Nitrogen, not phosphorus, enrichment controls biomass production in alpine wetlands on the Tibetan Plateau, China

Little is known about the effect of nitrogen (N) enrichment on alpine wetland plant N and phosphorus (P) limitations and N:P stoichiometry. We assessed the effects of N and P additions on plant nutrient limitations and N:P ratio in an alpine wetland complex on the Tibetan Plateau, China using a 3-year long factorial experiment. N and P were applied twice per year at an annual rate of 10 g m(-2) yr(-1) and 5 g m(-2) yr(-1), respectively. N addition significantly increased aboveground biomass, plant leaf N, plant leaf N:P ratio and soil available N:P ratio, but had no effect on plant leaf P and soil available P. Although N addition increased the plant N:P ratio it was still < 14. P addition increased plant and soil available P, decreased plant and soil N: P ratio, but had no effect on plant biomass. N + P additions had no additive effect on plant biomass over N additions alone, plant N or P, or soil available N or P. These results indicate that N, rather than P, is likely the limiting nutrient for biomass production in alpine wetlands in this region of Tibet and nutrient limitation does not shift from N to P with increasing N deposition. Thus, N deposition could have a significant influence on vegetation growth and biomass production.