Nutrient limitation of alpine plants: Implications from leaf N : P stoichiometry and leaf δ15N

Nitrogen (N) deposition can affect grassland ecosystems by altering biomass production, plant species composition and abundance. Therefore, a better understanding of the response of dominant plant species to N input is a prerequisite for accurate prediction of future changes and interactions within plant communities. We evaluated the response of seven dominant plant species on the Tibetan Plateau to N input at two levels: individual species and plant functional group. This was achieved by assessing leaf N : P stoichiometry, leaf delta N-15 and biomass production for the plant functional groups. Seven dominant plant species-three legumes, two forbs, one grass, one sedge-were analyzed for N, P, and delta N-15 2 years after fertilization with one of the three N forms: NO3-, NH4+, or NH4NO3 at four application rates (0, 7.5, 30, and 150 kg N ha(-1) y(-1)). On the basis of biomass production and leaf N : P ratios, we concluded that grasses were limited by available N or co-limited by available P. Unlike for grasses, leaf N : P and biomass production were not suitable indicators of N limitation for legumes and forbs in alpine meadows. The poor performance of legumes under high N fertilization was mainly due to strong competition with grasses. The total above-ground biomass was not increased by N fertilization. However, species composition shifted to more productive grasses. A significant negative correlation between leaf N : P and leaf delta N-15 indicated that the two forbs Gentiana straminea and Saussurea superba switched from N deficiency to P limitation (e. g., N excess) due to N fertilization. These findings imply that alpine meadows will be more dominated by grasses under increased atmospheric N deposition.