In karst areas, in order to ascertain the relationship between soil active organic carbon fractions and soil nutrient elements, topsoil samples (the sampling points were set at 80-m intervals) were collected for the analysis of soil physicochemical properties. In the sampling area, land use was divided into upland, paddy, and shrub land. The results showed that dissolved organic C (DOC) and microbial biomass C (MBC) contents in shrub land soil were higher than those values in upland and paddy soils (p<0.05). The total nitrogen (TN) content in paddy was lower than in upland and shrub land (p<0.05). The mean value of total phosphorus (TP) in upland and shrub land was approximately 1.5 times higher than that in paddy. Available nitrogen (AN) in shrub land soil was higher than in upland, whereas Olsen-P was lowest in shrub land in all soils (p<0.05). The C:P and N:P ratios in upland and paddy were lower than in shrub land (p<0.05). At plot scale, random forest analysis revealed that pH and soil organic carbon (SOC) were the most important variables determining DOC content in croplands (upland and paddy) and shrub land, respectively. The accumulation of SOC accelerates the growth of microbial biomass in upland. In shrub land, increases in SOC and total nitrogen were in favor of microbial growth. At the small-watershed scale, C:P and N:P ratios had a significant and positive effect on the content of DOC and MBC, respectively. The results implied that high C and N availability, especially in combination with low P availability, is helpful for increasing soil microbial biomass.