The underground leakage process of soil patches around bedrock outcrops in a karst rocky desertification area traced using rare earth elements

Abstract

The loss of soil through underground leakage is a unique erosion process in the rocky karst desertification areas with abundant bedrock outcrops. Although some effort has been made to understand soil leakage loss, until now, the processes and ways that soil leakage loss occur have continued to puzzle us in this area. To solve this problem, three typical soil patches around bedrock outcrops were selected on a sloped farmland in a rocky karst desertification area located in Guizhou, China. Stable rare earth element tracing techniques combined with a streaking and marking method were used to observe the soil leaking process under three intervals of natural rainfall (accumulated rainfall was 104.5 mm, 151.2 mm and 332.7 mm for each interval). We found that rainstorms or heavy rainstorms are the key factors that cause the loss of soil around outcrop bedrock outcrops, where the main form of soil leakage is the downwards creeping of the soil layer. The soil creep displacement associated with the rainfall total of 104.5 mm (which consisted of one rainstorm, two moderate rain events and several light rain events) could account for more than 62.5% of the total creep associated with the rainfall total of 332.7 mm. However, the rainfall total of 181.5 mm (which consisted of a heavy rain episode, four moderate rain events and several light rain events) accounted for less than 15.0%. During different rainfall events, the concentrations of Ce, La and Sm in patches of soil around bedrock outcrops are negative compared with the background values in soil layers with treated with rare earth elements (REEs), while the concentrations of Ce, La and Sm used for comparison have a certain presence in other soil layers. Additionally, the concentrations of Ce, La and Sm at the rock-soil interface are not always higher or lower than those of the corresponding soil layer. It is proven that the soils in both the rock-soil interface and soil layer can leak downwards with a certain degree of probability. During the leaking processes, these soils do not move down uniformly or as a whole, but some particles in certain layers migrate to the lower layer separately. Therefore, improving soil anti-erodibility can prevent the creep leakage of soil patches to a certain extent and further preserve a sufficient material basis for plant restoration in rocky desertification areas.