Disintegration characteristics in granite residual soil and their relationship with the collapsing gully in South China

The climate is a significant factor affecting the collapsing gully in weathered granite areas, and most of the surface layers of the collapsed area comprise granite residual soil. Granite residual soil has complex disintegration characteristics under different initial water content conditions. Besides, its disintegration characteristic is an essential factor for collapsing gully. Therefore, disintegration tests, triaxial shear tests, nuclear magnetic resonance tests, and hydraulic conductivity tests are conducted under torridity and rainstorm conditions in order to study the disintegration characteristics of granite residual soil. The results of disintegration test showed that the initial disintegration rate of granite residual soil increased rapidly with the decrease in water content, while the relationship between disintegration rate and water content in the later stage of disintegration is unclear. When soaked, the maximum decrease in cohesion was 44.48%, the hydraulic conductivity became six times larger, and the amplitude of the T2 curve increased by about 40%, which reduced the strength of the soil and provided better access for rainwater infiltration to deeper stratum. The results show that the microstructure of granite residual soil would be damaged and the disintegration would occur after a rainstorm at low water content. Micropores would be formed inside the sample after soaking, resulting in destroying the continuity of the material.

Response of Soil Water Dynamics to Rainfall on A Collapsing Gully Slope: Based on Continuous Multi-Depth Measurements

Soil water conditions play an important role in the formation of a collapsing gully, but we are still at the early stages of understanding how the soil water changes on the slope after different rainfall events due to a lack of high-frequency continuous field observations. This study aimed to reveal the response of soil water dynamics to rainfall events for different slope aspects and positions based on continuous multi-depth observations of soil water on a typical collapsing gully slope from 2017 to 2019 in Wuhua County, Guangdong Province, China. The vegetation characteristics and soil properties were investigated, and the storage of soil water was also calculated. The results showed that the dynamics and storage of soil water varied with the slope aspect, slope position and vegetation cover. The response time of the soil water to intensive rainfall events on the sunny slope was shorter than that on the shady slope, while soil water storage in the sunny slope was significantly lower than in the shady slope (p < 0.01). For the different slope positions, the soil water response time to the intensive rainfall events on the upper slope was shorter than that in the middle slope, while the soil water storage in the middle slope was significantly higher than on the upper slope. This was mainly due to the redistribution runoff from the upper slope to middle slope, delaying the process by which rainwater infiltrated into the soil layers. Moreover, vegetation significantly allayed the response of soil water dynamics to an intensive rainfall event but increased the storage of soil water, owing to the protection of soil surface from rain and conservation of high soil clay content. The bare area in the middle position of the sunny slope was speculated to be the potential source of the collapsing gully because it lacked the cover of vegetation. Our findings highlight the importance of soil water dynamics on the formation of a collapsing gully and provided valuable insights for the optimization of soil conservation and management practices for collapsing erosion.

Degradation Characteristics of Soil-Quality-Related Physical and Chemical Properties Affected by Collapsing Gully: The Case of Subtropical Hilly Region, China

In the subtropical hilly areas of China, a collapsing gully, a particular type of permanent gully, poses a great threat to the productivity and sustainability of the local ecological and agricultural systems. However, few studies have been performed regarding the effects of collapsing gully erosion on soil degradation. The aim of this study was to evaluate the effects of collapsing gully erosion on soil-quality-related physical and chemical properties. The collapsing gullies that were severely affected by erosion processes were considered at three stages (initial, active and stable stages) and corresponding soil samples were collected to analyze the spatial variation of the soil physical and chemical quality at each stage. The changes in the properties were assumed to be considerable in the regions affected by the erosion process compared with those unaffected by this process. Soil physical properties were more susceptible than soil nutrients to collapsing gully erosion in different spatial locations. The soil quality index (SQI) system consists of total nitrogen (TN), total phosphorus (TP), pH, capillary porosity (CP), sand content (SA), soil cohesion (SC) and root density (RD). Collapsing gully erosion was found to affect the soil physical and chemical properties by progressively reducing the SQI. The mean SQI value was the lowest in the active stage of the collapsing gully, with a higher soil degradation. For the different spatial positions in the collapsing gullies, the scour channel showed the lowest SQI value. The limiting indicators varied in the different stages or spatial sites in the collapsing gullies.