Background Plant soil fixation is mainly achieved through the roots to fix the soil, plant roots cross and entangle in the soil to formed "reinforcement" and "anchoring" to enhancing soil strength. Under the highly heterogeneous topographic and geomorphological conditions of Karst areas, most of the roots exist in the form of curved or even spiraling, and the morphology is different, thus the difference between the tensile characteristics of curved roots and straight roots needs to be further explored.

Methods In order to explore the mechanical properties of root systems with different degrees of curvature, the 5-year slope greening tree species Cassia bicapsularis was taken as the research object, the roots was classified into three diameter levels of 1-2, 2-3 and 3-4 mm, and according to the curvature, the roots was classified as straight root, slightly curved root and curved root. Finally, the roots was made into a specimen with a gauge distance of 100 mm, and a single tensile test was used to explore the tensile deformation characteristics of curved root.

Results 1) The tensile force and tensile strength of the roots of C. bicapsularis with different curvature were shown as straight roots>slightly curved roots>curved roots, and the ultimate elongation was curved root (29.26 %)>slightly curved roots (21.15 %)>straight roots (12.14 %). 2)The tensile deformation process of the C. bicapsularis curved roots was complex and diverse. According to the shape of stress-strain curve, the curves were divided into single peak type and multi-peak type, the roots deformed in different ways to resist external forces, including elastic deformation, plastic deformation, yielding and root skin tearing. In the stress-strain curve, curved roots was more than one pre-positioning stage compared with straight roots, and the stress range was 0.3-5.7 MPa. In addition, the curved roots was "concave type" in the elastic deformation stage, while the straight roots was "convex type", showing the buffering of the slope protection of the curved roots. 3) The initial force displacement of the curved roots was positively correlated with the curvature. Curved roots shared a portion of the soil pressure more than straight roots during force deformation, and the magnitude of the force was directly affected by the curvature.

Conclusions In this study, the tensile deformation process of the curved roots of C. bicapsularis is quite different from the straight roots. The curved roots is conducive to the occlusion of the root-soil complex to enhance the strength of the root-soil complex, when shallow landslides occur, the curved roots of C. bicapsularis have a superior slope protection capacity than straight roots. The results of this study provide reference significance for the evaluation of mechanical properties of curved roots.