A coupled Hydro–Biogeochemical framework for evaluating lateral loss of soil organic carbon under land-use change at the basin scale

The 'Grain for Green' (GFG) project is a key ecological restoration initiative in the Loess Plateau. The land use changes induced by GFG project have the potential to alter the spatial distribution of soil organic carbon (SOC), yet its impact on the lateral loss of SOC was not well understood or insufficiently quantified. This study was to develop a comprehensive framework using a coupled hydro-biological model (SWAT-DayCent) together with incorporating an empirical carbon enrichment coefficient for quantitatively assessing soil and SOC losses in a typical watershed in the Chinese Loess Plateau-the Weihe River Basin (WRB). The results revealed that the GFG project reduced cropland area from 58.64 × 103 km2 in 1995 to 54.55 × 103 km2 in 2020, while forest area expanded from 21.13 × 103 km2 in 1995 to 22.14 × 103 km2 in 2020. Grassland area initially declined from 50.20 × 103 km2 in 1995 to 49.62 × 103 km2 in 2000, before increasing to 51.39 × 103 km2 in 2020. Areas with high soil erosion and SOC loss in the basin are predominantly located in its western and southern regions, while low-value areas are mainly concentrated in the northern and central-eastern regions, exhibiting strong spatial heterogeneity. The GFG project significantly reduced the soil erosion and SOC loss in WRB, thereby enhancing regional soil carbon sequestration capacity. Compared to cropland-to-grassland conversion (CTG), the reduction in soil and SOC losses was more pronounced for cropland-to-forest conversion (CTF). Specifically, under the CTF program, soil erosion decreased from 1179.13 t km-2 yr-1 in the baseline period (1995, before GFG) to 15.24 t km-2 yr-1 (2015, about 99% reduction), and 98% reduction was found for SOC (from 10.29 t km-2 yr-1 to 0.22 t km-2 yr-1). For the CTG program, soil loss decreased by 76% (from 965.29 t km-2 yr-1 to 232.95 t km-2 yr-1), and SOC loss decreased by 74% (from 8.68 t km-2 yr-1 to 2.25 t km-2 yr-1). The findings of this study can be valuable for soil conservation and carbon sink management in the Loess Plateau, and the framework we developed can be potentially applicable in other areas.