Evaluating soil conservation from root distribution of Caragana Korshinskii Kom in the Loess region of Xining Basin

Caragana korshinskii Kom is one of the most important dominant shrub species in the loess region of Xining Basin. This study aims to quantitatively evaluate the mechanical effects of the roots on soil reinforcement and slope protection. Three growth stages were also selected (early age (<6 years old), middle age (6-14 years old), and old age (>14 years old)). The root morphological indexes and root geometric distribution were investigated at different ages via in situ excavation method and in situ pull-out test. The tensile strength of single roots was obtained via the single-root tensile test. In addition, the WWM model was used to evaluate the enhancement effect of roots on the soil shear strength at different growth stages. Moreover, the finite element (FE) simulation was carried out to determine the contribution of reinforcement and anchoring effect of C. korshinskii roots at different growth stages for the better stability of the loess slope. The results showed that the root depth, root width, total root length, and taproot diameter of the C. korshinskii were 3.610-5.500, 7.831-28.603 m, and 11.520-22.043 mm, respectively, indicating an increasing trend with the increase of age. Among them, the root length and root number were distributed mainly in the range of 0-0.6 m soil depth. Specifically, there was a significant decrease in the early and old age C. korshinskii with the increasing depth, whereas, the root length and root number at the middle age initially decreased with the soil depth and then increased with the turning point at the soil depth of 0.3-0.6 m. The root area ratio (RAR) and root additional cohesion at early and old age decreased with the increasing soil depth. Furthermore, the middle age C. korshinskii increased first and then decreased, indicating the most significant effect on the shear strength of the soil. The roots of C. korshinskii at different growth stages reduced the total displacement of the slope shoulder and central position of the slope, as well as the maximum total displacement of the slope. The maximum total displacements of C. korshinskii slope in early, middle and old age were 0.10%, 1.40%, and 1.88% lower than those of bare slope, respectively. The stability coefficient of the plant slope was in the range of 1.072-1.080, which was 0.19%-0.93% higher than that of the bare slope, indicating an increasing trend with the increase in plant age. The roots of C. korshinskii at different growth stages improved the slope stability coefficient, but the increase was limited. The dominant mechanical effect of C. korshinskii roots gradually evolved from the soil reinforcement to the anchoring effect from early to old age, indicating the strong influence of the plant growth stages. The finding can offer theoretical guiding to implement the ecological restoration and vegetation slope protection projects in Xining Basin and surrounding areas.