Abstract: A fertilization experiment was conducted in a 5-year-old Chinese fir (Cunninghamia lanceolata) plantation in Huitong, Hunan Province, China, to study effects of fertilization on soil organic carbon and enzymatic activity. In this experiment, four treatments were: no fertilization as control (CK); 200 kg N·hm^-2·a^-1 (N); 50 kg P·hm^-2·a^-1 (P); and 200 kg N·hm^-2·a^-1and 50 kg P·hm^-2·a^-1 (NP). The results showed that sucrase and catalase activities in 0-10 cm soil layer were decreased in NP treatment by 40.2% and 36.5% compared to the CK treatment. In 0-10 cm soil layer, β-glucosidase activity was decreased by 31.1% in NP treatment than in P treatment. In 10-20 cm soil layer, urease activity was significantly decreased in N treatment compared with the CK. Sucrase activity in 10-20 cm soil layer was significantly decreased by 46.9%, 37.8%, and 42.4% in N, P and NP treatments, respectively. Catalase activity in 10-20 cm soil layer was decreased by 22.6% in NP treatment than in P treatment. The concentration of dissolved  organic carbon in 0-10 cm soil layer was significantly decreased in the N and P treatments by 24.1% and 29.4% compared to the CK treatment, respectively. In addition, soil microbial biomass carbon in 0-10 cm layer was significantly lower in the NP treatment than in the CK treatment. The effects of fertilization on labile organic carbon were mainly found in 0-10 cm soil layer. Labile organic carbon in 0-10 cm soil layer was higher than in 10-20 cm soil layer. Correlation analysis showed that sucrase activity had a significant negative correlation with nitrate nitrogen content, indicating that the increasing nitrate nitrogen content by NP treatment suppressed the activity of sucrase. Dissolved  organic carbon was positively related to urease activity. There were positive relationships between soil microbial biomass carbon and urease, catalase, β-glucosidase activities. Our findings suggest that the release of soil carbon to the atmosphere is probably suppressed by decreasing soil microbial biomass carbon, dissolved  organic carbon content and soil enzyme activities, thus enhancing the potential of carbon sequestration in forest ecosystems.

Key words: maize, physiological characteristics., combined stress, seed germination