Soil Texture Influences the Magnitude and Depth Persistence of Alfalfa Reinforcement and Its Contribution to Slope Stability

Vegetation-based slope stabilisation depends on coupled mechanical and hydraulic processes that vary with soil texture and depth. This study linked a five-month long, 0.5 m column experiment to annual two-dimensional transient seepage and analytical infinite-slope modelling to quantify how alfalfa (Medicago sativa L.) and a mycorrhiza-containing formulation affect shallow root-zone stability in silty clay (SC) and silty sand (SS). Eighteen columns (n = 3 per soil–treatment combination) received unplanted control (C), alfalfa (A), or alfalfa plus mycorrhiza formulation (A + MF). Biomass, saturated hydraulic conductivity (ks), and saturated direct-shear response at six depths were measured. Relative to A, A + MF increased recovered root dry mass by 24.5% in SC and 53.2% in SS, and ks by 46.6% and 47.4%. Factorial ANOVA detected a soil × treatment interaction for ks (p = 0.00052), but not biomass. Exponential apparent-cohesion profiles separated reinforcement amplitude from depth persistence; fitted decay depths for A/A + MF were 0.167/0.176 m in SC and 0.064/0.22 m in SS. The measured mechanical and hydraulic parameters were applied to a 365-day model of a representative 1V:3H natural infinite slope. All planted cases retained higher annual minimum root-zone factors of safety than their controls, but retention, uptake and storm-timing sensitivities changed the A versus A + MF ranking. Reinforcement magnitude, depth persistence, and soil-specific hydrology should therefore be represented together when evaluating biologically treated shallow slopes.