The area of forest established through afforestation/reforestation has been increasing on a global scale, which is particularly important as these planted forests attenuate climate change by sequestering carbon. However, the determinants of soil organic carbon (SOC) sequestration and their contribution to the ecosystem carbon sink of planted forests remain uncertain. By using globally distributed data extracted from 154 peer‐reviewed publications and a total of 355 sampling points, we investigated above‐ground biomass carbon (ABC) sequestration and SOC sequestration across three different climatic zones (tropical, warm temperate, and cold temperate) through correlation analysis, regression models, and structural equation modeling (SEM). We found that the proportion of SOC sequestration in the ecosystem C sequestration averaged 14.1% globally, being the highest (27.0%) in the warm temperate and the lowest (10.7%) in the tropical climatic zones. The proportion was mainly affected by latitude. The sink rate of ABC (RABC) in tropical climates (2.48 Mg C ha−1 year−1) and the sink rate of SOC (RSOC) in warm temperate climates (0.96 Mg C ha−1 year−1) were higher than other climatic zones. The main determinants of RSOC were the number of frost‐free days, latitude, mean annual precipitation (MAP), and SOC density (SOCD) at the initial observation; however, these variables depended on the climatic zone. According to the SEM, frost‐free period, mean annual temperature (MAT) and MAP are the dominant driving factors affecting RSOC in Chinese plantations. MAT has a positive effect on RSOC, and global warming may increase RSOC of temperate plantations in China. Our findings highlight the determinants of SOC sequestration and quantitatively reveal the substantial global contribution of SOC sequestration to ecosystem carbon sink provided by planted forests. Our results help managers identify and control key factors to increase carbon sequestration in forest ecosystems.

Context Studies of afforestation have traditionally neglected the influences of plant microhabitats on the growth and carbon sink capacities of planted forests. Aims We investigated the potential mechanisms related to the relationship of afforestation elevation to soil organic carbon density (SOCD). Methods The carbon density of three plantation ecosystems and barren land soils were evaluated at two elevations in the Donglingshan Mountains of Beijing, with structural equation modelling and variation partitioning analyses used to identify the environmental factors that influenced the carbon densities of plantation ecosystems. Key results Afforestation elevation was related to the vegetation phenology of plantation forests. Specifically, growth periods at higher elevations were delayed relative to those at lower elevations, while different growth periods affected growth rate of diameter at breast height (RDBH), in addition to the carbon and nitrogen contents of ground surface litters. Consequently, lower elevation afforestation reduced the carbon sink capacity of coniferous plantation ecosystems in the study area. Lower plantation elevations were associated with significantly reduced RDBH values of Pinus tabuliformis. Further, biomass carbon density (BCD) and SOCD of Larix principis-rupprechtii plantations were significantly lower due to decreased elevations. Soil nitrogen concentrations, litter nitrogen density (LND), soil phosphorus concentrations, and BCD were the primary drivers of plantation SOCD. Conclusions Overall, different plantation elevations were associated with different vegetation phenologies and RDBH values, which further affected LND and BCD, thereby ultimately affecting variation of SOCD. Implications This study provides important insights into the selection of afforestation plots to maximise plantation carbon sequestration capacities.