Effects of vegetation restoration on deep soil carbon sequestration and its controlling mechanisms on the Loess...

The Loess Plateau, the largest loess deposit region globally, plays a pivotal role in regulating regional and global carbon cycles through variations in its soil carbon pool. In recent decades, large-scale vegetation restoration programs have been widely implemented across the Loess Plateau, and numerous studies have investigated their effects on soil carbon dynamics. However, how vegetation restoration affects the accumulation of deep soil carbon remains unclear. In this study, the Caijiachuan watershed in Jixian County, Shanxi Province, was selected as the study area. Using a space-for-time substitution approach, we examined the effects of different vegetation restoration types including grassland, Pinus tabulaeformis, Platycladus orientalis, Robinia pseudoacacia, and mixed forests of Platycladus orientalis and Robinia pseudoacacia on the 0-300 cm soil carbon pool, with cropland serving as the control. Compared with cropland, vegetation restoration increased soil organic carbon (SOC) content and stock in the shallow (0-100 cm) and deep (100-300 cm) soil layer, with mixed forests exhibiting the greatest SOC sequestration potential. The increase in deep SOC was associated with enhanced dissolved organic carbon (DOC), suggesting that vertical transport of DOC may have contributed to subsoil carbon accumulation. Vegetation restoration increased the content and stock of soil inorganic carbon (SIC) in deep soil layers, while the trend in shallow soils was not consistent. Among the vegetation types, grassland showed the highest potential for inorganic carbon sequestration. In addition, vegetation restoration significantly increased CO₂ concentration (P < 0.05) and decreased pH in shallow soils, which may have facilitated the dissolution of lithogenic carbonate. The dissolved products may have subsequently migrated downward with percolating water and reprecipitated in deeper soil layers, potentially increasing the storage of pedogenic carbonate in the 100-300 cm profile. This study is expected to provide important theoretical support for enhancing soil carbon sequestration through vegetation restoration on the Loess Plateau, particularly for understanding the formation and transformation of deep SIC.