Nitrogen (N) mineralization is an important component of the soil N cycle, which is important for supplying the available N in soil to plants in addition to maintaining the plant productivity of terrestrial ecosystems.Especially, when most of grassland ecosystems were confronted with lacking of soil available N, it is more meaningful to focus on the soil N mineralization. In this study, we analyzed 3 different grasslands located along the Northeastern China Transect (NECT); specifically, a typical grassland, a transitional grassland, and a desert grassland. The soils in the 0-20 cm layer of these grasslands were incubated in the laboratory at different temperature (5, 10, 15, 20 and 25 ℃) and moisture levels (30%, 60%, and 90% saturated soil moisture (SSM)) to evaluate differences in soil N mineralization and temperature sensitivity (Q₁₀), and to determine the main factors that regulate this process. The results showed that the total carbon, N, and phosphorus, microbial biomass carbon, and microbial biomass N of soils declined with grassland type, from typical grassland, to transitional grassland, to desert grassland. Furthermore, the total carbon, N, phosphorus, microbial biomass carbon, and microbial biomass N in soils were all significantly different among the three different grassland types (P < 0.001).The different soil properties in different grasslands indicated that the quality of the soil substrate tended to decline from typical grassland, to transitional grassland, to desert grassland. In parallel, net soil N mineralization and soil nitrification rates decreased gradually when the soils were incubated at the same temperature and moisture levels. For example, at an incubation temperature of 20 ℃ and moisture level of 60% SSM, the soil net N mineralization rate of the three grasslands was in the following order: typical grassland (0.715 mg N kg^-1 d^-1) > transitional grassland (0.507 mg N kg^-1 d^-1) > desert grassland (0.134 mg N kg^-1 d^-1). In contrast, the Q₁₀ values increased in the following order: typical grasslandA) decreased in the following order: typical grassland > transitional grassland > desert grassland. The Q₁₀ values appeared to be negatively correlated with the substrate quality index (A). Both grassland type and incubation moisture had a significant effect on the net N mineralization and nitrification rates of the soils, with significant interaction effects (P = 0.017). In contrast, these two factors (grassland type and incubation moisture) had no significant effect on soil ammonification rates. The models, using incubation temperature and moisture as variables, clearly showed the changes in the soil N mineralization rate (P < 0.0001), with R² ranging from 92% to 96%. This models demonstrated that within a certain threshold improving incubation temperature and moisture could promote the rate of soil N mineralization. Soil N mineralization exhibited a regular spatial pattern with the succession of grassland types from typical grassland to desert grassland, and was shown to be closely related to incubation temperature and moisture level. These data provide a new line of evidence explaining the noticeable spatial pattern of Inner Mongolian grasslands from the perspective of soil N availability.