Abstract:

In this research we studied the nitrogen responses of grassland plant communities in meadow steppe， typical steppe and desert steppe grasslands of Inner Mongolia. A gradient of seven nitrogen addition treatments was set up， namely： CK （0 g N·m^-2·yr^-1）， N₁ （5 g N·m^-2·yr^-1）， N₂ （10 g N·m^-2·yr^-1）， N₃ （15 g N·m^-2·yr^-1）， N₄ （20 g N·m^-2·yr^-1）， N₅ （25 g N·m^-2·yr^-1）， N₆ （30 g N·m^-2·yr^-1）， One-way ANOVA and correlation analyses were used to evaluate the difference in vegetation biomass and soil carbon and nitrogen in the different grassland types under the different rates of nitrogen addition， and relate these to other factors of influence. It was found that： 1） Nitrogen addition did not significantly affect the underground biomass of any of the three grassland types （P>0.05）， but significantly increased aboveground biomass in meadow and desert steppe （P<0.05）. As a preliminary assessment based on these results， the N₃ treatment is the approximate response saturation threshold. At this level of nitrogen addition， the average biomass increase of the three Inner Mongolian grassland types was 29.66%， compared to CK. The response to added nitrogen was most obvious in the dry desert steppe grassland. Nitrogen application significantly reduced the root∶shoot ratio in the meadow steppe grassland （P<0.05） and significantly increased the root∶shoot ratio of the typical steppe grassland under the N₃ treatment （P<0.05）， but had no significant effect in desert steppe grassland （P>0.05）. 2） The soil carbon and nitrogen contents of 0-10 cm and 10-30 cm soil layers were analyzed to determine the effects of nitrogen addition on soil organic carbon and total nitrogen content in the three grassland types and it was found that in the meadow steppe grassland nitrogen addition had no significant effect on these parameters （P>0.05）. However， for the typical steppe and desert steppe grasslands， effects of added nitrogen on soil carbon and nitrogen content were significant （P<0.05）， and the 0-10 cm soil layer response was more obvious. 3） The aboveground biomass was significantly correlated with soil C∶N ratio， annual precipitation and nitrogen application （P<0.01）； underground biomass and total biomass were significantly correlated with soil total nitrogen content， organic carbon content， soil C∶N ratio， annual mean temperature and annual average precipitation （P<0.01）. Overall， the biomass and soil carbon and nitrogen contents of the grassland ecosystem in the three types of grassland responded differently to nitrogen fertilization. This means that nutrient addition should be considered as a tool in grassland restoration and management.

Key words: nitrogen addition, grassland ecosystem, biomass, soil organic carbon, soil total nitrogen