Shear strength of soil-root system under different vegetation restoration modes on the Loess Plateau of China

The soil-root system shear strength (SRSS) is a useful dynamic parameter for evaluating soil detachment capacity by flowing water and slope stability. The changes in soil properties and root traits induced by vegetation restoration likely affect SRSS considerably. Nevertheless, few in-situ shear tests have been conducted to quantify the effects of restoration mode on SRSS in semi-arid region. This study was conducted to quantify the impact of restoration mode on SRSS of 0–50 cm soil layer at sites of one slope farmland (maize) and five vegetation restored lands (abandoned farmland, korshinsk peashrub, black locust, Chinese pine, and the mixed forest of amorpha and Chinese pine) and further to identify the related dominant contributing factors on the Loess Plateau. SRSS was reflected by the strain energy calibrated to 15% soil water content (SEc), peak shear stress (PSS) and the displacement of PSS (DPS) determined from the measured shear stress-displacement curves. The results showed that SEc decreased with soil depth (0–50 cm) for all vegetation restored lands. PSS increased with soil depth, except for slope farmland and abandoned farmland. DPS also decreased with soil depth as a whole. The mean SEc of 0–50 cm soil layer of different vegetation restored lands were 2.29–2.67 times greater than that of slope farmland. Chinese pine with a mean SEc of 138.3 J·m−2 was the best restoration mode to enhance soil erodibility. Soil water content significantly impacted SRSS and it decreased logarithmically with soil moisture. Plant roots could improve the ductility of soil, making shear failure with a great displacement. Root surface area density was the most sensitive parameter to reflect the effect of root system on soil strain energy. These results are helpful for understanding the mechanical mechanism of vegetation restored soils to mitigate erosion and to enhance slope stability in semi-arid region.