As one of the most active components of forest soil, soil enzymes contribute to the processes of soil organic matter decomposition and synthesis and influence all the biochemical reactions of soil, directly or indirectly. Human activities have dramatically increased the quantity of nitrogen fixed in terrestrial ecosystems, due to fossil fuel combustion, production and use of chemical fertilizers, and live stock ranching. Nitrogen (N) addition may rapidly influence soil carbon and nitrogen turnover during litter and soil organic matter (SOM) decomposition processes by changing the soil enzyme activities. Specifically, we hypothesized that adding N to N-limited ecosystems would increase the activity of hydrolytic enzymes and decrease that of oxidase. In the present study, we explored the effects of different forms and levels of nitrogen addition on extracellular enzyme activities in a temperate forest soil with dominant Quercus liaotungensis (light loam, mean annual temperature 11.7 ℃), in the Xi Mountain Forestry Station, China. Nitrogen loading was designed using three N forms (NO₃^--N, NH₄⁺-N, and NH₄NO₃), each containing a low-N (50 kg N hm^-2 a^-1, N50) and high-N (150 kg N hm^-2 a^-1, N150) treatment plot; a treatment plot of 0 kg N hm^-2 a^-1 (N0) served as a control. Each treatment comprised three replicate plots of 10 m × 10 m on the forest floor, with each plot having similar vegetation and biological soil crust cover. Soil urease, acid phosphatase, alkaline phosphatase, β-glycosidase, polyphenol oxidase, and catalase activities were analyzed to investigate the impacts of N forms and levels on soil enzyme activities from the temporal pattern. The results showed a significant increase of NH₄NO₃-N treatment in soil urease activity (+24.20% upper NO₃^--N, N50), and NH₄⁺-N treatment in acid phosphatase activity (+13.82% upper NO₃^--N, N150), and a considerable increase of N50 level in soil urease activity (+38.90% and +24.20% upper NH₄⁺-N and NH₄NO₃ in N0, respectively) (P < 0.05). However, N forms and levels did not affect the activities of alkaline phosphatase, β-glycosidase, polyphenol oxidase, and catalase. Increasing N (N50 and N150) resulted in higher alkaline phosphatase activity (+20.2% and +11.5% upper N0, respectively) and polyphenol oxidase activity (+64.3% and +41.8% upper N0, respectively); and, with the increase in N addition, both the alkaline phosphatase and polyphenol oxidase activities increased at N50 addition, and decreased at N150. We observed a significant increase in β-glycosidase activity (P < 0.05) with the different forms of N addition (NH₄⁺-N > NH₄NO₃-N > NO₃^--N), whereas N forms had no effect on alkaline phosphatase, polyphenol oxidase, and catalase activities. Furthermore, the highest activities of soil enzymes including urease, acid phosphatase, alkaline phosphatase, and polyphenol oxidase occurred in summer, but the highest β-glycosidase activity and the lowest catalase activity were found in the winter and autumn, respectively. However, N addition made little difference on temporal variation. Significant correlations of soil enzyme activities with soil microbial biomass carbon, NH₄⁺-N, and NO₃^--N were established (P < 0.05). These results suggested that N deposition could affect the soil carbon and nutrient flow by influencing environmental factors and microbial enzymatic activities in temperate forest ecosystems.