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.

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Land use change and slope position are commonly identified as the key factors affecting the soil organic carbon (C), total nitrogen (N), and total phosphorus (P) traits in distinct ecological scales. However, the directions of these effects are still unclear in some fragile terrestrial ecosystems. This study aimed to determine the characteristics of soil C, N, and P concentrations and stoichiometry as affected by different land uses and slope positions in a representative purple soil hillslope in Three Gorges Reservoir Area (TGRA), China, which is experiencing severe soil erosion and non-point source pollution. A total of 108 soil samples were collected from secondary forest, orchard plantation, and cropland on the upper, middle, and lower slopes, respectively. Soil C, N, and P concentrations and their stoichiometric ratios were determined. The results showed that soil C concentration was not affected by land use, while soil N and P concentrations were both the highest in orchard plantation rather than in secondary forest and cropland, resulting in the lowest C:N, C:P, and N:P ratios in the orchard plantation. Soil C and N concentrations synchronously decreased from upper slope to the lower slope, and soil P concentration was not markedly different among slope positions. This also caused the insignificant difference in soil C:N ratio and the remarkably lowest C:P and N:P ratios on the lower slope. There were significant interactive effects of land use and slope position on the study soil variables except soil P concentration. Our results highlight the effects of land use and slope position on soil C, N, and P traits and point to the decoupling of linkages between soil P and soil C as well as N due to the severe soil erosion and overuse of fertilization in the TGRA.

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