Composite root-soil mechanics of native vegetation in Central-Western Inner Mongolia

This study aims to enhance the biomechanical database concerning nitrogen fixation and plant erosion resistance in arid and semi-arid regions, providing a scientific foundation for the selection of native shrub and herb species and the optimization of mixed planting patterns for desertification control and reclamation of the Haidaigou open-pit coal mine dump in central-western Inner Mongolia. Five typical native species were examined: the shrubs Caragana korshinskii and Hippophae rhamnoides, the semi-shrub Hedysarum fruticosum, and the herbs Medicago sativa and Astragalus adsurgens. Experiments utilized loess soil from the dump site, characterized by a natural moisture content of 7.5% and a dry density of 1.56 g/cm³, with local field soil serving as a control. A ZJ-type strain-controlled direct shear apparatus was employed to test root-soil composites containing fine roots (1-1.5 mm in diameter) across five moisture content levels (3.5%, 7.5%, 11.5%, 15.5%, and 19.5%), all below the saturation point of 21.8%. Key mechanical parameters, including shear strength, cohesion, and internal friction angle, were measured under vertical loads of 12.5 kPa (simulating an 80 cm soil layer) and 25 kPa (simulating 150 cm). Results indicated that, at natural moisture content, the increase in shear strength of root-soil composites was greatest for Caragana korshinskii (60%), followed by Astragalus adsurgens (49%), and lowest for Hedysarum fruticosum (29%). Cohesion exhibited the highest increase in Astragalus adsurgens (46%) and the lowest in Hippophae rhamnoides (28%). As soil moisture content increased, both shear strength and cohesion initially rose before declining, with species-specific optimal moisture levels identified as follows: 7.5% for Caragana korshinskii, 11.5% for Hippophae rhamnoides, 7.5-11.5% for Medicago sativa, 11.5% for Hedysarum fruticosum, and 7.5-11.5% for Astragalus adsurgens. The internal friction angle of root-soil composites decreased slightly (-1% to -25%) compared to the control, confirming that cohesion is the primary contributor to the enhancement of shear strength. Under a 12.5 kPa load, root-induced soil consolidation was pronounced; however, at 25 kPa, this effect diminished, likely due to sparse root distribution in deeper layers. Furthermore, mixed-species plantings enhanced surface soil mechanical stability more effectively than single-species stands. These findings elucidate the biomechanical behavior and moisture response of root-soil complexes in native plants of arid and semi-arid regions, providing scientific and technical support for optimizing mixed "shrub- semi-shrub -herb" configurations in desertification control and mine reclamation.