Effects of herbaceous plant roots on tensile strength of root-soil composite in loess hilly region

Among the various failure modes associated with slope instability and soil erosion, tensile failure of the soil mass represents a critical aspect that should not be overlooked. Herbaceous species contribute significantly to mechanical reinforcement through their root systems. However, previous research has predominantly focused on quantifying the effect of roots on soil shear strength, while their influence on the tensile strength of the root-soil composite has been seldom investigated. This study aims to investigate the effects of root parameters from different mixed-species pattern on the tensile strength of root-soil composites in the loess hilly region. Field plots were established with Agropyron cristatum and Artemisia gmelinii in three different mixed planting ratios (1:3, 2:2, and 3:1), alongside a bare-soil control. In-situ field tensile tests were conducted, and soil physicochemical properties and root parameters were measured simultaneously. The results showed that among the tested plots, the 2:2 mixture of Agropyron cristatum and Artemisia gmelinii exhibited the optimal soil-reinforcing performance. Its tensile strength (27.74 kPa) was 2.36, 1.63, and 1.87 times that of the bare land, the 1:3 mixture, and the 3:1 mixture, respectively. Soil properties were the decisive factor affecting the tensile strength of the root-soil composite, with a higher contribution rate (54.67 %) than root parameters (45.33 %). Specifically, soil clay content was the largest single contributing factor (31.60 %). In the mixed taproot and fibrous root system patter, the reinforcement effect of fibrous roots (53.13 %) exceeded that of taproots (46.87 %). Among the root parameters, the root surface area density and root length density of fibrous roots contributed most significantly.