Abstract: We measured soil moisture, soil organic carbon fractions and carbon storage within 1 m depth in Picea crassifolia plantations (with stand ages of 13, 35, 80, and 100 years, denoted as 13P, 35P, 80P, and 100P respectively) and high-density Larix principis-rupprechtii plantation (with a stand age of 39 years, denoted as 39L) with similar site conditions in the northeastern margin of the Qinghai-Tibet Plateau. The aim of this study was to provide scientific basis for assessing carbon sequestration and ecological benefits of typical plantations. The results showed that the average soil moisture in the 0-80 cm layer of Picea crassifolia plantations showed the following order: 13P (30.99%) < 35P (33.85%) < 80P (38.61%) < 100P (41.74%). Soil moisture at 60-100 cm depth under the 39L was significantly lower than that in 35P (P<0.05), indicating that water depletion occurred in deep soil layers. With increasing stand ages, the contents of soil organic carbon (SOC) and particulate organic carbon (POC) in Picea crassifolia plantation increased significantly (P<0.05), mineral-associated organic carbon (MAOC) showed no significant changes, and the SOC stability decreased. The SOC stability of topsoil in 39L was higher than that of 35P, but the active organic carbon content was significantly lower than that of 35P. Correlation analysis revealed different driving mechanisms for SOC accumulation and stability after afforestation at the surface versus deep soil layers. Afforestation with Picea crassifolia had a high carbon sequestration capacity in biomass, soils, and litter layer, which can play a role of carbon sink for a relatively long time. After 39 years of afforestation with high-density plantations of Larix principis-rupprechtii, biomass carbon storage exceeded that of 100-year-old Picea crassifolia plantations (100P). It is recommended to adopt thinning and tending to achieve sustainable carbon sink function.

Key words: plantation forest, soil moisture content, soil organic carbon, carbon storage, organic carbon stability