Abstract: Fertilizer application is a common grassland management technique and plays an important role in the maintenance of grassland productivity and the health of its ecosystems. In this study, five kinds of fertilizer treatment were applied to determine their effects on soil nitrogen forms and transformations in the Leymus chinensis grasslands of Guyuan, Hebei Province. The results showed that, during the whole forage-growing season, the soil nitrate and ammonium nitrogen contents in grassland fertilized with both nitrogen and phosphorus were lower than that in grassland fertilized only with nitrogen. The application of nitrogen fertilizer changed the process of soil nitrogen mineralization. Grasslands with nitrogen fertilization showed stronger soil nitrification than the other treatments during the period from May to August, and higher net nitrogen mineralization during the period from May to July, which accounted for 57.58%-68.97% of total net nitrogen mineralization over the whole forage-growing season. Positive correlations were observed between soil ammonium nitrogen content, soil nitrate nitrogen content, net nitrogen nitrification, net nitrogen mineralization, soil temperature and soil moisture (r=0.491, P<0.01). High levels of nitrogen fertilizer application could increase the loss of nitrate nitrogen by leaching and thus the risk of environmental pollution.