Response of biomass and species diversity to nitrogen addition in alpine meadows

In order to explore the effects of air nitrogen deposition on alpine meadows, four nitrogen levels (N0, 0; N5, 14.3 g·m-2; N10, 28.6 g·m-2; N20, 57.1 g·m-2) were established in alpine meadows in the eastern Tibetan Plateau, and the biomass and species diversity for each level were measured after three consecutive years of nitrogen addition. The aboveground biomass increased from N0 to N20, reaching the maximal value at N20. Regarding the spatial pattern, the aboveground biomass showed a trend of upward shift, that is, the plants grew taller. The belowground biomass increased initially but decreased afterwards as nitrogen addition increased, peaking at N10, and it showed a trend of shifting towards topsoil (0-10 cm depth). Among the three functional groups (grass, sedge and forbs), aboveground biomass of grass increased from N0 to N20, peaking at 20, and so did the ratio between the aboveground biomass and total aboveground biomass of grass. In addition, plant species diversity decreased gradually as nitrogen addition increased. In conclusion, nitrogen addition exerted an evident influence on the structure of alpine meadows.