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

Abstract Soil organic carbon (SOC) content and its 13C 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 13C 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 δ13C value than the bare land, grassland or forest at the top 10 cm soil depth. Regardless of the land use type, the mean soil 13C values in the subsurface soil layers depleted and δ13C 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 δ13C 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.