Abstract: Sediment is the product of soil erosion, so it can reflect the characters of erosion process. In this study, six red soil samples, derived from three parent materials (Quaternary red clay, Shale and Granite) were selected and subjected to simulated rainfall experiment. The particle size distribution and cementing material of sediments were tested. From the compare of the physic-chemical properties of different sizes sediment, the main results as followed: Compared to the particle size distribution of test soils, there were more 0.002-0.02 mm particles in sediments, which occupied 26.68%-60.33% in sediments. It means that the runoff was preferred to transit particles between 0.002 mm to 0.02 mm. The proportion of 0.02-0.002 mm and < 0.002 mm particles were far greater than that in test soil from Shale and Granite while the proportion was lower than test soil from Quaternary red clay. After chemical dispersion treatment, the particle size distribution of sediment were different from the test soil, and there were more < 0.02 mm particles in sediments than test soils. The large particles (> 0.25 mm) in sediment from high-clay soils(derived from Quaternary red clay) is abundant, and most of which were stable aggregates. After chemical dispersion, there were more primary particles in sediments from high -sand soils (derived from Granite) and high-silt soils (derived from Shale) than in high-clay soils, and more clay and silt than test soils. The content of Fe_d, Al_d, Al_o and Si_o decreased with the increase of particles size in sediments derived from Granite, while Fe_d, Al_d and Al_o increased with the increase of particle size in sediments from Shale and Quaternary red clay. As a result, the large particles of sediments from Shale and Quaternary red clay were more stable than that from Granite. However, no obvious distribution regularities of Si_o were found in different size particles in three sediments.The content of organic matter also increased with the increase of particles size of sediment from shale and Quaternary red clay. However, the enrichment was not obvious for sediments from Granite, the content of organic matter of which was higher in 0.25-0.05 mm particles than in other sizes particles. The ration of MWD (mean weight diameter) before dispersion and after dispersion was positively correlated with the contents of oxide except Si_o, which was negatively correlated with the ration. It indicated that iron-aluminum oxides (Fe_d, Al_d and Al_o) are important stable factors of red soil aggregates.

Key words: red soil, simulated rainfall, sediment, mean weight diameter, oxide, organic matter