Abstract
Fires are an integral part of many terrestrial ecosystems and have a strong impact on soil properties. While reports of topsoil magnetic enhancement after fires vary widely, recent evidence suggests that plant ashes provide the most significant source of magnetic enhancement after burning. To investigate the magnetic properties of burnt plant material, samples of individual plant species from Iceland and Germany were cleaned and combusted at various temperatures prior to rock magnetic and geochemical characterization. Mass-normalized saturation magnetization values for burnt plant residues increase with the extent of burning in nearly all samples. However, when normalized to the loss on ignition, fewer than half of ash and charcoal samples display magnetic enhancement relative to intact plant material. Thus, while magnetic mineral concentrations generally increase, changes in the total amount of magnetic material are much more variable. Elemental analyses of Icelandic samples reveal that both total plant Fe and saturation magnetization are strongly correlated with Ti and Al, indicating that most of the Fe-bearing magnetic phases originate from inorganic material such as soil and atmospheric dust. Electron microscopy confirmed that inorganic particulate matter remains on most plant surfaces after cleaning. Plants with more textured leaf surfaces retain more dust, and ash from these samples tend to exhibit higher saturation magnetization and metal concentrations. Magnetic properties of plant ash therefore result from the thermal transformation of Fe in both organic compounds and inorganic particulate matter, which become concentrated on a mass basis when organic matter is combusted. These results indicate that the soil magnetic response to burning will vary among sites and regions as a function of 1) fire intensity, 2) the local composition of dust and soil particles on leaf surfaces, and 3) vegetation type and consequent differences in leaf morphologies.
Highlights
Wildfires and anthropogenic burning affect many aspects of soils, including their magnetic properties
The shape of the χ vs. T curves are different for char and ash produced from the same plant sample, suggesting that heating to the higher temperatures used for ashing produces magnetic phases with somewhat different compositions than those resulting from lower temperature treatments
A close correlation between total Fe and Ti in the Icelandic plant samples indicates that a significant amount of the iron in plant ash is derived from inorganic particulate matter embedded in plant surfaces, which dominates the magnetic properties of the ash
Summary
Wildfires and anthropogenic burning affect many aspects of soils, including their magnetic properties. Fire-induced magnetic enhancement may occur through thermal transformation of weakly magnetic ferric oxide and oxyhydroxide soil minerals such as ferrihydrite, lepidocrocite, goethite, and hematite (Hanesch et al, 2006; Till et al, 2014; Jiang et al, 2018; Till and Nowaczyk, 2018) to strongly magnetic phases such as magnetite or maghemite. Such reactions likely contribute to the enhanced magnetic susceptibility observed in some burnt soils compared with unburnt soils (Gedye et al, 2000; Blake et al, 2006), in forest fires and similar high-intensity burning events. A study by Roman et al (2013) on grassland fire temperatures concluded that fire is an unlikely source of significant magnetic enhancement in prairie soils based on the relatively low temperatures experienced by the soil compared with temperatures necessary to produce mineralogical changes during laboratory heating
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