Influence of hedgerows on the particle size distribution of eroded sediment on purple soil slope

The hedgerow is a row of dense and upright vegetation that is planted along the contours of the slope. Among them, the crops can be produced between the hedges. Planting hedgerows on slopes can be the function of soil stabilization and fertilizer preservation, sediment containment and pollution control, soil quality improvement, and biological terrace formation. Particularly, the hedgerow is a vital soil and water conservation for the purple soil sloping farmland in the Three Gorges Reservoir Area (TGRA). Much effort has been made into the effects of hedgerows on the erosion process, soil and water conservation benefits. However, it is still unclear on the influence of hedgerows on the sediment particle size distribution and sediment sorting, especially for soil with uneven texture, like the purple soil in the TGRA. Taking the purple soil in the TGRA as the research object, this study aims to explore the effects of hedgerows on sediment particle size distribution and sediment sorting. Therefore, the simulated rainfall experiments were conducted with intensities of 60 and 90 mm/h at a fixed slope gradient (15°) for three slope conditions (CK-Control check, P-Hedgerow, and R-Only hedgerow roots). Then, the influence of hedgerows on the particle size distribution of eroded sediment on purple soil slope was evaluated to compare the sediment particle size distribution, enrichment ratio and agglomeration ratio under different slope conditions. The results indicated that the sand was the main particle in the rainfall erosion sediment. The influence of slope condition on particle size distribution was greater than that of rainfall intensity. Compared with the CK, the R slope condition only reduced the loss of clay and silt particles by 0.25%-5.48%, while the P slope condition significantly reduced the loss of sand particles by 30.75%-47.28% percentage points, resulting in a“diminution effect” of sediment particles. By contrast, the rainfall intensity led to the “coarsening effect” of sediment particles under different slope. The sediment sorting mainly existed at the early rainfall period for the CK slope condition. The influence of R slope condition on sediment sorting for effective particle was more pronounced during early rainfall period, indicating the large value of enrichment ratio of clay, fine silt, and coarse silt, compared with the CK. There was the pronounced influence of P slope condition on sediment sorting for effective particle during the whole rainfall period. But the influence effect was stable with the increase of rainfall duration. In terms of particle transport form, the sediment particles were mainly transported via aggregates on the CK, whereas, more aggregates were broken by raindrops and then dispersed into single grains under the R slope condition. Also, more fine particles were aggregated to form aggregates on the P slope, but the aggregates tended to disperse into single grains in the middle and late periods of rainfall. Overall, the sediment sorting was more outstanding under the hedgerow slope. Therefore, the hedgerow can be cooperated with other measures (such as earth banks, tillage measures, and straw mulching) for better performance. The finding can be used to further understand the effect of hedgerow on the sediment sorting of purple soil slopes. Scientific basis and data support can be offered to construct the regional soil erosion model for the efficient use of soil and water resources.