Distinct Spatial Distributions and Drivers of Soil Black Carbon and Dissolved Black Carbon in the Xinfengjiang Watershed, South China

Increasing evidence shows that the aging of black carbon (BC) releases dissolved black carbon (DBC) and influences carbon cycling and environmental quality. However, the spatial distributions of BC and DBC in soils and their driving factors at the watershed scale remain poorly understood. Here, we investigated the distribution and environmental determinants of BC and DBC in soils in the Xinfengjiang Watershed in South China. Results revealed decoupling of BC and DBC spatial patterns within the watershed. The BC content was significantly higher in grasslands than in shrublands, while the DBC content did not differ significantly across land-use types. Cropland soils had the highest average DBC-to-dissolved organic carbon, DBC-to-BC, and DBC-to-soil organic carbon ratios and the lowest benzene hexacarboxylic-to-pentacarboxylic acid ratios for both BC and DBC among land-use types. It indicates higher oxidative degradation and lower aromatic condensation of BC in cropland soils. Total nitrogen and dissolved total nitrogen were the strongest positive factors influencing the BC content, while bulk density was a major negative factor. For the DBC content, the absorbance coefficient a₂₅₄ of soil leachate was the primary predictor. These findings provide insights into watershed BC and DBC dynamics and implications for environmental management.