soil 15N of the investigated grasslands was positively related to soil N turnover and indicated strong N effects of productivity, species composition and diversity of the investigated grasslands. Leaf 15N values of 22 different non N2-fixing plant species were positively related to soil 15N values across sites. Within a site variability of leaf 15N values, was explained by differences in the species' NO3 to NH4 uptake ratios. The observed differences in NO3 to NH4 among species were, however, not species specific but depended on site characteristics and abundance of a species when tested across the investigated sites, suggesting high plasticity of the investigated species with regard to NO3 vs. NH4 uptake. In addition, the range of leaf 15N values among species in a site increased with increasing diversity indicating that niche specification of plants for soil N increases with increasing diversity or interspecific competition. Our study shows that the structure and function of the investigated grasslands are strongly influenced by N dynamics. Further, our study gives strong evidence that co-occurring plants partition different forms of soil N and that plants show a high degree of plasticity with respect to N uptake.