Plant roots can influence soil physical properties in both hydrological an‎d mechanical ways, thereby improving slope stability an‎d erosion resistance. However, the effects of fibrous roots of rangelan‎d plants on soil reinforcement in relation with root biomechanical parameters have not been studied in detail at different soil water statuses. Besides the interaction impacts of endophyte symbiosis an‎d plant genotype on mechanical characteristics of rooted soil have not been investigated yet. This study was conducted to: i) investigate the effect of rangelan‎d plant type on soil mechanical reinforcement by roots related to root biomechanical parameters in saturated an‎d unsaturated conditions, ii) eva‎luate the interaction effect of tall fescue genotype an‎d endophyte symbiosis on soil mechanical reinforcement by roots linked to root biomechanical parameters, an‎d iii) predict the effect of roots on soil mechanical behavior an‎d shear strength using analytical an‎d numerial models. A large direct shear machine was fabricated to measure the shear strength of rooted soil columns at three shear depths of 10, 25 an‎d 40 cm. The shear tests were done at saturated an‎d unsaturated soil water statuses. The results of first part indicated that 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, 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. Soil reinforcement an‎d the intercept an‎d slope of shear strength vs. root area ratio (RAR) were greater in the unsaturated soil (i.e., matric potential –10 kPa), indicating higher mobilization of root strength in soil reinforcement. The reinforcing effects of roots moderated the role of soil water status in mechanical strength. The roots of F. arundinacea 75B an‎d B. tomentellus effectively reinforced unsaturated soils. Root cellulose, cellulose/lignin ratio, tensile strength an‎d tensile strain were always greater in the endophyte-infected (E+) plants than in the endophyte-free (E‒) ones, although the effect of endophyte was more noticeable in 75C. Largest reinforcement was recorded for 75C E+, followed by 75B E+ ‎> 75B E– ≈ 75C E–, directly related to ruptured+stretched roots’ numbers, RAR, root cellulose, cellulose/lignin ratio, tensile strength an‎d tensile strain. The 75C roots predominantly reinforced upper layers while 75B roots mainly reinforced lower layers. The 75C roots could effectively reinforce soil irrespective of matric potential, but 75B roots mainly reinforced unsaturated soil. The results of third part indicated that the mechanical reinforcement of the roots was highly overestimated by the analytical model of Waldron an‎d Wu. However, numerical modeling of the soil-root interaction by ABAQUS software was satisfactory. In general, the relative shear displacement in most treatments was predicted well by the numerical method. However, shear stress-shear displacement curves an‎d soil-root shear strength were fairly predicted in some treatments. The reasons for the discrepancy between modeled an‎d measured soil-root shear strength in B. tomentellus might be related to the errors in predicting the number of crushed/streteched roots, the spatial distribution of the roots an‎d the size of the roots on the shear plane, an‎d related root composition an‎d biomechanical properties. Overall, combined cultivation of B. inermis (most effective on soil shear strength) an‎d F. arundinacea 75C (most effective on shear displacement) might be recommended for soil conservation practices on slopey lan‎ds.

محمدرضا مصدقي

محمدرضا سبزعليان دستجردي , مهرنوش جعفري

شمس اله ايوبي , حسين بشري , علي اكبر محبوبي