Soil cementation function degradation and aggregate stability decline after fire disturbance and their effects on erodibility and erosion intensity

Previous studies have confirmed that fire disturbance is one of the most important driving factors influencing changes in soil erodibility. However, the underlying mechanisms and quantitative relationships between fire disturbance and soil erodibility remain insufficiently understood. To further investigate the effects of different fire severity levels on soil aggregate stability, soil cementation function, and soil erodibility, as well as to reveal the spatial distribution characteristics of post-fire soil erosion, this study selected the burned area of the Jinyun Mountain wildfire in Chongqing, China, which occurred in August 2022, as the research site. By integrating multi-source remote sensing data with field-measured soil data, a comparative analysis was conducted among four fire severity treatments, including a control group (CK), low severity fire (I), moderate severity fire (II), and high severity fire (III). Variations in soil organic carbon (SOC), total nitrogen (TN), aggregate stability, and the soil erodibility factor were systematically examined. Furthermore, path analysis was employed to identify the intrinsic driving mechanisms responsible for post-fire changes in soil erodibility. The results demonstrated that fire disturbance significantly weakened the soil cementation function and altered key soil physicochemical properties. With increasing fire severity, SOC content exhibited a continuous decreasing trend, indicating substantial losses of organic matter caused by thermal combustion and decomposition. At the same time, total nitrogen content decreased significantly compared with the control treatment, reflecting the considerable volatilization and depletion of nitrogen-containing compounds during the fire process. These reductions in organic matter and nutrient availability weakened the binding capacity between soil particles, thereby impairing the overall structural integrity of the soil. In addition, fire disturbance substantially modified the physical structure of the soil. Compared with the control group, post-fire soils showed evident fragmentation of soil aggregates. The proportion of macroaggregates (>2 mm), which play a crucial role in maintaining soil structural stability and resistance to erosion, decreased significantly as fire severity increased. Conversely, the proportion of aggregates within the > 0.25–2 mm size range increased after fire disturbance, indicating the breakdown of larger aggregates into smaller structural units. As a consequence, the mean weight diameter, an important indicator of aggregate stability, decreased significantly across burned sites. These findings suggest that increasing fire severity accelerates aggregate disintegration and weakens the soil’s resistance to external erosive forces.