Land use drives the chemical differentiation of soil dissolved organic matter in a drinking water source watershed.

Land-use change can influence watershed carbon cycling and drinking water quality by altering the quantity and composition of soil dissolved organic matter (DOM). To characterize land-use-related variations in soil DOM, this study investigated the Xinfengjiang Reservoir watershed, a major drinking water source in South China. Surface soils (0-20 cm) were collected from representative land-use types, including forest, grassland, shrubland, and cropland. Ultraviolet-visible absorption spectroscopy and excitation-emission matrix fluorescence spectroscopy, and parallel factor analysis (PARAFAC) were used to characterize soil DOM concentrations, optical properties, fluorescent components, and their environmental controls. Land-use effects were more pronounced for DOM quantity and dissolved C-N composition than for optical and fluorescent characteristics. Grassland soils had relatively high dissolved organic carbon (DOC) concentrations, with DOM characterized by lower aromaticity and smaller apparent molecular size. Cropland DOM tended to exhibit greater humification and a relatively higher contribution of terrestrially derived high-molecular-weight humic-like material, whereas forest DOM showed a tendency toward greater microbial- and protein-like contributions. Soil DOM variation was mainly explained by C-N stoichiometry, nutrient availability, and soil physical properties, with C/N contributing the most to the explained variation. These findings provide new insights into land-use related soil DOM dynamics and their potential implications for water quality in drinking water source watershed.