The evaluation of soil quality evolution in the process of vegetation degradation in karst mountain area

To evaluate karst mountain soil quality due to the degradation of vegetation, this paper selected two representative microhabitat soils, i.e. the surface and rocky gully soils, as evaluation units for the construction of karst mountain soil quality evaluation system. 16 indexes were selected from 29 indexes using the canonical correspondence analysis, and the soil quality was discussed by using the comprehensive evaluation method of factor analysis. The results showed that, in the process of vegetation degradation in karst forest, using weighting calculation with the areas of the said two microhabitats to obtain the soil quality synthesis score can more accurately reflect the changes of soil quality due to the vegetation degradation at various phases; and these changes can be specifically divided into 3 phases. During these phases, the concentrations of soil active organic carbon, available N, available P, available K, available Mn, available S and urease all significantly decreased, while the available Fe, sucrase and clay contents significantly increased. In Phase I with vegetation represented by original-secondary forest, no significant difference was found in soil indicators between the sample plots. In Phase II (as represented by shrub wood), the contents of soil active organic carbon, available N, available P, available K, available S and urease and the content of available Mn respectively decreased by 33.73%, 22.41%, 57.66%, 37.72%, 44.59%, 46.57% and 12.52% in average, compared to those of Phase I; and the sucrase activity and soil clay content respectively increased by 81.38% and 47.69% in average. In Phase III (as scrub-grassland soil), the concentrations of soil available P, urease and alkaline phosphatase and available Mn decreased by 42.82%, 44.42%, 28.45% and 20.55%, respectively, compared to Phase II; the content of soil available Fe and clay content respectively increased by 56.40% and 57.49% in average. In this phase, the contents of soil available P and available K become very scarce, which leads to significant decrease of the soil quality.