Soil moisture and hysteresis affect both magnitude and efficiency of root reinforcement

Authors: ZHU, JIN-QI - Nanchang University; MAO, ZHUN - University Of Montpellier; WANG, YUNQI - Beijing Forestry University; WANG, YUJIE - Beijing Forestry University; TONG, LI - Beijing Forestry University; WANG, KAI - Beijing Forestry University; Langendoen, Eddy; ZHENG, BOFU - Nanchang University

Submitted to: Catena
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/11/2022
Publication Date: 12/1/2022
Citation: Zhu, J., Mao, Z., Wang, Y., Wang, Y., Tong, L., Wang, K., Langendoen, E.J., Zheng, B. 2022. Soil moisture and hysteresis affect both magnitude and efficiency of root reinforcement. Catena. 219, 106574. https://doi.org/10.1016/j.catena.2022.106574.
DOI: https://doi.org/10.1016/j.catena.2022.106574

Interpretive Summary: Vegetation is an environment-friendly method to stabilize hill slopes by providing additional soil strength through its root system. The effects of soil water content on root physical properties and resulting soil reinforcement remain poorly understood. Scientists at the USDA, ARS, National Sedimentation Laboratory in collaboration with researchers at Beijing Forestry University, China have investigated the change in root strength and soil reinforcement of Symplocos setchuensis Brand, a species used for afforestation in southwest China, for five soil water content levels: 17%, 22%, 29%, 38%, and 47%. The effect of soil water content on root reinforcement was mainly twofold: (1) modifying root tensile strength (especially for larger roots) and (2) changing the proportion of roots that fail either by slippage or breakage modes (especially for smaller roots). The findings can be used by hydraulic and agricultural engineers when evaluating bio-engineered slope stability measures.

Technical Abstract: Plant roots are widely recognized for protecting soil against shallow landslides. The mechanics of rooted soil failure under varying subsurface hydrologic regimes remains poorly understood. The goal of our study was to characterize the impact of soil water content on root mechanical traits and soil reinforcement. Measurements included root mechanical traits and soil-root interactions during the shearing process, e.g., root tensile strength, root water content, soil shear strength, soil reinforcement provided by roots, and root failure mode (slippage versus breakage). One-year-old Symplocos setchuensis Brand trees were replanted in shear boxes (0.40 × 0.40 × 0.50 m), which were placed in a canopy gap on Jinyun Mountain, Chongqing, China. After three years of vegetation growth, root mechanical measurements and soil direct shear tests were performed under different soil volumetric water content varying from 14.9 to 44.4%, which was controlled by either wetting or drying the soil. Root area ratio and root failure modes were examined as a function of root diameter. We found a unimodal shape relationship between shear strength of rooted soil per unit of root area ratio and soil water content. Soil shear strength was positively correlated with root area ratio in a linear manner. At higher soil water content (> 28%), the slope of this relationship decreased rapidly with increasing soil water content. Our results suggested that the effect of soil water content on root reinforcement was mainly twofold: its modification of root tensile strength (especially for larger roots) and the change in proportion of root failure modes (breakage and slippage) for roots smaller than 5 mm in diameter. These findings reveal the vital role of soil water content on the effectiveness of root reinforcement, which helps us to more accurately incorporate volumetric soil water content in quantifying soil-root interactions and modeling slope stability.