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The interaction between soil minerals and soil organic matter (SOM) plays an important role in governing carbon release and sequestration in soil, yet understanding their behavior during wildfires remains poorly understood. This study examined the evolution of humic acid (HA, a representative of SOM) under simulated wildfire heating conditions (30–900 °C) in the presence of two representative soil minerals: montmorillonite (Mnt) and ferrihydrite (Fhy). Whereas Fhy accelerated the mineralization of HA, Mnt enhanced its preservation. These disparities stemmed from variations in the surface reactivity, structure, and transformations of Fhy and Mnt. Lewis acid sites, more abundant on Fhy surfaces than on Mnt surfaces, enhanced the decarboxylation of HA and caused carbon losses as CO2. However, Brønsted acid sites, which are more abundant on Mnt surfaces than on Fhy surfaces, enhanced carbon preservation by promoting HA isomerization and aromatization. Above 350 °C, lattice oxygen release from Fhy promoted the oxidative decomposition of HA, while Fhy itself underwent reduction to form magnetite, wüstite, and zero-valent iron. The confinement of HA within the micro/mesopores created by Mnt’s inert nanolayers prevented the thermal degradation of HA, enhancing carbon preservation. These findings advance our understanding of the specific roles of soil minerals in the decomposition, transformation, and preservation of SOM during wildfires. 

Forested watersheds are instrumental in providing purified and reliable water to millions of people worldwide. The changing climate has increased the frequency and severity of global fire events. Forested watersheds and their ecosystem functions are greatly disrupted during fire activity. Postfire concerns in forested watersheds include unpredictable and potentially simultaneous alterations in source water quality and hydro-biogeochemical processes. The degree of fire severity can complexly modify water quality through the production of fire-transformed constituents on the burned forest floor (i.e., nutrients, metal(loid)s, dissolved organic matter, and the formation of disinfection byproducts). Correspondingly, fire severity and postfire rainfall patterns can refine hydro-biogeochemical processes that influence the transport of the fire-transformed constituents (i.e., vegetation function, soil structure, hydrological pathways, and microbial communities). Postfire alterations to water quality and hydro-biogeochemical processes introduce further complexity with varying temporal influence, which ranges from months to decades. As postfire water quality and watershed response research progresses, it is essential to homogenize interdisciplinary expertise to bridge knowledge gaps between fields ranging from forest ecology, hydrology, microbiology, and geochemistry. A multidisciplinary approach in wildfire research will facilitate a comprehensive perception of the diverse water quality risks associated with fire activity and mitigate fire concerns on a global level.