Soil-Matrix-Dependent Root Reinforcement by Chrysopogon zizanioides: Direct Shear and Statistical Evidence from Andean Soils.

Plant roots can modify soil mechanical behaviour through anchorage, interlocking, and stress-transfer mechanisms; however, the magnitude of this effect is not uniform and depends on the soil matrix in which the root system develops. This study evaluates the biomechanical reinforcement induced by Chrysopogon zizanioides (L.) Roberty roots in seven contrasting Andean soil matrices classified as CL-ML, MH, CH, OH, OL, CL, and ML. Bare-soil and Vetiver-rooted specimens were compared across three depth intervals: 0.00-0.50 m, 0.50-1.50 m, and 1.50-2.00 m. Shear-strength parameters were determined through direct shear tests under controlled normal stresses of 50, 100, and 200 kPa, and the relative amplifications of apparent cohesion (Δc) and internal friction angle (Δφ) were calculated. ANOVA, Tukey HSD post hoc tests, effect-size analysis, descriptive depth-dependent regression analyses, and shear-resistance sensitivity analysis were then applied to identify soil-dependent reinforcement patterns. The results showed that the mechanical contribution of Vetiver roots was primarily controlled by soil type. The highest relative cohesion amplification was obtained in OL soil, where Δc reached 100.0%, whereas the strongest friction-angle response was observed in CH soil, with Δφ reaching 20.1%. MH soil exhibited an intermediate and technically balanced response, combining moderate cohesion gains with relevant frictional improvement. One-way ANOVA screening indicated that soil type was the main differentiating factor controlling the Vetiver-induced response, while depth had a less dominant effect. Tukey HSD comparisons confirmed that OL exhibited a distinct cohesion-amplification pattern relative to the other soil matrices. These findings indicate that C. zizanioides root reinforcement should not be interpreted as a uniform plant effect but as a soil-matrix-dependent biomechanical response governed by root-soil interaction. The initial mechanical condition of the soil, rather than root content alone, determines whether Vetiver reinforcement is expressed primarily as a cohesion gain, a friction-angle gain, or a combined response. Because the statistical evidence is screening-level owing to the absence of replication within soil-depth cells, these patterns should support soil-specific interpretation rather than be applied as universal amplification factors.