Objective This study aimed to explore the effects of seasonal nitrogen (N) addition on soil inorganic N pool and N mineralization rates in temperate steppe grasslands. It focused on revealing monthly dynamics, interannual variations, and environmental driving mechanisms to support understanding soil N cycling and its relationship with atmospheric N deposition.

Method A field experiment was established with N addition applied in different seasons: autumn, winter, and the growing season. Inorganic N content and N mineralization rates were measured by top-cover PVC cylinders method to explore the effects of seasonal N addition on net N mineralization potential.

Result Compared with control, N addition significantly increased inorganic N pools but declined over time. Compared with N addition in autumn and winter, N addition in growing season significantly increased inorganic N by 226.5%. The response of ammonium N, nitrate N and inorganic N to N addition in different seasons showed obvious seasonal pattern. Meanwhile, N addition in different seasons significantly increased the rate of N mineralization. At the end of growing season, net ammonification rate showed a trend of decreasing first and then increasing after N addition in growing season. N addition in growing season significantly increased net nitrification rate and net N mineralization rate at the end of growing season. N mineralization rate showed significantly monthly and interannual changes to N addition in different seasons. The response of mineralization rate to N addition in different seasons was mainly influenced by environmental factors. It was found that temperature had a negative effect on nitrification rate and net N mineralization rate, while precipitation had a positive effect. N addition in different seasons was main factor causing the difference of N mineralization rate.

Conclusion Seasonal N addition significantly alter soil inorganic N pools and mineralization rates in temperate grasslands, showing notable seasonal and interannual patterns with experimental time. In conclusion, study of the effects of different seasonal N addition on soil N mineralization provides theoretical support for in-depth understanding of response rule and mechanism of soil N conversion process to atmospheric N deposition. It also helps to comprehensively evaluate the relationship between seasonal effects of atmospheric N deposition and soil N cycle.