Background A forest fire occurred in Hutou village, Xiema street, Beibei district, Jinyun Mountain National Nature Reserve, Chongqing.In order to further study the harm caused by forest fires and promote their post-disaster reconstruction, the author carried out an investigation in the burned area of Jinyun Mountain Nature Reserve, collected soil samples to determine the saturated permeability coefficient and soil-water characteristic curve of soil, and studied the response mechanism of fire process to soil hydraulic characteristics, in order to provide reference for the prevention and control of geological disasters in the burned areas.

Methods The basic physical properties such as liquid limit, bulk density and saturated water content under different fire degrees were measured by equilibrium cone method and ring knife method. The saturated permeability coefficient k_s of shallow root soil was measured by soil variable head permeability test. The particle size distribution curve was measured by laser particle size analyzer and the soil water characteristic curve was fitted based on van-Genuchten (VG) model.

Results 1) After the forest fire, the bulk density, saturated water content, liquid limit, porosity and other physical properties of the shallow surface soil increased, and the number of roots in the burned area decreased. Among them, the number of fine roots 0-2 mm changed the most, which decreased by about 2-3 times. 2) For the three degrees of mild, moderate and severe fire, the saturated permeability coefficient of Pinus massoniana forest and Phyllostachys pubescens forest increased after fire. In particular, the permeability coefficient of the surface soil in the severely burned area of P. massoniana forest increased by 58.26 times compared with that of the unburned shallow soil, and the permeability coefficient of the surface soil in the severely burned area of P. pubescens forest increased by 162.24 times. 3) The results showed that the soil particle size in the fire area was smaller than that in the non-fire area, and the air intake value parameter α and the pore size distribution parameter n of the soil water characteristic curve of the shallow root soil in the fire area decreased with the increase of the degree of fire, and the pore size distribution parameter n increased with the increase of the degree of fire.

Conclusions Forest fires lead to a decrease in the number of fine roots in the surface soil and an uneven distribution in the soil layer, and affect the physical properties of the surface soil, so that the pore size distribution changes, which further leads to the increase of the saturated permeability coefficient and the weakening of the water holding capacity. In the post-disaster recovery of geological disasters in burned areas, the response mechanism of the above soil hydraulic characteristics should be considered.