Soil Science Society of America Journal Soil Sci. Soc. Am. J. 77:257–267 doi:10.2136/sssaj2012.0273 Received 28 Aug. 2012. *Corresponding author (shizhihua70@gmail.com) © Soil Science Society of America, 5585 Guilford Rd., Madison WI 53711 USA All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher. Effects of Mulch Cover Rate on Interrill Erosion Processes and the Size Selectivity of Eroded Sediment on Steep Slopes Soil & Water Management & Conservation S oil erosion by water is not only associated with on-site land degradation but also greatly contributes to negative downstream of-site impacts such as looding, pollution, and siltation of water bodies. Soil erosion by water in- volves the detachment, transport, and deposition of soil materials due to the erosive forces of raindrops and runof, and these processes are commonly divided into rill and interrill components depending on the source of eroded sediment (Meyer and Wischmeier, 1969). In regions of the world where rainfall intensities are not high, rates of interrill erosion can be considerable; even where rainfall intensities are high, interrill areas occupy a pivotal position in the erosion system, acting as links between incident rainfall and those areas of concentrated low (rills and gullies) where most erosion occurs (Issa et al., 2006). Interrill soil erosion processes tend to be size selec- tive, and the particle-size distribution (PSD) of eroded sediment can provide basic information regarding erosion processes (Loch and Donnollan, 1983; Miller and Baharuddin, 1987; Mitchell et al., 1983; Proitt and Rose, 1991; Meyer et al., 1992; Z.H. Shi * State Key Lab. of Soil Erosion and Dryland Farming on the Loess Plateau Institute of Soil and Water Conservation Chinese Academy of Sciences Yangling, Shaanxi 712100 China and College of Resources and Environment Huazhong Agricultural Univ. Wuhan 430070 China B.J. Yue L. Wang N.F. Fang D. Wang State Key Lab. of Soil Erosion and Dryland Farming on the Loess Plateau Institute of Soil and Water Conservation Chinese Academy of Sciences Yangling, Shaanxi 712100 China F.Z. Wu College of Resources and Environment Huazhong Agricultural Univ. Wuhan 430070 China Mulching with vegetative residue is an effective soil conservation practice. A better understanding of sediment characteristics associated with various mulch rates would improve the use of this practice for soil conservation. An experiment was conducted to evaluate the effects of straw mulch on runoff, erosion, and the particle-size distribution (PSD) of eroded sediment. Straw mulch rates of 0, 15, 30, 50, 70, and 90% cover were tested using simulated rainfall. The effective PSD of sediment (undispersed) was compared with equivalent measurements of the same samples after dispersion (ultimate PSD) to investigate the detachment and transport mechanisms involved in sediment mobilization. The maximum stream occurred at a different time from the peak sediment concentration during rainstorms under low mulch rates, which indicated the predominance of supply-limited conditions. However, at higher mulch rates the erosion processes were typical of a transport- limited sediment regime. The ratio of the sediment transported as primary clay to the soil matrix clay content was always less than 1, meaning that most of the clay was eroded in the form of aggregates. Transport selectivity was relected by the silt enrichment, and silt-sized particles were transported mainly as primary particles since their effective–ultimate ratio was close to 1. The enrichment ratios for the sand-sized fractions decreased from 0.98 to 0.38 with increased mulch rates, and effective–ultimate ratios for sand-sized particles were always greater than 1, indicating that most of these particles were predominantly aggregates of iner particles, especially at high mulch rates. The indings reported in this study have important implications for the assessment and modeling of interrill erosion processes. Abbreviations: CEC, cation exchange capacity; ER, enrichment ratio; ESP, exchangeable sodium percentage; NSLR, normalized soil loss rate; PSD, particle-size distribution; SLR, soil loss ratio.