Dramatical variation in rill erosion resistance during two-year continuous straw incorporation on sloping farmland

Straw incorporation has been increasingly recommended to control rill initiation and development. However, the unpredictable impact of continuous straw incorporation on residual straw and soil physicochemical properties leads to uncertain variations in rill erosion resistance. In our experiment, maize straw was annually incorporated into runoff plots for two consecutive years. The plots were configured with three fixed factors: straw length (0–2 cm and 2–5 cm), straw amount (4000 kg ha^-1 and 8000 kg ha^-1), and straw incorporation depth (15 cm and 20 cm). Two rill erosion resistance parameters, soil critical shear stress (τ_c) and rill erodibility (k_d), were measured using a submerged jet apparatus after maize harvesting. The results revealed a 10.98% decrease in τ_c and a 93.03% increase in k_d after two rounds of straw incorporation compared to after the first incorporation, indicating that soil resistance to rill erosion decreased during continuous straw incorporation. Structural equation modeling suggested that the incorporated depth was the dominant contributor to variations in rill erosion resistance following the first straw incorporation, primarily by influencing total porosity and saturated water content. As soil agglomeration progressed driven by straw decomposition, the straw amount had an increasingly indirect effect on rill erosion resistance, with primary factors shifting to capillary porosity, straw residues, water-stable aggregates, humic substances, and the humus fraction. Following continuous straw incorporation, both τ_c and k_d increased with the straw amount but decreased with the incorporated depth, indicating that excessive or shallow incorporation of straw can effectively prevent rill scouring under low shear stress but is less effective under high-stress conditions. Our study contributes to a deeper understanding of the dynamics of rill erosion resistance during continuous straw incorporation and provides a valuable reference for optimizing straw-returning strategies on sloping farmland.