This study was conducted in a deciduous broad-leaved Phoebe forest, a conifer Cunninghamia forest, and a mixed evergreen broadleaf forest in Dujiangyan Region, Sichuan, China. Our objectives were: (1) to compare the pool sizes of different soil N components and their seasonal variations in the three forest types; (2) to explore the relationship between the different N pools in these forests, and (3) to examine how N transformations were linked to vegetation, soil properties, and microorganisms. The results showed that there was obvious seasonal dynamics in soil ammonia concentration in all the three forest types with the maximum values occurring in winter (December). Soil ammonia concentration decreased gradually during the growing season. We observed that the soil nitrate concentration varied greatly with seasons, but it followed different temporal patterns with the soil ammonium concentration. There were significant differences in soil nitrate concentration among the three forest types. Significant differences in soil total nitrogen existed among the three forest types with the highest value in the conifer forest and the lowest in the Phoebe forest. Substantial seasonal variations of soil microbial biomass nitrogen were also observed. Soil temperature and moisture contributed to the temporal and spatial variations of soil microbial biomass N. Our results suggest that N transformations were related closely to soil properties and vegetation types.