| Soil active organic carbon (SAC) refers to the fractions of organic carbons that are easy to move and to be oxidized and mineralized, and they are quite available to plants and soil microorganisms. Soil dissolved organic carbon (DOC) and microbial biomass carbon (MBC) are considered to be the useful indicators of SAC. Soil active organic carbon is involved in all biological and biochemical processes in soil, the minor change of SAC can be detected before the content of total organic carbon (TOC) in soil has changed, although it accounts for small fraction of TOC. The types of vegetation influence the size and quality of soil organic matter. Over the last several centuries, extensive areas of native vegetation of karst areas in Maolan, Guizhou Province have been converted to croplands and grasslands. The impacts of these land use changes on soil organic carbon (SOC) are unclear. In order to assess the impacts of land use changes on active soil organic carbon, we compared the DOC, MBC concentrations in adjacent plots of native forest, cropland, grassland with the same elevation, exposure and soil type. The results showed that the mean forest DOC were higher than that of grassland and cropland 25%, 48% respectively. From May to August, they all increased with the increase of temperature. The forest and cropland DOC reached the highest value in August while the grassland reached the peak value in October. The forest MBC was higher than that of grassland and cropland 81%, 45% respectively. Land use changes play an important role in the changes of DOC and MBC in soils. The relationship between active soil organic carbon and environmental factors is complicated which still needs to be further studied.The soil dissolved organic carbon plays an important role in the karst ecosystem. DOC is involved in all biological and biochemical processes in soil, the minor change of DOC can be detected before the content of total organic carbon(TOC) in soil has changed, although it accounts for small fraction of TOC. The types of vegetation influence the size and quality of soil dissolved organic carbon. The profile distribution of soil DOC under different land use patterns as affected by human practice is necessary for assessing the alteration of soil quality. 4 types of land use (forestry field, the grassland field, the depression field, the hillside maize field), which are widely distributed in karst area of Guizhou Province, southwest of China, were selected to investigate profile distribution of DOC. In each pedon at 0-75cm depth, soil samples were collected from 4-5layers. The results showed that the profile distribution of DOC was different under different land use patterns, indicating the significant effect of land use patterns on DOC.DOC in the forest field were higher than that of in the grassland field, the depression field and the hillside maize field. The soils under grassland field contained a larger proportion of DOC to organic carbon. The results obtained could potentially provide theoretic support for understanding the effects of land use change on soil DOC and establishment of sustainable land use patterns.We also compared the contents of major nutrient elements of soil under different land use patterns and the relationship with DOC. The results showed that under three land use patterns, different nutrient elements such as N,P,K have different evolve trends during the year studied. The contents of nutrient elements beneath 20cm of the soil always higher than that of beneath 50cm of the soil.And according to the analysis of the variation of pH, the contents and the composition of organic matter major nutrients(N,P,K) and the microbial biomass of the soils ,we found that the soil quality degenerated quickly with the development of karst rocky desertification. The method of hierarchical cluster analysis were used to evaluate the early warning indicators of soil quality degeneration in the process of karst rocky desertification.
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