As more and more nitrogen fractions are discharged into the atmosphere by human activities, the nitrogen content in the ecosystem gradually becomes saturated, and subtropical region has become one of the regions with the most serious nitrogen deposition in the world. Soil organic nitrogen (SON) fractions can directly reflect the response of soil nitrogen availability to nitrogen deposition. The effects of different nitrogen application levels on SON fractions were studied in a natural Castanopsis carlesii forest in the Castanopsis kawakamii Nature Reserve located in Sanming, Fujian Province. The results showed that: (1) SON content under high nitrogen treatment (1.23 g/kg) was significantly higher than that under control treatment (0.89 g/kg). The content of soil labile nitrogen fraction (LPⅠ-N + LPⅡ-N) in high nitrogen treatment (0.28 g/kg) was significantly higher than that in low nitrogen (0.24 g/kg) and control treatments (0.22 g/kg). Soil recalcitrant nitrogen (RP-N) contents in high nitrogen (0.94 g/kg) and low nitrogen (0.82 g/kg) treatments were significantly higher than that in control (0.68 g/kg). (2) Compared with control and low nitrogen treatment, high nitrogen treatment significantly increased the contents of soil total nitrogen (TN), microbial biomass nitrogen (MBN), ammonium nitrogen (NH₄⁺-N) and β-N-acetylglucosaminidase (NAG) activity (p<0.05). The contents of nitrate nitrogen (NO₃^--N), dissolved organic nitrogen (DON) and the activities of β-glucosidase (βG) and cellulolytic enzyme (CBH) showed an increasing trend with the increase of nitrogen application level. (3) Redundancy analysis (RDA) showed that soil TN and NAG enzymes were positively correlated with organic nitrogen and its fractions, while soil MBN and NH₄⁺-N were correlated with SON fractions. Overall, nitrogen deposition in this region improves soil nitrogen availability and microbial activity, which is helpful to further understand the soil nutrient transformation and cycling mechanism in subtropical forests under global climate change.