Effects of soil conservation practices on soil erosion and the size selectivity of eroded sediment on cultivated slopes

Soil conservation practices can greatly affect the soil erosion process, but limited information is available about its influence on the particle size distribution (PSD) of eroded sediment, especially under natural rainfall. In this study, the runoff, sediment yields, and effective/ultimate PSD were measured under two conventional tillage practices, downhill ridge tillage (DT) and plat tillage (PT) and three soil conservation practices, contour ridge tillage (CT), mulching with downhill ridge tillage (MDT), and mulching with contour ridge tillage (MCT) during 21 natural rainfall events in the lower Jinsha River. The results showed that (1) soil conservation practices had a significant effect on soil erosion. The conventional tillage of DT caused highest runoff depth (0.58 to 29.13 mm) and sediment yield (0.01 to 3.19 t hm(-2)). Compared with DT, the annual runoff depths and sediment yields of CT, MDT and MCT decreased by 12.24%-49.75% and 40.79%-88.30%, respectively. (2) Soil conservation practices can reduce the decomposition of aggregates in sediments. The ratios of effective and ultimate particle size (E/U) of silt-and sand-sized particles of DT and PT plots were close to 1, indicating that they were transported as primary particles, however, values lower/greater than 1 subject to CT, MDT and MCT plots indicated they were transported as aggregates. The ratios of E/U of clay-sized particles were all less than 1 independently of tillage practices. (3) The sediments of soil conservation practices were more selective than those of conventional tillage practices. For CT, MDT and MCT plots, the average enrichment ratios (ERs) of clay, silt and sand were 1.99, 1.93 and 0.42, respectively, with enrichment of clay and silt and depletion of sand in sediments. However, the compositions of the eroded sediments of DT and PT plots were similar to that of the original soil. These findings support the use of both effective and ultimate particle size distributions for studying the size selectivity of eroded sediment, and provide a scientific basis for revealing the erosion mechanism in the purple soil area of China.