Moderate Fire Temperatures Affect the Structure of Clayey Oxisol Aggregates

ABSTRACT Wildfires have become increasingly frequent and intense worldwide, raising concerns about their impacts on soil systems. However, the thermal responses of clay‐rich Oxisols, dominated by Fe and Al oxides, remain poorly understood. In this study, samples from the 0 to 5 cm layer of six Oxisols were subjected to controlled heating at temperatures ranging from 100°C to 600°C, and two complementary indicators of soil structural quality were evaluated: aggregate stability (AS) and tensile strength (TS). A critical threshold was observed at 300°C, when soil organic matter decreased sharply and TS dropped by more than 30%, accompanied by the appearance of microcracks in aggregates. In contrast, AS increased progressively with heating, reflecting the contribution of mineral transformations at higher temperatures. These contrasting responses demonstrate that AS and TS are governed by distinct mechanisms, with AS primarily influenced by mineral transformations and TS by organic matter degradation under thermal stress. The findings highlight the vulnerability of Oxisols to moderate fire temperatures and provide insights to guide soil recovery strategies after burning.