The role of straw mulching in shaping rills and stabilizing rill network under simulated extreme rainfall

Abstract

Straw mulching has been widely demonstrated as a useful practice for soil and water conservation, but its effectiveness in controlling rill erosion has rarely been systematically studied. The objective of the current study was to evaluate the effects of straw mulching on rill development and rill network evolution. A cinnamon soil was mulched with corn straw segments at five different rates of 0, 2.1, 4.2, 6.3 and 8.4 t ha−1, to attain the surface coverages of 0%, 10%, 20%, 30% and 40%, respectively, and received four 30-min simulated rainfalls at the intensity of 90 mm h−1. The structure from motion (SfM) photogrammetry was applied to measure soil surface elevation changes upon each rainfall. Straw mulching was found to retard runoff initiation, but decreased soil losses were typically acquired at the mulched treatments with 20% or more straw coverages. Nevertheless, this conservation effect did not monotonically increase with mulching rate, because a larger number of straw segments were more likely to induce rill erosion via surface flow convergence, thereby increasing total soil loss. Straw mulching also changed the course of rill development. Head retreat dominated rill erosion at the control without mulching throughout the entire experiment, suggesting an early stage of rill erosion. With the straw segments around and at the bottom, the initially equivalent contributions by headcutting, bed scouring and sideward erosion quickly shifted to the predominance of bed incision and then to sidewall expansion, successively. The rapid transition of rill erosion subprocess in this way, together with the dendritic and stabilized networks, indicate a mature stage of rill development by the end of the experiment. The rill depths and main channel widths, as a consequence, were typically smaller at the mulched treatments. These findings demonstrate the effectiveness of adequate straw mulching in stabilizing rill network in addition to reducing soil erosion at the plot scale, and hold important implications for agricultural management in slope farming systems.