Abstract: Alpine meadow is a fragile and vulnerable ecotone in northern Tibet, characterized by high altitude, low temperatures and limited precipitation. Low soil nitrogen availability in these meadows is a key factor limiting the productivity of the grassland. Increasing nitrogen deposition can change C and N cycling dynamics in this ecosystem. The research reported in this paper explored the impact of N addition on soil microbial biomass C and N for four grassland categories in the alpine meadow region of Zoige County: non-degradation grassland (NDG), light degradation grassland (LDG), moderate degradation grassland (MDG), and serious degradation grassland (SDG). In 2015 and 2016, urea was applied to plots during the return of green growth in spring, at rates of nitrogen concentrations 0, 5, 10 and 20 g·m^-2 (denoted CK, N₅, N₁₀ and N₂₀, respectively), in which the nitrogen concentrations of 5, 10 and 20 g·m^-2 were equivalent to urea application amounts of 10.78, 21.55, 43.10 g·m^-2. The results showed that soil microbial biomass C and N were sensitive to N addition but this response diminished with incrfeasing level of grassland degradation. N addition changed the relationship between soil microbial biomass C, microbial biomass N, and also other soil parameters, such as available phosphorus, nitrate nitrogen, total phosphorus, total nitrogen, total organic carbon, pH, and soil moisture content. Correlation of soil microbial biomass C or N with and other environmental factors was not significant, especially for the N₂₀ treatment. Grassland degradation and nitrogen addition can also greatly impact on the ratio of soil microbial biomass C and soil microbial biomass N (SMBC∶SMBN). For the CK treatment, SMBC∶SMBN was significantly higher in serious degradation grassland than in the other three grassland degradation categories. Under the N₅ and N₁₀ treatments, SMBC∶SMBN showed no significant difference between plots of the four degradation categories. However, in non-degradation grassland, the SMBC∶SMBN for the N₂₀ treatment was 33.7% higher than in the CK treatment. In contrast, in serious degradation grassland, SMBC∶SMBN decreased 62.5% for the N₂₀ treatment, compared to the CK treatment. The results indicate that the structure and composition of the soil microbial community is significantly changed following N fertilizer addition to the alpine meadow grassland.

Key words: nitrogen addition, alpine degraded grassland, soil nutrient, soil microbial biomass, soil microbial biomass C/N