137Cs and 210Pbex tracing of soil erosion and organic carbon dynamics in relation to the Grain-for-Green project in the Hilly area of the Central Sichuan Basin

The Grain-for-Green project (GFGP) is one of the world’s largest ecological restoration projects and has profoundly decreased soil erosion and increased soil organic carbon (SOC) accumulation. No clear conclusion regarding changes in soil erosion and the impacts of soil erosion on carbon dynamics in the Sichuan Basin due to the GFGP has been reached. Four sample sites, reference land (RL), cultivated land (CL), and land where the GFGP was implemented in 1999 (GFGP99) and 1972 (GFGP72), were selected to investigate the impacts of the GFGP on soil erosion and SOC dynamics via combined application of ¹³⁷Cs and ²¹⁰Pb_ex in the Sichuan Basin, Southwest China. The different depth distributions and landscape positions of ¹³⁷Cs, ²¹⁰Pb_ex and SOC in CL, GFGP99 and GFGP72 indicate that the GFGP significantly reduced soil erosion and increased the SOC concentration. The eroded soil thickness decreased by 325 % and 255 % in GFGP99 and GFGP72, respectively, relative to CL. The soil redistribution rates estimated by the revised ²¹⁰Pb_ex mass balance model ranged from 65.33 to −25.18 Mg ha⁻¹ yr⁻¹ and from 18.83 to −138.74 Mg ha⁻¹ yr⁻¹ before and after GFGP implementation, respectively. The SOC densities of GFGP99 and GFGP72 were 14.83 % and 36.84 % greater than those of CL, respectively. Notably, relatively low correlations were found among the ¹³⁷Cs, ²¹⁰Pb_ex and SOC densities, which is possibly related to the coupled effects of SOC transport and input during soil erosion. More attention should be given to the limitations of estimating soil redistribution rates and SOC dynamics using fallout radionuclides, such as ¹³⁷Cs and ²¹⁰Pb_ex, on slopes following reforestation due to land-use changes.