Magnetic Properties of Plant Ashes and Their Influence on Magnetic Signatures of Fire in Soils

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.

A Review of Speleothems as Archives for Paleofire Proxies, With Australian Case Studies

“…Till et al. (2021) also found that (for the Icelandic samples) total plant iron and saturation magnetization strongly correlated with titanium and aluminum, suggesting that inorganic materials (soil and dust) were the source of iron‐bearing magnetic phases. They conclude that vegetation ash in soil is a function of burn extent, season, chemical composition of dust and soil and plant type (Till et al., 2021).…”

“…Magnetic enhancement noted in fire pits has previously been attributed to inputs of ferromagnetic materials from burned vegetation (McClean & Kean, 1993). More recently, experimental burning of plant samples from Iceland and Germany demonstrated that magnetic mineral concentrations increased with the extent of burning, although the amount of magnetic material did not always scale proportionally (Till et al., 2021). Till et al.…”

Discovery and potential ramifications of reduced iron-bearing nanoparticles—magnetite, wüstite, and zero-valent iron—in wildland–urban interface fire ashes

“…These findings suggest that wildfires contribute to the magnetic enhancement of topsoil as reported elsewhere. 29,93 Understanding the impact of fires on soil magnetism is important for accurate interpretation of magnetic paleoenvironmental proxies.…”

“…A recent study demonstrated the magnetic properties of plant ashes, which result from the thermal transformation of Fe in both organic and inorganic particulate matter. 29 Another study demonstrated the presence of magnetite (γ-Fe 2+ (Fe 3+ ) 2 O 4 ) and/or maghemite (γ-(Fe 3+ ) 2 O 3 ) particles in burned soils and plants characterized by spherical shapes and sizes typically between 0.1 and 2 μm. 30 The degree of pyrogenic magnetic enhancement of fire-affected soils is strongly related to the wildfire severity.…”

Magnetic Susceptibility Profile, Fe Content and pH Analysis of Apple Orchard Soil and the Relation to Fruit Diameter

“…The magnetic properties are controlled by the types, size of grain, quantity, and morphology of magnetic mineral in the soil. The benefits of Fe element in the plants have also been explored among them as the controller of the photosynthesis process (Briat et al, 2010;Donnini et al, 2013;Till et al, 2021). The existence of the iron element in the soil and its relation with soil acidity properties can be described that the more acidic the soil, the smaller the iron mineral since iron dissolves in acidic substance.…”