Soil‐Root Shear Strength of Gullies Covered by Different Vegetation Types on the Loess Plateau of China

ABSTRACT Gully erosion dominates soil degradation in small watersheds. While vegetation mitigates gully development, its effectiveness depends on type, partly through altering soil‐root shear strength. Few in situ studies assess this effect. Hence, this study quantified soil‐root shear strength differences between gullies covered by grass versus grass‐shrub mixed communities on the Loess Plateau. In situ shear tests measured peak shear stress and displacement, generating shear stress‐displacement curves. Peak shear stress and strain energy derived from these curves were calibrated to 15% soil moisture to characterize soil‐root shear strength. Results showed the average peak shear stress for grass‐covered gullies (9.32 kPa) was significantly higher than for mixed vegetation gullies (6.25 kPa). Shear strength decreased initially then increased with depth. The critical depths for this peak shear stress transition were 20 cm (grass) and 30 cm (mixed). Variations in shear strength with vegetation type and depth were primarily controlled by soil properties and root attributes. Peak shear stress exhibited a significant positive association with soil cohesion. Strain energy showed significant positive relationships with root mass density and effective root density. Soil organic matter content and aggregate stability enhanced strain energy through direct and indirect effects. These findings provide insights into the mechanical mechanisms by which vegetation type enhances gully soil strength and controls land degradation in semi‐arid regions.