[Objective] The response of soil labile organic carbon components and enzyme activities to natural forest conversion was analyzed in order to provide a scientific basis for predicting regional soil health evolution and environmental change. [Methods] A native evergreen broad-leaved forest and a converted conifer plantation, an orchard, a sloping tillage area, and a paddy with similar geographical background and clear land use history were studied. The responses of labile organic carbon fractions and enzyme activities in topsoil to native forest conversion were investigated using various physical, chemical, and biochemical techniques. [Results] The native forest conversion to an orchard, a sloping tillage area, and a paddy significantly reduced the soil organic carbon content, labile carbon fraction contents, and enzyme activities by 42%—67%, 47%—88%, and 36%—89%, respectively. Notably, the readily oxidizable organic carbon, microbial biomass carbon, and invertase activity experienced the greatest reduction compared with the native forest conversion, sensitively indicating the reduction in soil organic carbon content and its lability. Readily oxidizable organic carbon was, therefore, considered to be a practical index. Reductions in soil labile organic carbon fractions and enzyme activities were lower for the conversion of native forest conversion to plantation than for the native forest conversion to orchard and sloping tillage area, showing the preservation capacity of soil labile carbon in the plantation. The ratio of labile organic carbon to total organic carbon in soil was significantly reduced after the native forest conversion to plantation, sensitively reflecting the reduction in soil carbon quality. [Conclusion] Native forest conversion led to substantial reductions in labile organic carbon quantity, carbon quality, and related enzyme activities in soil, suggesting a degradation of soil biological health and a decline in soil quality.