Abstract: Abstract Aims Soil inorganic nitrogen, including ammonium nitrogen (NH4+-N) and nitrate nitrogen (NO3--N), is an essential nutrient for the growth of terrestrial plants. Grazing affects soil physical and chemical properties and microbial activity through livestock feeding, trampling and manure return, changes soil inorganic nitrogen content, and then affects plant productivity. However, under different grazing intensities, the seasonal dynamics and interannual differences of soil inorganic nitrogen are not clear. Methods This study takes the grassland in the agro-pastoral ecotone of northern Shanxi as the research object, relying on the experimental platform of different grazing intensity established in August 2016 (no grazing UG, light grazing LG:2.35 sheep unit hm-2 growing season, moderate grazing MG:4.8 sheep unit hm-2 growing season, heavy grazing HG:7.85 sheep unit hm-2 growing season) By measuring the seasonal variation of soil inorganic nitrogen content in the growing season from 2017 to 2021 (May-September), the seasonal dynamics and interannual differences of soil inorganic nitrogen under different grazing intensity were studied. Important findings The results showed that: (1) different grazing intensity had no significant effect on the seasonal mean of soil inorganic nitrogen content, but significantly decreased plant biomass, which was related to the change of grazing effect in different periods of growing season. (2) the content of soil inorganic nitrogen increased at first and then decreased in the whole growing season. (3) the content of soil inorganic nitrogen showed significant interannual difference, which was related to the change of interannual precipitation. The results of this study showed that the response of soil inorganic nitrogen content in northern agro-pastoral ecotone to short-term grazing intensity was not significant, and the change of seasonal precipitation was the main reason for the interannual variation of soil inorganic nitrogen content. in the future, grassland management should pay more attention to the effect of precipitation changes on soil inorganic nitrogen.

Key words: Key words Grazing intensity, Ammonium nitrogen, nitrate nitrogen, interannual variation, Plant biomass