Background The root system has significant influence on the mechanical properties of soil and can play a role in soil erosion and shallow landslide. After the occurrence of forest fires, the surface soil was affected, and the slope-fixing effect of plant roots was significantly weakened. However, the effects of forest fires on the mechanical properties of the root-soil system and root corruption have not been fully studied.

Methods To reveal the weakening mechanism of fire on root soil fixation ability, on August 21, 2022, Chongqing Jinyun mountain national nature reserve of fire occurring has carried out investigation. According to the fire intensity classification criteria, the burned severity was divided into three grades: low, moderate and high. In this study, plant root investigation and soil sample collection were carried out on two kinds of forest types and three burn severity of fire areas within one month after the disaster. Collecting soil samples from different degree of fire area, electronic universal testing machine was used to measure root tensile strength, ZJ direct shear device to measure shear strength of soil-root systems.

Results 1) There was a small difference in the number of fine roots between the lowly burned area and the unburned area, while the number of fine roots in the moderately and the highly burned area decreased significantly, on the roots of the diameter of less than 2 mm, theropencedrymion in lowly, moderately and highly burned area, respectively, than those unburned area decreased by 16%, 28% and 44%, bamboo forest decreased by 17%, 25% and 50% respectively. 2) The root tensile strength decreased as a power function with the diameter. Compared with the root tensile strength before and after fire, the root tensile strength decreased with the increase of fire intensity. 3) When the normal stress was the same, the shear strength of the root-soil complex in the unburned area of theropencedrymion and bamboo forest was significantly greater than that in the burned area, and the shear strength decreased significantly in the moderately burned area and the highly burned area. 4) The soil cohesion was significantly affected by the burned severity. The soil cohesion of 0-10 cm in the highly burned area of coniferous and theropencedrymion was 86% lower than that in the unburned area, and the soil cohesion of 20-30 cm in the highly burned area of bamboo forest was 72% lower.

Conclusions This study revealed the change of mechanical properties of root-soil complex and root failure mechanism of different burned severity and forest types after forest fire. The result may provide a basis for the study of the effects of forest fires on the mechanical properties of root-soil complex and its related mechanisms, and provide data support for the assessment of the vulnerability of geological disasters such as shallow landslides after disasters, as well as the prevention and reduction of post-disasters.