The effect of nitrogen addition on community structure and productivity in grasslands: A meta-analysis

The ongoing increase in anthropogenic nitrogen (N) addition may have a substantial impact on grassland ecosystems. Understanding the dynamics of the productivity and community structure of grassland ecosystems following N addition is important. We conducted a meta-analysis of the data from 2516 quadrats described in 56 published N-addition studies and aimed to address these questions and further describe the productivity and community structure of grassland ecosystems in response to N addition in China. Our results showed the critical threshold for N-induced productivity loss to grassland was variable. In meadow steppe, when the range of N addition occurred at rates between 7.5 and 20 g N m−2 yr−1, the aboveground net primary production (ANPP) increased, and below this range, the biomass of the functional groups (forb, grass, and sedge) decreased. Furthermore, above the range, both productivity and structure were negatively related to the rate of N addition. The critical threshold for the N-induced changes in the productivity of the desert steppe was 10.5 g N m−2 yr−1, at which production increases with no reduction in diversity. Additionally, the increase in ANPP with N addition stabilizes above approximately 12 g N m−2 yr−1 in desert steppe, and the decrease in species richness with N addition stabilizes above approximately 11 g N m−2 yr−1. Thus, at approximately 11 g N m-2 yr−1, further N addition causes no increase in production and a clear reduction in diversity. Our results contribute to a better understanding of the consequences of future climate change in grassland ecosystems.