Abstract: [Objective] To provide scientific basis for evaluating the soil carbon stock and its management in afforested soils, we investigated the changes in soil organic carbon and its labile fractions after afforestation by the grain for green program.[Method] Cryptomeria fortumei plantation respectively 4, 8, and 12 years after planting by the grain for green Program and a farmland without trees as the control were chosen in Xuanwei Municipality, Yunnan Province. Soil, litter and fine roots were collected at different soil layers (0-20,20-40 and 40-60 cm) from the selected plantations and the control plot . Soil organic carbon content and its labile fractions, soil densities, soil nitrogen, as well as litter and fine root biomass were analyzed. [Result] Compared with the farmland, the content of soil organic carbon in the 0-60 cm layer deceased by 20.07%, 19.29%, and 11.52%, respectively for 4, 8 and 12 years of age, indicating that the soil organic carbon content significantly decreased in the early 4 years and then gradually increased thereafter, but not increased to the level in the farmland at the 12th year after planting. For the highest labile fraction of the soil organic carbon the maximum value was 4.46 g·kg^-1 before planting, and the minimum was 2.67 g·kg^-1 at the 4th year after planting; for the second highest labile fraction, the maximum was 12.03 g·kg^-1 at the 8th year after planting and the minimum was 4.61 g·kg^-1 at the 4th year after planting; for the least labile fraction the maximum was 20.94 g·kg^-1 at the 8th year after planting and the minimum was 9.12 g·kg^-1 at the 12th year. The contents of both soil organic carbon and its labile components decreased along with the depth of soil layer, and there were significant differences (P<0.05) among different soil layers; the minimum content (11.14 g·kg^-1) of soil organic carbon was found in the soil layer of 40-60 cm at the 8th year. The correlation coefficients of the soil organic carbon, the highest labile fraction, and the second highest labile fraction with the total soil nitrogen were 0.894, 0.756 and 0.755, respectively, which were all significantly positive. The coefficient of soil organic carbon with soil density was -0.664, indicating a significantly negative correlation.[Conclusion] The soil organic carbon and its labile fractions in the 0-60 cm soil layer showed a decrease in early years and an increase in later years in Cryptomeria fortumei plantations. The age and the litter content were important factors affecting the changes. We suggest reduce forest logging and retain the forest litter as far as possible in order to have a good management of forest carbon sink.

Key words: Grain for Green Program, Cryptomeria fortumei plantation, soil organic carbon, lability