Differential mechanisms of soil reinforcement by plants in Northern and Southern China

This study aims to compare the impact mechanisms of soil reinforcement by plants in the Jinyun Mountain area of Chongqing and the loess hilly area of Yan'an City, Shanxi Province, China. A systematic analysis was performed on the root morphological and soil mechanical parameters of trees (Rhus typhina and Ulmus pumila) and shrubs (Vitex negundo and Ziziphus jujuba). Four species were then selected to determine the soil shear strength. The RBMw model was employed to calculate the soil reinforcement benefits provided by these plants. The soil reinforcement of different plant species was evaluated to explore the components and contributions of soil reinforcement benefits. The results showed that: 1) The V. negundo shared the largest values for the root length per unit root volume, total root surface area, number of root forks, and root tips, while the R. typhina was the smallest values among the four species. There were no significant differences in the average root diameter of the four species. Moreover, the root length, number of root forks, and number of root tips of the R. typhina and U. pumila planted in Chongqing were significantly higher than those in Yan'an. While the V. negundo and R. typhina showed no significant differences. 2) The root tensile strength was inversely related to the root diameter in a power-law function. The V. negundo shared the highest average tensile strength. There was no significant difference of tensile strength of roots from the same plant species between different regions. By contrast, the V. negundo roots exhibited the higher tensile strength in Chongqing, compared with Yan'an. 3) The root systems of the four plant species were provided soil reinforcement efficiencies ranging from 0.65 to 4.12 kPa, with the significant differences among species. Among individual plants, V. negundo in Chongqing exhibited the highest soil reinforcement effects, while Z. jujuba was the lowest in Yan'an. The highest soil reinforcement efficiency was observed in the R. typhina in Chongqing, whereas, the lowest was found in the U. pumila in Yan'an. The root-soil composite effective cohesion of the two tree species planted in Chongqing was higher than that of bare soil (by 10%), while the shrubs was generally slightly lower. The root-soil composite shear strength in the V. negundo planting area was approximately 50% lower than that of the bare soil. The soil effective cohesion significantly decreased in the first year of V. negundo planting in Chongqing. The root system of V. negundo in Chongqing was contributed 30% to the root-soil composite effective cohesion, followed by the R. typhina in Chongqing (21.30%), and the U. pumila in Yan'an was the lowest (3.63%). The soil reinforcement effects and efficiencies of the four tree species planted in Chongqing were significantly higher than those in Yan'an. There was no variation in the exception of the individual soil reinforcement efficiency of Z. jujuba. The soil under R. typhina exhibited the highest comprehensive soil cohesion, while the high number of roots of V. negundo was resulted in the extremely low soil shear strength, leading to the soil cohesion lowest in the V. negundo planting area. As such, plants can be expected to reinforce the soil via their root systems. The finding can provide the theoretical basis to select the tree species for soil reinforcement and slope protection in different regions.