Post-Wildfire Soil Properties Changes: Insights Into Hillslope Erosion After the March 2024 Yajiang Fire

Soil property changes influence material transport from hillslopes to channels after the wildfire and may indirectly trigger debris flow initiation. This study investigates post-fire soil property evolution and its role in hillslope erosion following the 15 March 2024 Yajiang Fire, integrating field measurements with laboratory simulations to quantify temperature- and duration-dependent soil changes and their controls. Results demonstrate that wildfire-driven soil organic matter alteration is governed predominantly by peak heating temperature and not exposure duration. Post-fire soil profiles are stratified into three thermal impact zones: (a) a high-temperature zone (>600°C), characterized by complete organic matter combustion; (b) a water-repellent (WR) zone (100–600°C), subdivided into a highly WR layer, an aggregate stability (AS)-enhancement layer, and a low WR layer; and (c) an unaffected zone (<100°C). The high-temperature zone and highly WR layer, mobilized by wind and gravity, accumulate in channels as dry ravel deposits, forming the primary source of immediate post-fire debris flows. The AS-enhancement layer, exhibiting improved aggregate stability, temporarily mitigates raindrop splash and interrill erosion of underlying soils prior to the first post-fire rainfall event. However, this layer delays but does not prevent deeper soil mobilization during subsequent intense runoff. These findings clarify temperature thresholds controlling post-fire soil zonation, highlight mechanisms linking soil property changes to debris flow initiation, and provide valuable data on post-fire hillslope erosion processes.