Study of factors influencing soil water repellency under simulated forest fire

Yunnan pine, as the prominent arboreal species in the southwestern mountains, exhibits abundant resin content and displays susceptibility to forest fires. Soil in fire-affected areas typically exhibits water repellency after a forest fire, leading to increased surface runoff and erosion within the affected area, ultimately triggering post-fire debris flows. In order to investigate the influencing factors and formation mechanism of soil water repellency, unburned soils in the pine forest vegetation area of Lawo Township, Mianning County, Sichuan Province was selected as the research project. Indoor simulated fire experiment were conducted, and orthogonal experiments were designed considering factors such as turpentine content, soil debris content, initial organic content, heating temperature, and heating period. X-ray diffraction and scanning electron microscopy were used to analyze changes in chemical elements and microstructure, studying the impact of forest fires on soil water repellency. The results showed that the degree of influence of various factors on soil water repellency was ranked as follows: initial organic matter content, heating temperature, heating period, pine resin content, and debris content. The initial organic matter content and heating temperature on soil water repellency was highly significant, while heating time and pine resin content was significant, and debris content had no significant impact. All factors could cause changes in organic compounds in the soil, thereby affecting soil water repellency. Specifically, after moderate burning (temperature ≤ 400 °C), organic compounds in the soil underwent chemical changes, forming dense hydrophobic organic matter films covering the surface of mineral particles and filling the interstitial space of the particles, resulting in enhanced soil water repellency. Excessive burning (temperature > 400 °C) resulted in the consumption of organic matter, the destruction of soil microaggregate structure, loose stacking of mineral particles, and a decrease in soil water repellency. The research results can provide a basis for the soil erosion pattern of soil in pine forest fire-affected areas and the formation mechanism of post-fire debris flows.