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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. 

Abstract Nutrient foraging by fungi weathers rocks by mechanical and biochemical processes. Distinguishing fungal-driven transformation from abiotic mechanisms in soil remains a challenge due to complexities within natural field environments. We examined the role of fungal hyphae in the incipient weathering of granulated basalt from a three-year field experiment in a mixed hardwood-pine forest (S. Carolina) to identify alteration at the nanometer to micron scales based on microscopy-tomography analyses. Investigations of fungal-grain contacts revealed (i) a hypha-biofilm-basaltic glass interface coinciding with titanomagnetite inclusions exposed on the grain surface and embedded in the glass matrix and (ii) native dendritic and subhedral titanomagnetite inclusions in the upper 1–2 µm of the grain surface that spanned the length of the fungal-grain interface. We provide evidence of submicron basaltic glass dissolution occurring at a fungal-grain contact in a soil field setting. An example of how fungal-mediated weathering can be distinguished from abiotic mechanisms in the field was demonstrated by observing hyphal selective occupation and hydrolysis of glass-titanomagnetite surfaces. We hypothesize that the fungi were drawn to basaltic glass-titanomagnetite boundaries given that titanomagnetite exposed on or very near grain surfaces represents a source of iron to microbes. Furthermore, glass is energetically favorable to weathering in the presence of titanomagnetite. Our observations demonstrate that fungi interact with and transform basaltic substrates over a three-year time scale in field environments, which is central to understanding the rates and pathways of biogeochemical reactions related to nuclear waste disposal, geologic carbon storage, nutrient cycling, cultural artifact preservation, and soil-formation processes. 

Nanomaterials are critical components in the Earth system’s past, present, and future characteristics and behavior. They have been present since Earth’s origin in great abundance. Life, from the earliest cells to modern humans, has evolved in intimate association with naturally occurring nanomaterials. This synergy began to shift considerably with human industrialization. Particularly since the Industrial Revolution some two-and-a-half centuries ago, incidental nanomaterials (produced unintentionally by human activity) have been continuously produced and distributed worldwide. In some areas, they now rival the amount of naturally occurring nanomaterials. In the past half-century, engineered nanomaterials have been produced in very small amounts relative to the other two types of nanomaterials, but still in large enough quantities to make them a consequential component of the planet. All nanomaterials, regardless of their origin, have distinct chemical and physical properties throughout their size range, clearly setting them apart from their macroscopic equivalents and necessitating careful study. Following major advances in experimental, computational, analytical, and field approaches, it is becoming possible to better assess and understand all types and origins of nanomaterials in the Earth system. It is also now possible to frame their immediate and long-term impact on environmental and human health at local, regional, and global scales.