Quantifying the lateral transport of soil organic carbon induced by soil erosion and sediment yield into rivers under CMIP6

Climate change is reshaping the water and soil carbon cycles, exerting a profound influence on water and food security at the basin scale. However, the estimation of soil organic carbon into rivers (SOCR) under both streamflow-driven and sediment-carrying conditions remains widely controversial. This uncertainty adds to the complexity of predicting water quality changes and carbon balance estimates in basins. To address these issues, this study proposes a new research framework to quantify the lateral transport of soil organic carbon due to soil erosion and sediment yield (SESY) into rivers under the CMIP6 climate model. The framework was applied to the Dongjiang River Basin (DRB), Southern China. Surface runoff and SESY were simulated using the SWAT model for the baseline period (1973-2012) and four shared socioeconomic pathways (SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5) scenarios in the basin. Furthermore, the SOCR was estimated under baseline and future climate scenarios using the sediment yield combined with the soil organic carbon content (SOCC) model, and the correlation between SOCR and meteorological factors was subsequently analyzed. The results revealed that compared with the baseline period, surface runoff and sediment yield in the near future exhibited the largest decreases of 45.48 % and 51.69 % under SSP3-7.0 and the smallest decreases of 21.76 % and 22.04 % in the far future under SSP5-8.5, respectively. Compared to the multi-year average SOCR in the baseline period, the SOCR under the four climate scenarios decreased by 17.64 %, 21.48 %, 26.60 %, and 13.64 %, respectively, indicating that future water quality in the basin will be less affected by SOCR. The main factors affecting SOCR were precipitation and surface runoff, with correlation coefficients exceeding 0.68 and 0.32, respectively. This study provides a scientific basis for water quality management and carbon cycle estimations in the basin. The proposed framework is applicable to the assessment of water environment problems in other basins.