Abstract:Desert steppe belongs to the arid type of grassland, with low nutrient content, fragile ecological environment and sensitive response to disturbance. The gradual increase of global nitrogen deposition changed the process of carbon and nitrogen transformation in the ecosystem and affected soil properties. In order to explore the effects of nitrogen addition on biological properties of soil in desert steppe, we took the desert steppe in Yanchi County as the research object and set up 5 nitrogen treatments as N0, N1, N2, N3 and N4.These treatments were set based on the pure nitrogenaddition levels of 0, 2.5, 5, 10 and 15 g/(m2·a), by applying with urea as nitrogen source. Results showed that:(1) The NH4+-N content in 0-20 cm soil layer of desert steppe increased with the increase of nitrogen application rate, while the NO3--N content increased firstly and then decreased. In 0-10 cm soil layer, compared with N0, the NH4+-N contents under N1, N2, N3 and N4 treatments were increased by 109.61%, 136.52%, 197.19% and 198.88%, respectively. The NO3--N content was the highest in N2 treatment, which was significantly higher than those in N0, N3 and N4 treatments (P<0.05). (2) Compared with N0, the MBN contents in 0-10 cm soil layer were significantly decreased by N1, N2, N3 and N4 treatments, which were reduced by 37.54%, 38.11%, 28.56% and 29.81%, respectively. (3) With the increase of nitrogen addition, the number of fungi in 0-10 cm soil layer of desert steppe gradually decreased, among treatments, the N0 and N1 were significantly higher than N3 and N4 (P<0.05). PCoA analysis showed that the microbial community structure in amoA and nirK zones of desert steppe soil were significantly changed by high nitrogen addition. Based on this research, the high nitrogen addition (N3, N4) had negative effects on soil microbial community in desert steppe, which were as follows:the number of fungi decreased, NO3--N content decreased, α diversity and dominant bacteria abundance of nitrogen-transforming microorganisms decreased, and the microbial community structure of nitrogen-transforming changed significantly.