More Like this

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

   https://doi.org/10.3389/feart.2020.592659  

   Till, Jessica L.; Moskowitz, Bruce; Poulton, Simon W. (January 2021, Frontiers in Earth Science)

   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. 

   more » « less
2. Wildfire Ash Composition and Engineering Behavior

   https://doi.org/10.1061/JGGEFK.GTENG-11683  

   Wirth, Xenia; Antunez, Vanessa; Enriquez, Dezire; Arevalo, Zuleyma; Kanaan, Ramzieh (June 2024, Journal of Geotechnical and Geoenvironmental Engineering)

   After a wildfire event, ash is a newly formed surficial soil layer with microscale properties such as roughness, morphology, and chemical composition that may impact how ashes form fabrics in situ and so affect the overall hydrological conditions of a burned area (infiltration capacity, permeability, etc.). To examine the effects of ash microscale properties on macroscale behavior, eight wildfire ash samples from California were characterized physically (specific gravity, specific surface area, particle size, etc.), chemically (elemental composition, organic and inorganic carbon content, etc.), and geotechnically (strength, compaction, saturated hydraulic conductivity, etc.). The tested ashes were found to contain predominantly organic unburned carbons and carbonates derived from the combustion of calcium-oxalate rich fuels in temperatures likely ranging from 300°C to 500°C. Ashes had high specific surface areas because morphologically, particles had highly texturized and porous surfaces. Additional water was necessary to coat the particle surfaces, which led to high liquid limits and compaction optimum moisture contents. Hydraulic conductivity values were within range for silty sands (10^−5–10^−3  cm/s), and specimens had friction angles near 30°. However, tested ashes consistently demonstrated high void ratios and low bulk densities during testing for strength, hydraulic conductivity, and compaction. These anomalies were attributed to unusual carbonate morphologies; the high interparticle friction of these phases allowed ashes to form looser fabrics than a typical silty sand and contributed to the measured high void ratios, low maximum dry unit weights, and high friction angles. Overall, we hypothesize that the relative amounts of inorganic versus organic constituents in our wildfire ash samples affected how the ashes formed fabrics and so affected their geotechnical properties. 

   more » « less
3. Engineering properties of wildfire ashes

   Wirth, X; Antunez, V; Fregoso-Sanchez, D; Enriquez, D; Arevalo, Z (June 2023, Proceedings of the 9ICEG)

   While a large amount of research has been performed in recent years on characterizing wildfire ashes and determining the hydrological properties of slopes post-wildfire, fewer studies have concentrated on the overall engineering behavior of ash. This study addresses this need by examining the compaction, shear strength, and hydraulic behavior of wildfire ash and ash/soil layered specimens. Unique chemical and physical characteristics of wildfire ashes were shown to influence the ash engineering behavior. Ashes in the as-received condition have a silty sand grain size distribution with higher than expected surface areas (>1 m2/g). Chemically, ashes contained silica, aluminum, calcium (in the form of carbonates) and residual organic carbon from incomplete combustion. Maximum dry unit weights ranged from 13 – 16 kN/m3 at optimum moisture contents between 20 and 30%. Hydraulic conductivity of samples varied between 10^-4 and 10^-5 cm/s. A wet deposited layer of fine-grained ash on top of compacted sand reduced the hydraulic conductivity of sand by 1 – 3 orders of magnitude. Shear strength of ash/sand layered specimens demonstrated that ash was fairly stiff, with an average friction angle of 28 degrees. Void ratios of specimens were consistently higher than expected for a silty sand fabric (usually above 1.0). Ash particles were irregular in shape with fibrous textures and had electrostatic attractive tendencies. The authors suspect that the unique chemistries present in ash (notably carbonates and organic char) contributed to the loose fabric structure and engineering properties that were atypical of a silty sand grain size distribution 

   more » « less
4. Discovery and potential ramifications of reduced iron-bearing nanoparticles—magnetite, wüstite, and zero-valent iron—in wildland–urban interface fire ashes

   https://doi.org/10.1039/d2en00439a  

   Baalousha, Mohammed; Desmau, Morgane; Singerling, Sheryl A.; Webster, Jackson P.; Matiasek, Sandrine J.; Stern, Michelle A.; Alpers, Charles N. (November 2022, Environmental Science: Nano)

   The increase in fires at the wildland–urban interface has raised concerns about the potential environmental impact of ash remaining after burning. Here, we examined the concentrations and speciation of iron-bearing nanoparticles in wildland–urban interface ash. Total iron concentrations in ash varied between 4 and 66 mg g −1 . Synchrotron X-ray absorption near-edge structure (XANES) spectroscopy of bulk ash samples was used to quantify the relative abundance of major Fe phases, which were corroborated by transmission electron microscopy measurements. Maghemite (γ-(Fe 3+ ) 2 O 3 ) and magnetite (γ-Fe 2+ (Fe 3+ ) 2 O 4 ) were detected in most ashes and accounted for 0–90 and 0–81% of the spectral weight, respectively. Ferrihydrite (amorphous Fe( iii )–hydroxide, (Fe 3+ ) 5 HO 8 ·4H 2 O), goethite (α-Fe 3+ OOH), and hematite (α-Fe 3+ 2 O 3 ) were identified less frequently in ashes than maghemite and magnetite and accounted for 0–65, 0–54, and 0–50% of spectral weight, respectively. Other iron phases identified in ashes include wüstite (Fe 2+ O), zerovalent iron, FeS, FeCl 2 , FeCl 3 , FeSO 4 , Fe 2 (SO 4 ) 3 , and Fe(NO 3 ) 3 . Our findings demonstrate the impact of fires at the wildland–urban interface on iron speciation; that is, fires can convert iron oxides ( e.g. , maghemite, hematite, and goethite) to reduced iron phases such as magnetite, wüstite, and zerovalent iron. Magnetite concentrations ( e.g. , up to 25 mg g −1 ) decreased from black to gray to white ashes. Based on transmission electron microscopy (TEM) analyses, most of the magnetite nanoparticles were less than 500 nm in size, although larger particles were identified. Magnetite nanoparticles have been linked to neurodegenerative diseases as well as climate change. This study provides important information for understanding the potential environmental impacts of fires at the wildland–urban interface, which are currently poorly understood. 

   more » « less
5. Persistence of Soil Water Repellency After the 2022 Bolt Creek Fire

   https://doi.org/10.3390/geosciences15120472  

   Demir, Mustafa; Robichaud, Peter R; Akin, Idil Deniz (December 2025, Geosciences)

   Wildfire ash and water-repellent soil are new materials that are formed after a wildfire that change the mechanical and hydraulic behavior of wildfire-burned slopes. Wildfire ash is known to be typically hydrophilic and to retain water, whereas the water-repellent soil layer acts as a hydraulic barrier. However, there is limited in situ soil water content data to understand the short- and long-term impacts of wildfire ash and a water-repellent soil layer on the hydromechanical behavior of burned slopes. This study investigates the trends in water content of wildfire ash, water-repellent soil, and subsurface soil after the 2022 Bolt Creek Wildfire near Skykomish, WA. The ash deposit averaged 10 cm, with a maximum 30 cm thickness in channels immediately after the fire, which allowed the in situ measurement of ash water content. Soil water content sensors were installed in the ash and subsurface soil layers, and changes in the water content were monitored for a year after the fire. The surface ash layer was above a thin (<1 cm) water-repellent soil layer, which was followed by the soil that did not show any apparent effects from the fire. The results showed a reduction in ash thickness and the persistence of the water-repellent layer over a year. 

   more » « less