Abstract: Saline-alkali soil is a vital component of terrestrial ecosystems in arid regions of northwest China, and nitrogen application is the most important approach to enhance crop yield in these areas. To investigate the effects of nitrogen addition on saline-alkali soil ecosystems in arid regions, this study examined the changes in soil physicochemical properties, microbial diversity and community structure, and soil enzyme activities under nitrogen addition （0, 0.2, 0.6, and 1 g/kg NH₄NO₃） in loess soils with varying degrees of salinity-alkalinity （light, moderate, and severe） in Longzhong region. The results showed that nitrogen addition reduced the soil pH from 8.28~8.46 to 7.71~8.21, while the electrical conductivity exhibited an increasing trend. The α-diversity indices of both fungi and bacteria significantly decreased （p≤0.05）, and β-diversity analysis revealed that nitrogen addition amplified the differences among microbial communities. Specifically, the relative abundances of Bacteroidota and Basidiomycota in severely saline-alkali soils decreased by 59.27% and 16.32%, respectively. The abundances of Nocardioides and Fusarium, which are involved in nitrogen metabolism, also declined markedly. The activities of soil urease （URE）, N-acetylglucosaminidase （NAG）, and β-glucosidase （β-GC） were reduced. These findings indicate that nitrogen addition significantly alters the microbial diversity and community structure of saline-alkali loess, with high-concentration nitrogen application （0.6 g/kg） inhibiting microbial growth and reducing soil enzyme activities. This study provides a scientific basis for evaluating and predicting the changes in saline-alkali loess micro-ecosystems under nitrogen deposition scenarios.