Abstract: The objective of this study was to examine the effects of nitrogen (N) application on labile organic carbon and carbon pool management index of the Stipa baicalensis steppe in Inner Mongolia, China. The treatments in the manipulative experiment included N additions at rates of 0 (control), 50, 100, 200 and 300 kg N·ha^-1·yr^-1 (designated N₀, N₅₀, N₁₀₀, N₂₀₀ and N₃₀₀, respectively). It was found that, compared to N₀, the N₅₀, N₁₀₀, N₂₀₀ and N₃₀₀ treatments significantly increased the contents of readily oxidized soil organic carbon and dissolved organic carbon, while decreasing the content of microbial biomass carbon. The N₁₀₀ treatment significantly increased the proportion of readily oxidized organic carbon and the proportion of dissolved organic carbon, and significantly increased the soil carbon activity, carbon pool activity index and carbon pool management index. The N₂₀₀ and N₃₀₀ treatments significantly decreased the proportion of readily oxidized organic carbon, and significantly decreased the soil carbon activity, carbon pool activity index and carbon pool management index. A correlation analysis indicated that there was a significant positive correlation between the proportion of readily oxidized organic carbon, the proportion of dissolved organic carbon, the proportion of microbial biomass carbon, the carbon pool index and the carbon pool management index. Chronic increase in N deposition may reduce the quality of the carbon pool in the S. baicalensis steppe.

Key words: Stipa baicalensis steppe, nitrogen addition, soil, labile organic carbon, carbon pool management index