Variability in mollic epipedon thickness in response to soil erosion–deposition rates along slopes in Northeast China

The Mollisol region of Northeast China is one of the most significant grain-producing areas. Intense soil erosion has led to a sharp decline in both the mollic epipedon (ME) thickness and soil quality, and these processes have in turn resulted in a serious threat to the national food security. Soil erosion–deposition processes influence the soil thickness. However, the quantitative relationship between the ME thickness and soil erosion–deposition rate along slopes remains unclear. In this study, the ME thickness and soil erosion–deposition rate along three typical long, gentle slopes in Heilongjiang Province were investigated by using ground-penetrating radar (GPR) and 137Cs tracing technology. Variations in the spatial characteristics of the ME thickness and soil erosion–deposition rate on slopes were obtained. Wavelet analysis was employed to analyse the soil erosion–deposition rate on these slopes. The results indicated that the soil erosion rate along the three slopes was periodic and oscillated between high and low values. The range of primary cycle variation was 73–80 m, and the range of secondary cycle variation was 44–47 m. A negative correlation was found between the soil erosion–deposition rate and ME thickness within each cycle. The periodic variability in the soil erosion rate on these slopes also yielded a periodic variability in the ME thickness. Fitted equations were derived based on the ME thickness, slope length and soil erosion–deposition rate. The error range for soil thickness prediction was 0.30–9.40 %. This study could provide an efficient and accurate method to predict ME thickness change patterns along slopes and to help arrange soil erosion control measures at appropriate locations.