Grassland desertification is one of the most serious environmental problems in China. However, many studies examining changes in nitrogen characteristics during grassland desertification have focused on arid or semi-arid regions, and few studies have considered semi-humid regions. In this study, the soil nitrogen content during the process of desertification in a northwest Sichuan alpine area, a semi-humid region, was analyzed. Desertification had a significant impact on soil nitrogen in the 0-100 cm soil layer, and total nitrogen, available nitrogen, NH₄⁺-N, NO₃^--N, and microbial biomass nitrogen (MBN) decrased significantly during the process of grassland desertification by 73.95%, 77.72%, 76.75%, 79.77%, and 84.12%, respectively). Total nitrogen, available nitrogen, NH₄⁺-N, NO₃^--N, and MBN decrased most strongly in the 0-20 cm soil layer among all layers within 0-100 cm by 86.43%, 83.52%, 82.11%, 88.82%, and 91.77%, respectively. As the soil depth increased, both the content and degree of change during grassland desertification gradually decreased. The extent of the decrase in soil nitrogen content varied during the progression of grassland desertification. During the early degradation stage, soil nitrogen decreased the most; The total nitrogen, available nitrogen, NH₄⁺-N, NO₃^--N and MBN decrased 41.18%, 35.17%, 46.74%, 43.46%, and 46.88%, respectively. Based on these results, the changes in soil nitrogen observed as the degree of desertification increased in semi-humid regions were similar to those observed in previous studies for arid and semi-arid regions. Plants are important for the sequestration of soil nitrogen, not only because they act as nitrogen sources, but also because they protect soil containing nitrogen from blowing away. A correlation analysis showed that total nitrogen, available nitrogen, NH₄⁺-N, NO₃^--N and MBN were all significantly positively correlated with community coverage (P < 0.01). In addition, total nitrogen, available nitrogen, NH₄⁺-N, NO₃^--N and MBN were all significantly positively correlated with silt and clay, which have high nitrogen contents (P < 0.01). In contrast, coarse sand lacking nitrogen was significantly negatively correlated with total nitrogen, available nitrogen, NH₄⁺-N, NO₃^--N and MBN (P < 0.01). Based on these results, we inferred that soil nitrogen loss was caused by a gradual decrease in community coverage during grassland desertification, and the loss of silt and clay via wind erosion. Consequently, efficient measures should be taken to protect silt and clay from wind erosion, and to improve community coverage during the desertification of grassland. In addition, potential grassland desertification, particularly during the early stage, should be managed using ecological methods to avoid the negative effects of increasing grassland desertification.