Distribution characteristics of eroded sediment particles in purple slope cultivated land under different straw mulch amount

Straw mulching can dominate the sorting effect of the runoff on the sediment under the secondary rainfall. Thus, there is the aggregate distribution of eroded sediment. This study aims to clarify the influence of straw mulching on the aggregate distribution of the eroded sediment in the purple slope cultivated land. Different amounts were adopted using straw mulching (0, 0.2, 0.4, 0.6, and 0.8 kg/m²) and gradients (10°,15°, and 20°). A systematic investigation was implemented to determine the content of each particle size (<0.002, 0.002-0.05, and 0.05-2 mm) and its enrichment rate (ER) and median particle size (D_0.5) in the eroded sediment under the indoor artificial rainfall. The results show as follows: 1) There was a stable variation trend of <0.002 mm eroded sediment aggregates with the rainfall duration. There was a gradual decrease and increase trend of 0.002-0.05 mm and 0.05-2 mm eroded sediment aggregates, respectively. 2) The aggregate content of <0.002 mm increased first and then decreased, with the increase of the straw cover amount. While the aggregate content of 0.002-0.05 mm increased as a whole. Compared with the bare slope, 0.8 kg/m² straw mulching significantly reduced the <0.002 mm aggregate content by 44.87%-124.50%. It indicated that the purple slope under straw mulching mainly lost <0.002 mm aggregate content. The aggregate content of 0.2 kg/m² on the 15° slope under straw mulching was significantly lower by 29.09%, 26.73%, 35.15%, and 23.89% than that of 0, 0.4, 0.6 and 0.8 kg/m², respectively. At the 10° and 15° slope surfaces, the 0.05-2 mm eroded sediment content of 0.2 kg/m² straw mulch was significantly higher than the rest straw mulch loads. 3) The median particle size initially increased and then decreased under the same slope, with the increase of straw cover amount. The median particle size with the slope was also dominated by the straw cover amount. Specifically, the median particle size content of 0.8 kg/m² on the 20° slope was significantly higher by 26.28%, 37.03%, and 34.48% than that of 0, 0.4, and 0.6 kg/m², respectively. 4) The ER_0.002-0.05 was greater than 1 under the slope and straw cover, while ER_0.002 and ER_0.05-2 were less than 1. Furthermore, ER<0.002 shared no difference at 10° and 20° under the coverage of 0 and 0.4 kg/m², respectively. While the rest of conditions shared the significant differences. The ER_0.002-0.05 of 0.2 kg/m² mulch was significantly lower than that of the rest stalks under the three gradients. There was a trend of first decreasing and then increasing with the increase of straw mulch. The ER_0.05-2 of 0.2 kg/m² coverage at each slope was significantly higher than that of the rest coverage (P<0.05). 5) There was a significant quadratic function relationship between the aggregate content and enrichment rate of <0.002 mm and the straw mulch amount. The major maximum was reached when the straw mulch amount was 0.2 kg/m². Once the amount of applied straw was greater than 0.4 kg/m², the loss of clay particles was reduced effectively. The finding can provide the scientific basis for the distribution and optimization of straw mulching on the eroded sediment in the sloping farmland from the purple soil area.