Abstract: Aims Soil organic carbon (C) pool plays an important role in stabilizing terrestrial C cycle and regulating global climate, etc. The purpose of this study is to provide data support for scientifically predicting the C sink function of the grasslands in semi-arid region under global change. Methods Based on a two-factor field experiment of precipitation change (–50%, –30%, natural, +30%, +50%) and nitrogen (N) addition (0 and 5 g·m–2·a–1) established in 2017 in a desert steppe in Ningxia, the response patterns and influencing factors of organic C characteristics (content, storage, and components) in 0–60 cm soils after 4 years of treatments. Important findings N addition had little effects on soil organic C characteristics. In contrast, precipitation changed soil organic C characteristics and the extent of the effect depended on N level and soil depth. Across the whole 0–60 cm, compared with the natural precipitation, both increasing and decreasing precipitation had less effects on the organic C characteristics under 0 g·m–2·a–1 N addition. In contrast, the 30% decrease in precipitation significantly increased easily oxidized organic C and dissolved organic C content and the 30% increase in precipitation significantly increased particulate organic C and light fraction organic C content under 5 g·m–2·a–1 N addition. Organic C and its storage were positively correlated with soil water content, cellobiohydrolase activity and alkaline phosphatase activity, while they were negatively correlated with soil NO3–-N. Easily oxidized organic C, particulate organic C and light fraction organic C content were positively correlated with soil leucine aminopeptidase activity and Simpson dominance index, while they were negatively correlated with microbial biomass N content. Dissolved organic C and microbial biomass C content showed the opposite relationship with the three indices above. These results show that, changing precipitation have little effects on soil organic C and its storage under N addition, but moderately increasing and decreasing precipitation will reduce organic C stability by changing soil water content, N availability, enzyme activity and plant community dominance, further leading to an increasing risk for soil C emission in desert steppes.

Key words: semi-arid region, changing precipitation pattern, increasing atmospheric nitrogen deposition, soil carbon emission, soil carbon stability