Effects of erosion-induced degradation on subsequent soil erosion and sediment sorting of a cultivated black soil under simulated heavy rainstorm

Soil erosion is a major cause of soil degradation, leading to soil quality deterioration. The resulting alternation of soil properties would in turn change the soil's responses in subsequent soil erosion events later, which, however, has rarely been studied. The objective was to examine the effects of such erosion-induced degradation on subsequent soil erosion and sediment size distribution of a cultivated black soil under heavy rainstorms. The plough layers of the black soil that had suffered from 0, 10, 30, 50, and 70 years of soil erosion in northeast China were artificially constructed, corresponding to five erosional degradation levels of none, slight, moderate, severe, and very severe degradations, successively. A simulated heavy rainfall at the 75 mm h(-1) intensity was performed, and runoff and sediment samples were collected every 6 min and analyzed for particle size distribution. Owing to the increasingly coarse source soil and thereby enhanced hydraulic conductivity, the steady-state runoff rate decreased from 1.06 to 0.77 mm min(-1), as the degradation level increased from none to very severe degradation. However, the fine-textured noneroded soil had a better water retention capacity, which decreased runoff, especially at the beginning of the rainfall. The maximum runoff in total, that is, 103.61 mm, was consequently observed in the slightly degraded soil. The sediment concentrations and yields here were also significantly greater than those of the other treatments (p <0.01), suggesting a relatively higher soil erodibility. Sediment sorting was observed at each treatment especially during the first half of the rainfall, except for the slightly eroded soil where the elevated runoff was believed sufficient to equally transport different-sized particles. The clay particles were enriched in the sediments eroded from the non- and moderately degraded soils, as the corresponding enrichment ratios (ERs) were typically above 1. Whereas for the severely and very severely degraded soils, ERs were around 0.5, indicating considerable depletion of these fine particles. These results demonstrate the diverse erosional responses of the soils exhibiting varying erosion-induced degradation levels and hold important implications for agricultural management and soil and water conservation in the black soil region of northeast China as well as other similar regions.