Response of shallow karst fissure soil quality to secondary succession in a degraded karst area of southwestern China

Vegetation restoration is one of the effective means to control soil erosion and an essential method of eco-environmental restoration in fragile karst rocky desertification areas. The shallow karst fissure (SKF) filled with soil has become one of the most important root habitats in this area. It is essential to scientifically evaluate the soil quality of SKF and to provide a basis for matching species with sites. Here, we explore and gain insight into the characteristics of the response of the physicochemical properties of SKF soil to different stages of vegetation succession. Additionally, we develop a synthetical assessment system to determine soil quality. The key results were as follows. Significant differences were found in the soil properties between the surface and the SKF. Compared with the surface, the SKF soil structural property was poor, but the water conditions were better. The soil nutrient content of the SKF was significantly (p < 0.05) lower than that of the surface, but the differences in soil nutrients inside the SKF were not significant. In addition, the soil quality indicator (SQI) values of the SKF soil were significantly (p < 0.05) lower than those of the surface and decreased gradually with soil depth. Moreover, secondary succession significantly (p < 0.05) improved the soil properties and, thus, the soil quality. With recovery of vegetation, the soil properties of the surface and the SKF were significantly improved. The difference in the soil properties and the SQI values between the surface and underground gradually narrowed with succession. Furthermore, the thickness of the SKF soil was more than twice that of the surface soil. Overall, the soil properties and the soil quality of the SKF were relatively poor, but they were greatly improved with vegetation restoration. The SKF habitat can be the preferred habitat for vegetation restoration and is more favourable for plants with deep roots and a higher root penetration ability. The results of this study can supply references for future studies on karst rocky desertification control and vegetation restoration in the karst area of southwestern China, northern and central Vietnam, the islands of Java, Indonesia and Kampot Province, southwestern Cambodia and other karst regions with the same ecological background.