LAUSR

Abstract Wetting rate is one of the major factors determining the magnitude of slaking force in soils. Differences in wetting rate result in varying degrees of soil aggregate breakdown that alters the distribution of particle sizes in sediment. The objective of the present study was to determine how various wetting rates affected soil aggregate breakdown and soil fraction distribution in a contour ridge system. In the simulated rainfall experiment, the breakdown of 16 sizes soil aggregate was evaluated under five wetting rates (10, 20, 30, 60, and 90 mm h− 1) at different stages of erosion. The greatest fraction of sediment was microaggregate (accounting for 63.66%–69.19% of the total), and 20–50 μm and 50–100 μm classes were dominant within microaggregate fraction. The obviously increased loss of sediment from these two sizes occurred mainly as a result of the breakdown of 2–5 mm and 1–2 mm fractions during the inter-rill stage of erosion, and it was induced by the collapse of 0.25–0.5 mm, 0.5–1 mm, and 100–200 μm aggregates during rill erosion. Critical wetting rate thresholds of 20 mm h− 1 controlled the breakdown of 2–5 mm and 1–2 mm fractions during the inter-rill stage. During the rill stage, when the wetting rate exceeded 10 mm h− 1, the breakdown of 0.25–0.5 mm and 0.5–1 mm sizes intensified and the critical threshold for the collapse of 100–200 μm aggregate was 20 mm h− 1. These findings enhance our understanding of the breakdown mechanism of soil aggregates and supply guidance for implementing contour ridge systems.