Effects of seasonal nitrogen addition on soil net nitrogen mineralization in typical temperate grasslands of Inner Mongolia, northern China

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.