Background Plant roots play the most important role in improving soil erosion resistance. Affected by the fluctuation of water level, the root-soil composite in the water-level fluctuation zone of the Three Gorges Reservoir undergoes alternating cycles of dry and wet, and the effect of its roots on soil shear resistance is weakening. The purpose of this study is to explore the effect of roots on soil shear resistance in the water-level fluctuation zone of the Three Gorges Reservoir.

Methods The water-level fluctuation zone of the Three Gorges Reservoir at an elevation of 165 m was selected as the study area, and the typical herb Cynodon dactylon and its root-soil composite were sampled during the period of vigorous vegetation. Direct shear tests and triaxial tests were conducted on undisturbed soil and remolded soil, and the internal friction angle and cohesion of soil were obtained. The effects of root morphological characteristics, root content and root layouts on soil shear resistance were discussed by regression analysis and principal component analysis.

Results 1) The internal friction angle has a significant linear positive correlation with root length density, and increases logarithmically with the increase of root surface area density and root volume density. There is a significant logarithmic positive correlation between cohesion and root morphological characteristics. The influence degree of root morphology index on soil shear resistance is as follows: Root surface area density > root volume density > root length density > root weight density > root diameter. 2) Compared with rootless soil, the shear strength of root-soil composites with root contents of 0.9, 0.6 and 0.3 mg/cm³ increase by 14.54%-49.56%, 9.09%-35.82% and 3.80%-17.58%. The internal friction angle increases by 13.95%, 13.07% and 1.98%, respectively. The cohesion increases by 29.37%, 24.48% and 15.34%, respectively. 3) Compared with vertical and horizontal layouts, the shear strength of cross layout increases by 4.22%-16.95% and 11.45%-32.61% respectively. Cohesion increases by 15.61% and 16.09% respectively. The maximum stress values increases by 3.78%-34.18% and 36.86%-41.44% respectively. The effect of root layouts on cohesion is greater than that of internal friction angle.

Conclusions To sum up, root morphological characteristics, root content and root layouts all have significant effects on the soil shear resistance of root-soil composite in the water-level fluctuation zone of the Three Gorges Reservoir. This study provides scientific reference for species screening, community construction, ecological construction of soil and water conservation, ecosystem reconstruction, and reservoir management in the water-level fluctuation zone of the Three Gorges Reservoir.