Superimposed effects of multiple rainfall events on soil structure

Rainfall is the main external force affecting soil surface structure. Many studies have evaluated the impact of a single rainfall event. However, the superimposed effects of multiple rainfall events on soil structure remain unclear. Therefore, surface soil from tillage in the black soil region of Northeast China was selected as the research object, and simulated rainfall and CT tests were combined to characterize the 3D indices of surface soil aggregates and pores under multiple rainfall events, uncovering the dynamic changes in the soil surface structure. The results showed that during rainfall, only a dense layer is formed on the upslope, while an alternating superposition of sedimentary–dense layer multilayers occurs on the downslope. For the upslope, as the rainfall frequency increased, the percentages of 0.25–0.053 mm aggregates in the dense layer with rainfall intensities of 40 mm/h and 70 mm/h exhibited opposite trends, with ranges ranging from 6.29 to 12.16 % and 45.21 to 70.71 %, respectively. For the downslope, when the sedimentary layer is at the top, the percentage of 0.25–0.053 mm aggregates decreases with increasing rainfall frequency, but the percentage of 0.25–0.053 mm aggregates increases when there is a dense layer on top of the sedimentary layer. In summary, the uppermost sedimentary layer at the downslope mainly shows fragmentation, while the lower sedimentary layer shows aggregation. The pore structure of the sedimentary layer downslope is influenced mainly by the fragmentation of its own large aggregates, whereas the pore structure of the dense layer is influenced mainly by the vertical downwards transport of the small aggregates of the upper layer of the sedimentary layer. The results of this study are beneficial for revealing the erosion of black soil slopes and the evolution of soil structure.