Soil mechanical reinforcement by the fibrous roots of selected rangeland plants using a large soil-root shear apparatus

Soil reinforcement by fibrous roots of rangeland plants is essential for soil conservation. This study was done to: i) determine the effect of two genotypes 75B and 75C of Festuca arundinacea, Bromus inermis, and Bromus tomentellus on soil reinforcement at saturated/unsaturated conditions, and ii) explore the relationships between soil shear strength, and root area ratio (RAR) determined by image analysis and root chemical composition and mechanical properties in a sandy clay loam soil. Two genotypes of F. arundinacea were chosen based on their different morphological/physiological responses to environmental stresses. Rooted soils were sheared at three depths, 10, 25, and 40 cm. Peak shear strength and shear displacement-at-failure in the rooted soils were about four times greater than in the non-planted ones. B. inermis (with highest numbers of stretched roots) showed the highest shear strength (i.e., 35.7 kPa), and the roots of B. tomentellus were as effective as F. arundinacea. However, their effects on shear displacement-at-failure were significantly lower. Highest and lowest shear displacements were observed in the rooted soils of F. arundinacea and B. tomentellus, respectively. Greatest soil reinforcement was observed in the shallow soil (i.e., depth 10 cm). Soil reinforcement and the intercept and slope of shear strength vs. RAR were greater in the unsaturated soil (i.e., matric potential –10 kPa), indicating higher mobilization of root strength in soil reinforcement and lower root slippage. Shear strength differences of rooted soil were better noticeable in unsaturated condition. More than fivefold decrease in shear strength was observed from unsaturated to saturated state in control, but it was in the range of 1.4–1.7 for planted soils. This revealed that the reinforcing effects of roots moderated the role of soil water status in mechanical strength. Little changes in the slopes of shear strength vs. RAR relations for F. arundinacea 75C and B. inermis imply that they are effective in soil reinforcement irrespective of water status. However, the slope of relations for F. arundinacea 75B and B. tomentellus considerably increased from saturated to unsaturated state, showing that the roots of these two plants effectively reinforced unsaturated soils. Highest soil reinforcement by the roots of B. inermis was partly related to their greater lignin and hemicellulose contents and tensile strain-at-failure. Highest and lowest values of shear displacement-at-failure were observed in F. arundinacea 75C and B. tomentellus, respectively; it means that the roots of F. arundinacea 75C with higher cellulose/lignin ratio, tensile strength, and strain-at-failure stretched greater under shear.