Afforestation changes soil organic carbon stocks on sloping land: The role of previous land cover and tree type

Afforestation contributes to soil organic carbon (SOC) sequestration and the restoration of degraded soil. However, the changes in SOC stock following afforestation on sloping land are poorly understood. Here we conduct a quantitative review that synthesizes 239 observations from 31 studies to evaluate the effects of slope gradients on the changes in SOC stock in three soil layers (0-20 cm, 20-40 cm and 40-60 cm) following afforestation, considering previous land cover (cropland and grassland) and tree type (broadleaf and conifer). In the topsoil layer, slope gradient has a significant effect on the changes in SOC stock: slope gradient positively relates to SOC stock change and significantly predicts the SOC stock change after controlling for temperature, precipitation, stand age, and initial SOC stock. Soil gradient explains 12.5% of the variance. We find a significant decrease in SOC stock after the afforestation of gentle slopes with a gradient lower than 10% but a significant increase in SOC stock on slopes with a gradient higher than 30%. Afforestation on sloping cropland results in an increase in SOC stock, especially in the upper soil, while afforestation on grassland results in an increase in SOC stocks only on very steep slopes (gradient >60%). Tree type has a limited effect on changes in SOC after afforestation on sloping land. However, conifers show a greater SOC sequestration potential in deeper soils and on steep slopes with a gradient higher than 60%, while broadleaf trees sequester more SOC in topsoil and on slopes with a gradient of 30-60%. The present study suggests that to increase SOC sequestration, priority should be given to afforestation with conifers on steep cropland.