Erosion-induced recovery CO2 sink offset the horizontal soil organic carbon removal at the basin scale

To improve soil carbon sequestration capacity, the full soil carbon cycle process needs to be understood and quantified. It is essential to evaluate whether water erosion acts as a net source or sink of atmospheric CO₂ at the basin scale, which encompasses the entire hydrological process. This study introduced an approach that combined a spatially distributed sediment delivery model and biogeochemical model to estimate the lateral and vertical carbon fluxes by water erosion at the basin scale. Applying this coupling model to the Dongting Lake Basin, the results showed that the annual average amount of soil erosion during 1980–2020 was 1.33×10⁸ t, displaying a decreasing trend followed by a slight increase. Only 12% of the soil organic carbon displacement was ultimately lost in the riverine systems, and the rest was deposited downhill within the basin. The average lateral soil organic carbon loss induced by erosion was 8.86×10¹¹ g C in 1980 and 1.50×10¹¹ g C in 2020, with a decline rate of 83%. A net land sink for atmospheric CO₂ of 5.54×10¹¹ g C a⁻¹ occurred during erosion, primarily through sediment burial and dynamic replacement. However, ecological restoration projects and tillage practice policies are still significant in reducing erosion, which could improve the capacity of the carbon sink for recovery beyond the rate of horizontal carbon removal. Moreover, our model enables the spatial explicit simulation of erosion-induced carbon fluxes using cost-effective and easily accessible input data across large spatial scales and long timeframes. Consequently, it offers a valuable tool for predicting the interactions between carbon dynamics, land use changes, and future climate.