Abstract: Taking the farmlands under different years (0, 5, 10, and 15 a) of poplar afforestation in semiarid region of Northwest China as test objects, this paper studied the effects of farmland afforestation on the ecosystem carbon (C) stock and its distribution pattern. The results showed that with increasing year of afforestation, the biomass C increased gradually, and most of it was allocated in tree stem. Compared with the control (CK1), the soil organic C content in 0-10, 10-20, 20-40, and 40-60 cm layers after afforested for 5 years was decreased by 7.65%, 36.66%, 51.94%, and 21.4%, while that after afforested for 10 years was increased by 69.63%, 62.46%, 17.07% and 66.94%, respectively. After afforested for 15 years, the soil organic C content in 0-10, 20-40 and 40-60 cm layers was increased by 31.96%, 81.50%, and 58.05%, respectively, but that in 10-20 cm layer was decreased by 13.38%, compared with the control (CK2). The C stock in 0-60 cm soil layer was decreased by 31% after afforested for 5 years, but increased by 47% and 41% after afforested for 10 and 15 years, respectively. The total C stock in 5 year-old poplar plantation was decreased by 10%, but that in 10- and 15-year-old poplar plantations was 3 and 6 times of the farmland. It was indicated that after farmland afforestation with poplar, the ecosystem C stock increased significantly, and most of the increased C was allocated in aboveground biomass C pool. At the early stage of afforestation, soil organic C stock tended to decrease, but thereafter, this stock increased gradually and was even higher than the control.

Key words: Pinus koraiensis, Stem respiration, Growth respiration, Maintenance respiration, Q₁₀