| With rapid development of global economics and population growth, fossil fuel consumption, atmospheric nitrogen deposition increased proportionally. Now, China has become one of the most seriously deposited nitrogen regions, followed by Europe and America. Betula luminifera has important effects on economic, ecological and social values, which were planted a lot for shelter forest in the project of returning land farming to forestry in Sichuan province from 2000a. Therefore, under the background of Nitrogen deposition increased remarkably in this region, studies on soil respiration, soil enzyme activities, litter decomposition, and their response to simulated atmosphere nitrogen deposition in B. luminifera forest were conducted as references to the management and supervision of sub-tropical broad-leaf eco-system.From December 2008 to February 2009, a simulation test was conducted in a B. luminifera forest in Rainy Area of West China. Four treatments were installed, i. e., control (CK,0 g N m-2·a-1), low nitrogen (L,5 g N m-2·a-1), medium nitrogen(M,15 g N m-2·a-1), high nitrogen(H,30 g N m-2·a-1). Twelve plots, each with 3m×3m dimensions were established, surrounded by a 3m wide buffer strip. Ammonium nitrate solution was sprayed monthly as 12 equal applications over the entire year.1. Litter decomposition and its responses to simulated atmosphere nitrogen (N) deposition for B.luminifera forest in Rainy Area of West China were studied from January 2008 to February 2009. Litter bag method was used to quantify the litter decomposition. A total of 540 bags were prepared from mesh (1mm×1mm) poly vinyl screen of approximately 20cm×20cm in dimension. Each bag was filled with 10g, air-dried mass, of litter. These litter bags were evenly and randomly distributed, and three bags were collected from each plot at about 2,4,6,7,10, and 13 months after the start of the study. Results showed that:(1) Rate of litter decomposition exhibited significant repressive effect depending on the increment of N, and this response revealed more significant following the length of exposure of litter to the N treatment. After decomposing for 13 months, the mass remains of CK, L, M, H was 33.6%, 46.5%,49.2%,53.2% respectively. (2) On the whole the decomposition of cellulose exhibited significant repressive effect, while the effect of N deposition on the lignin varied from positive to negative depending on stage of litter decomposition. The lignin mass covers total mass from 33.7% initially to 40.1% after decomposing for 13 months, So we suggest the lignin decomposition maybe the most restricted factor in the late period of litter decomposition. (3) Variation of peroxidase activity which was stimulated before July but was restricted later, exhibited significant relationships with rate of lignin mass loss(P<0.05, n=7), and polyphenol oxidase activity exhibited negative effect to N input. They are main soil enzymes for decomposing lignin and maybe an important biological factors to litter decomposition.2.0-10 cm soil samples were collected after moving litter bag every time, and the activities of soil uresae, invertase, amylase, hydrogen peroxidase, peroxidase, polyphenol oxisdas, cellulose, phosphatase were determined. Results showed that:the activities of invertase, amylase, hydrogen peroxidase, phosphatase and cellulose had apparent seasonal variation, with a peak of in summer. But the activities of uresae, peroxidase, polyphenol oxisdas didn't exhibit significant seasonal variation. After nitrogen deposition, the activities of invertase, amylase and cellulose also showed apparent seasonal tendency, while hydrogen peroxidase and phosphatase had no significant seasonal variation. Nitrogen deposition stimulated the activities of invertase and uresae, and following the increment of nitrogen input, for example, the activities of invertase and uresae of H treatment were lower than those of M. Nitrogen deposition stimulated the activities of invertase, amylase, phosphatase and cellulose in autumn and winter, and this result imply nitrogen deposition might stimulated the vigor of part microbes which are not active in autumn and winter. Therefore, the activities of soil enzyme correlating with organic matter, for instance invertase, amylase, phosphatase and cellulose in autumn and winter.3. The Barometric Process Separation(BaPS) technique and LI-8100 Automated Soil CO2 Flux System were used to quantify the monthly and daily variation of soil respiration respectively. Results showed that:Soil respiration exhibited a clear seasonal pattern, with the highest rate observed in July in every treatment. Soil respiration exhibited significant repressive effect following the increment of N. For instance, the treatment of L, M, H was 27.6%,35.9%,47.7% lower than CK in July. Also daily variation of soil respiration from July to November, exhibited a similar pattern with a single-peak curve. The mean values of daily variation also revealed a negative response to increasing level of N addition. This result might imply the N saturation-response mechanisms of B.luminifera forest in Rainy Area of West China. Temperature was the leading factor of soil respiration variation, could explain 50.1%-79.8% monthly variation of soil respiration; Then, following the N gradient the Q10 values decreased, indicated N deposition may reduces the temperature sensitivity of soil respiration.
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