Impact of Epichloë fungal endophyte-tall fescue symbiosis on mechanical characteristics of rooted soil depending on the host genotype

Abstract

Although the soil reinforcement by fibrous roots has been studied, 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 explore the effects of endophyte symbiosis (Epichloë coenophiala) with tall fescue (Festuca arundinacea) genotypes (75B and 75 C) on root reinforcement of a sandy clay loam soil. Glasshouse grown plants with (E+) and without (E−) endophytes were tested for shear characteristics (i.e., shear strength, S_f, relative shear displacement, RSD_f, and strain energy, E_strain) at depths of 10, 25 and 40 cm and two matric potentials (–10 and 0 kPa) measured using a large direct shear machine. Soil reinforcement by roots was related to root area ratio (RAR, ratio of root area to soil cross-section area), chemical composition and biomechanical characteristics (i.e., tensile strength, σ_Y, and tensile strain-at-failure, ϵ_Y, determined by an Instron universal tension-compression device). The roots increased S_f, RSD_f and E_strain by 4.1x, 4.5x and 14.5x compared to the root-free soil, respectively, with endophytes causing a further increase for 75 C but not for 75B. Cellulose and cellulose/lignin ratio were higher in the 75 C roots than the 75B roots. Cellulose, cellulose/lignin ratio, σ_Y and ϵ_Y were always greater in the E+ plants than in the E− ones, although the effect of endophyte was more noticeable in 75 C. Largest reinforcement was recorded for 75 C E+ (i.e., S_f of 36.5 kPa, RSD_f of 29.1 %, and E_strain of 5.89 kJ m⁻³), followed by 75B E+ > 75B E– ≈ 75 C E–, directly related to ruptured+stretched roots’ numbers, RAR, root cellulose, cellulose/lignin ratio, σ_Y and ϵ_Y. Compared to S_f and RSD_f, the E_strain better discriminated the reinforcing effect of root/endophyte. Lower reinforcement was observed at saturated condition, and root/endophyte moderated the influence of matric potential on soil reinforcement. The 75 C roots (irrespective of endophyte) could effectively reinforce the soil (high slopes of S_f–RAR relations). But, the reinforcing effect of 75B E– roots was low and significantly increased due to endophyte presence. The 75 C roots predominantly reinforced upper layers while 75B roots mainly reinforced lower layers. In 75 C, endophyte’s effect was significant for depths > 10 cm; however, the endophyte’s effect in 75B was significant for depth of 10 cm. The 75 C roots could effectively reinforce soil irrespective of matric potential, but 75B roots mainly reinforced unsaturated soil. Overall, genotype selection of tall fescue associated with endophyte inoculation could improve capacity of this species to reinforce unstable slopes.