Soil nitrogen (N) mineralization is a key process in the cycling of N and is sensitive to ongoing human activities and climate changes. However, little is known about how grazing and N inputs separately and/or jointly affect the soil net N mineralization of grassland ecosystems across large temporal scales in the context of climate change. Here, we conducted a 4-year in situ measurement campaign of soil net N mineralization within a longterm grazing and N addition treatments in a semi-arid grassland. We found an alternating seasonal pattern of soil N transformation between mineralization and immobilization. Relative to normal year, peak N mineralization was advanced one month in the wet year, but peak N immobilization was delayed one month in the dry year. Precipitation-induced changes in soil temperature and moisture determined the seasonal dynamics of N mineralization in wet and dry years. The effects of grazing and N addition on soil N mineralization were temporally dependent. At the seasonal scale, N addition significantly accelerated soil N mineralization and immobilization in the peak growing season and the period after the peak growing season, respectively. Dry climate weakened the response of peak soil N transformation to N addition. Grazing broadly had no effect on soil N transformation during most of the experimental period, except for some events during the wet years. At the interannual scale, grazing, N addition and their interactions had no influences on cumulative net N mineralization. Further analysis showed that the interannual variation in soil N mineralization was driven by the previous year's annual precipitation rather than the current year. Our results suggest that grazing and N addition could stabilize the soil N cycling in semi-arid grasslands over long-term time scales. Moreover, they emphasize the importance of precipitation legacy effects on soil N cycling, and thus for ecosystem productivity.