Significant soil carbon accumulation (about 10 %) under the anthropogenic N deposition has been observed in our 16-year-long field experiment. This project will mainly focus on the physiological processes underlying this response of soil carbon under the chronic N addition. Several hypotheses will be tested to exam which factor or factors mostly contribute to the soil carbon increase under chronic N addition, and whether the foliage litter or the fine root litter is more responsible for this increase.