Different types of sub-alpine grassland respond similarly to elevated nitrogen deposition in terms of productivity and sedge abundance
Increasing emissions of reactive nitrogen (N) compounds threaten the composition of species-rich communities even in remote alpine areas. In the few studies available from (sub-)alpine grassland, N deposition altered species composition in favour of sedges, which are assumed to be less limited by phosphorus (P) compared to other species. Is the magnitude of the sedges' relative increase modified by availability of other nutrients, which is in turn influenced by site-specific edaphic and climatic conditions?
Swiss Central Alps.
We selected ten sub-alpine sites that covered a broad range of communities and soil conditions, including both calcareous Seslerietum as well as acidic Nardetum pastures. As a model species, Carex sempervirens Vill. was present at each site. For two consecutive years, in addition to a control, two treatments were applied to the vegetation: (1) 50 kg·N·ha−1·yr−1 as NH4NO3 and (2) compound NPK fertilizer comprising the same amount of N. Total above-ground biomass, fraction of all sedges, leaf length and leaf N, P and potassium (K) content of C. sempervirens were recorded and analysed for their responses to treatments and interaction with variables linked to productivity and nutrient availability (including soil water potential, pH and soil P).
Biomass production was stimulated across all sites on average by 30% with elevated N deposition, and by 60% with NPK fertilizer application, revealing the sites to be similarly co-limited by N and by P and K. Also across all sites, the fraction of sedges was increased on average 33% by N addition, but remained constant with NPK application, suggesting stronger P and/or K co-limitation in non-sedge species. N treatment effects were consistent in their direction but varied between sites; nevertheless this variation could not be explained by the measured edaphic or climatic factors. However, by trend, sedges benefited more from N fertilization on sites with higher pH, but benefited less on very dry sites.
Our findings reveal that the observed increase of sedges is a general response of sub-alpine grassland to elevated N deposition and thus can be regarded as universal both for Seslerietum as well as Nardetum pastures.