Nitrogen (N) along with carbon is the most complex and crucial element essential for life, and plays a critical role in nutrient cycling. In terrestrial ecosystems, N availability is often a limiting factor that controls primary production and C storage. NH⁺₄-N, NO^-₃-N and microbial biomass N are different active N pools, and participate in short-term biological cycling within soil. N availability of soils is mostly influenced by these active N pools, which provide the information needed to determine the sustainability of land management. NH⁺₄-N and NO^-₃-N are the main soil nitrogen forms that can be directly absorbed and utilized by plants. Microbial biomass plays a critical role in nutrient cycling and is a sensitive indicator of changes in soil quality induced by management practices, and it contains the largest portion of the biologically active N in soil. Soil organic N turns into inorganic nitrogen with the help of soil microorganisms, which have the ability to degrade organic compounds and modify inorganic products, contributing in particular to the mineralization of nutrients. Any changes in the type or amount of organic matter entering the soil could directly affect the structure of the soil microbial community or the functions performed by the various microbial groups in soil. An investigation was carried out to study the differences in soil nitrogen fractions contents, culturable microbial counts and urase activity, and their relationships under natural evergreen broadleaved forest and its artificial regeneration forests of Sassafras tzumu, Cryptomeria fortunei and Metasequoia glyptostroboides in southern Sichuan Province, China. Soil samples were collected from each forest to determine soil nitrogen fractions, microbial counts and enzyme activity. Regression analysis was used to determine the relationship between soil nitrogen fractions, microbial counts and enzyme activity. The results showed that the contents of soil total nitrogen, microbial biomass nitrogen, NH⁺₄-N and NO^-₃-N, the counts of culturable bacteria, fungi and actinomycetes, and the activity of urase of all forests had the same seasonal pattern of autumn ＞ spring ＞ winter ＞ summer, and in all seasons and followed the order of natural evergreen broadleaved forest ＞ Sassafras tzumu plantation ＞ Metasequoia glyptostroboides plantation ＞ Cryptomeria fortunei plantation. Thus, artificial regeneration of natural evergreen broadleaved forest would result in decrease in all soil nitrogen fractions contents, microbial counts and urase activity, as well as the nutrient preserving and supplying capacity, but the size of change varies with regenerated forest types. The soil nitrogen fractions contents were positively and significantly correlated with microbial counts and urase activity, indicating that soil microbial counts and urase activity could be used to evaluate the soil nitrogen fractions contents for natural evergreen broadleaved forest and its artificial regeneration forests. The results are significance in protection of natural evergreen broadleaved forest, tree selection for its artificial regeneration, and soil management, moreover, tree selection when converting farmland to forestland.