Human activities had dramatically increased the quantity of atmospheric nitrogen deposition since the industrial revolution. Increased nitrogen deposition in forest ecosystems would not only have a direct impact on tree growth, but also indirect effects of soil carbon storage influencing the microbial degradation of soil organic matter. Soil enzymes produced by microbes play a key role in the biochemical functioning of soil organic matter decomposition and nutrient cycling. Soil enzyme activity is one of the indicators of soil microbial community activity. In recent years, numerous studies have shown that increasing atmospheric nitrogen deposition had significant effects on soil enzyme activities. However, soil enzyme activities in different ecosystems always showed different responses to increasing nitrogen deposition. Thus, it was necessary to assess the impact of nitrogen deposition on forest soil enzyme activities in two different ecosystems. In this study, four treatments were installed in a plantation and a natural forest of Pinus tabulaeformis at the Taiyue Mountain of Shanxi Province China, starting in August 2009 and including four treatments: a) control (CK, 0 kg N hm^-2 a^-1), b) low nitrogen (LN, 50 kg N hm^-2 a^-1), c) medium nitrogen (MN, 100 kg N hm^-2 a^-1), and d) high nitrogen (HN, 150 kg N hm^-2 a^-1). Each treatment comprised three replicate plots of 2m×2m which were sprayed with CO(NH₂)₂ solutions at the beginning of each month. Soil samples were collected at a depth of 0-20 cm in May, July and September since 2012, and the activities of invertase, cellulase, peroxidase, polyphenol oxidase, urease and neutral phosphatase were determined. Soil invertase and cellulase were assessed using the 3,5-dinitrosalicylic acid colorimetric method, peroxidase and polyphenol oxidase by pyrogallol colorimetric method, urease by phenol-sodium hypochlorite colorimetric method and neutral phosphatase by di-sodium phenyl phosphate colorimetric method. Urease and neutral phosphatase activities increased with increasing nitrogen levels; polyphenol oxidase activity of the natural forest and invertase activity of the plantation decreased significantly in the low nitrogen treatment (P < 0.05); medium and high nitrogen treatments significantly inhibited peroxidase and polyphenol oxidase in both ecosystems, and cellulase in the natural forest and invertase in the plantation (P<0.05). In conclusion, simulated N deposition inhibited the activities of soil peroxidase and polyphenol oxidase, and stimulated urease and neutral phosphatase activities. Cellulase activity in the natural forest and invertase activity in the plantation were reduced by simulated N deposition, yet invertase activity in the natural forest and cellulase activity in the plantation had no significant variation. The polyphenol oxidase activity which plays a key role in the degradation of lignin significantly associated with activities of cellulase and invertase (P < 0.05) and the inhibition of lignin degradation would decrease soil dissolved organic matter. Therefore the inhibition of cellulase and invertase may be caused by the reduction of the carbon source which can be utilized by soil microbes. Furthermore, polyphenol oxidase activity was more sensitive in the natural forest than in the plantation because the natural forest had a higher soil nitrogen content. Because the enzymes which were inhibited were related to degradation of soil organic matter, the inhibition of soil activities under simulated nitrogen deposition could slow down the degradation of soil organic matter, leading to the increase of soil organic matter.