Fire disturbance intensity regulates soil aggregate stability and water repellency in dry-hot valleys of southwest China

Fire disturbance regulates soil aggregate stability (AS) and water repellency (WR) by altering soil organic matter (SOM), thereby influencing soil and water conservation functions in ecologically fragile regions. However, the mechanisms underlying the dynamic responses of AS and WR to combined variations in fire intensities and land-use types remain unclear. This study investigated grassland, shrubland, and forestland to elucidate how fire disturbance affects soil AS and WR. By integrating field sampling, laboratory-controlled heating, Mantel tests, and partial least squares path modelling (PLS-PM), this study demonstrates that AS and WR exhibit non-linear responses to fire disturbance intensity. Under low fire disturbance (LFD), the proportion of water-stable aggregates >0.25 mm ( R 0.25 ), mean weight diameter (MWD), geometric mean diameter (GMD) and water drop penetration time (WDPT) increased by 7.77%, 13.43%, 20.68%, and 11.59%, respectively, compared with no fire disturbance (NFD). In contrast, moderate fire disturbance (MFD) and high fire disturbance (HFD) weakened these responses. Under HFD, all three land-use types exhibited reduced AS, and WR remained <5 s, indicating hydrophilicity behavior. Land-use type also influenced variations in AS, WR, and SOM to fire disturbance. Under LFD, forestland showed the largest increase in WR, with WDPT increasing by 19.85% relative to NFD. In contrast, high-intensity burning, shrubland exhibited the greatest losses in both SOM and AS, Under HFD, SOM decreased by 96.75% compared with NFD, while GMD declined to 0.48 mm, corresponding to a 35.74% reduction relative to NFD. SOM was identified as the primary factor driving the dynamic variation in AS and WR. PLS-PM analysis revealed that SOM exerted strong positive effects on WR (path coefficients: 0.47–1.53; P < 0.05) and AS (0.52–1.15; P < 0.05). Mantel tests further confirmed that SOM, soil water content (SW), and pH exhibited the strongest explanatory power for AS and WR ( r ≥ 0.4, P < 0.05). Overall, this study elucidates SOM-dominated changes in AS and WR under fire disturbance. These findings provide new insights into the combined effects of fire intensity and land-use type on soil properties and offer a scientific basis for post-fire soil restoration and soil and water conservation.