Changes in quantity and quality of dissolved organic carbon in purple soil: Roles of land use and soil depth

Dissolved organic carbon (DOC) is the substrate for microbial activities and regulates the biogeochemical cycle of carbon. Considerable areas of croplands have been converted to secondary forest and economic plantation following the implementation of Grain-for-Green Program projects in China, markedly altering the quantity and quality of topsoil DOC. However, the extent and magnitude of these land use changes affecting soil DOC remain largely unclear. This study aimed to elucidate the effects of different land uses and soil depths on topsoil DOC quantity and quality. Soil samples were collected from secondary forest (SF), orchards/plantations (OP), and cropland (CL) at 0-5, 5-10, 10-15, and 15-20 cm soil depths, respectively, from a representative purple soil hillslope, Southwest China. Soil DOC concentration and related UV-visible absorbance characteristics were determined to assesss respectively the quantity and quality of DOC. Results showed that DOC concentration was 44% lower in the OP than in the SF and CL. Higher ratios of UV absorbance value at 254, 260, and 400 nm to DOC concentration (i.e., SUAV(254), SUVA(260), and C:C ratio), electronic conductivity (EC), cation exchange capacity (CEC), soil organic carbon (SOC), and total nitrogen (TN) concentration were found in the OP rather than in the SF and CL. The SUAV(254), SUVA(260), SOC, and TN concentration were the highest at 0-5 cm soil depth. DOC concentration showed negative relationships with most UV absorbance values, SUAV(254), SUVA(260), and C:C ratio. Soil EC, CEC, and pH were closely correlated with the DOC concentration and its UV-visible variables. Our results indicated that DOC was relatively richer in aromatic and humic substances in the OP, providing valuable information for sustainable ecological restoration and land use management.