Land use changes can affect soil carbon storage in terrestrial ecosystems by altering either the biotic or the abiotic processes involved in carbon cycling, such as the adsorption of carbon on soil minerals. In this study, the organic carbon contents and their stable carbon isotopic compositions (delta C-13) of five soil profiles under different land uses in the Maolan karst region in Southwest China were analyzed. Four non-forest samples from the karst were selected and examined to determine whether farmlands or grasslands that were converted from forestland were more effective in sequestering carbon in soils. The soil organic carbon (SOC) contents decreased in the following order: virgin forest, burnt grassland, shrubland, grassland, and farmland. However, no differences were observed in the delta C-13 values at the bottom of the five profiles, which indicated that most of the soil organic carbon was derived from the original virgin forest. The percentages of SOC that resulted from C-4 plants in the virgin forest were significantly lower than the percentages in the other plots, while the percentages of SOC that resulted from C-3 plants in the virgin forest were significantly higher than the percentages in the other plots. The delta C-13 values in the topsoils of the various profiles decreased in the following order: burnt grassland, farmland, grassland, shrubland, and virgin forest. From the surface to the base of the farmland profiles, little variation occurred in the delta C-13 values of the SOC (from -25.1 to -29.1 parts per thousand). This variation in the delta C-13 values of the SOC in the surface soils was associated with changes in the vegetation cover. In addition, our results suggest that the plant functional type is important for controlling the depth-related changes in the SOC contents and the delta C-13 values of the SOC.