Profile distribution of soil organic carbon and its isotopic value following long term land-use changes

Soil organic carbon (SOC) content and its C-13 natural abundance along soil profile are sensitive to the changes of land use since these changes alter external carbon (C) inputs and belowground C turnover processes. A study was conducted to investigate how the conversion of cropland to forest, grassland or bare land (no vegetation) affect the distribution of SOC and its C-13 values in 200-cm soil profiles. After 28 years of land conversion, the cropland that was remained as cropland (treated as control) had a significantly higher delta C-13 value than the bare land, grassland or forest at the top 10 cm soil depth. Regardless of the land use type, the mean soil C-13 values in the subsurface soil layers depleted and delta C-13 signature decreased by 1% 5% compared with the top 10 cm soil layer, while the corresponded SOC content decreased by 3.5% to 91.2% as soil depth went down deeper. The SOC stock in the 0-200 cm profile was 16% and 12% higher in the grassland and forest, respectively, than in the control. The conservation of cropland to grassland and forest largely increased the SOC stock in the 20-40 cm soil layer compared with the control. The newly formed SOC was higher in the grassland than in the forest, and accounted for 15.4% and 9.7% of the SOC stock in the soil depth of 0-60 cm in the grassland and forest, respectively. Soil delta C-13 values and SOC contents were negatively correlated with pH and clay content, while they were positively correlated with nitrogen and C/N ratio along the soil profile. We infer that the conversion of cropland to forest or grassland is a beneficial practice for SOC storage, especially in the 0-60 cm depth. Such conversion has little impact on the SOC turnovers in deep soil layers during the study period.

Automated summary

Soil organic carbon content and C-13 natural abundance along soil profile are sensitive to land use changes, with conversion of cropland to forest or grassland beneficial for increasing SOC storage, especially in the 0-60 cm depth. The turnover of SOC in deep soil layers has little impact during the study period.