An official website of the United States government
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
This content will become publicly available on December 1, 2026
Persistence of Soil Water Repellency After the 2022 Bolt Creek Fire
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
- Award ID(s):
- 2421382
- PAR ID:
- 10656324
- Publisher / Repository:
- MDPI
- Date Published:
- Journal Name:
- Geosciences
- Volume:
- 15
- Issue:
- 12
- ISSN:
- 2076-3263
- Page Range / eLocation ID:
- 472
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
The Caldor fire burned ~222,000 acres of the Eastern Sierra Nevada during summer–fall 2021. We evaluated the effects of this “megafire” on the physical properties of a sandy soil developed from glacial tills to document fire-induced soil modifications in this region. We measured soil water retention and hydraulic conductivity functions as well as the thermal properties of five core samples from control (unburned) areas and eight core samples from burned soil of the same soil unit. Soil water repellency was measured in terms of water drop penetration time (WDPT) in the field and apparent contact angle in the laboratory on control and burned soil as well as ash samples. Soil organic matter (SOM) and particle and aggregate size distributions were determined on control and burned soil samples. Additionally, scanning electron microscopy (SEM) was used to image microaggregates of control and burned soil samples. We found a significant difference in SOM content and sand and silt aggregate size distribution between control and burned samples, which we associated with the disintegration of microaggregates due to the fire. We found no significant difference between soil water retention and hydraulic conductivity functions of control and burned soil but observed greater variation in saturated hydraulic conductivity and systematic shifts in thermal conductivity functions of burned compared to control samples. WDPT and apparent contact angle values were significantly higher for burned soils, indicating the occurrence of fire-induced soil hydrophobicity (FISH). Interestingly, the average apparent contact angle of the control soil was >90°, indicating that even the unburned soil was hydrophobic. However, the ash on top of the burned soil was found to be hydrophilic, having apparent contact angles <10°. Our results indicate that SOM and microaggregates were readily affected by the Caldor fire, even for sandy soil with a weakly developed structure. The fire seemed to have moderated thermal properties, significantly and soil wettability but had only minimal effects on water retention and hydraulic conductivity functions. Our findings demonstrate the complex nature of fire-soil interactions in a natural environment and highlight the need for additional investigation into the causes and processes associated with FISH and structure alterations due to fire to improve our ability to rapidly determine potential problem areas in terms of hazards commonly associated with fire-affected soils.more » « less
-
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 distributionmore » « less
-
Abstract Wildfire is a disturbance expected to increase in frequency and severity, changes that may impact carbon (C) dynamics in the soil ecosystem. Fire changes the types of C sources available to soil microbes, increasing pyrogenic C and coarse downed wood, and if there is substantial tree mortality, decreasing C from root exudates and leaf litter. To investigate the impact of this shift in the composition of C resources on microbial processes driving C cycling, we examined microbial activity in soil sampled from an Oregon burn 1 year after fire from sites spanning a range in soil burn severity from unburned to highly burned. We found evidence that postfire rhizosphere priming loss may reduce soil C loss after fire. We measured the potential activity of C‐acquiring and nitrogen (N)‐acquiring extracellular enzymes and contextualized the microbial resource demand using measurements of mineralizable C and N. Subsurface mineralizable C and N were unaltered by fire and negatively correlated with hydrolytic extracellular enzyme activity (EEA) in unburned, but not burned sites. EEA was lower in burned sites by up to 46%, but only at depths below 5 cm, and with greater decreases in sites with high soil burn severity. These results are consistent with a subsurface mechanism driven by tree mortality. We infer that in sites with high tree mortality, subsurface EEAs decreased due to loss of rhizosphere priming and that inputs of dead roots contributed to mineralizable C stabilization.more » « less
-
Wildfires can change ecosystems by altering solutes in streams. We examined major cations in streams draining a chaparral-dominated watershed in the Santa Ynez Mountains (California, USA) following a wildfire that burned 75 km2 from July 8 to October 5, 2017. We identified changes in solute concentrations, and postulated a relation between these changes and ash leached by rainwater following the wildfire. Collectively, K+ leached from ash samples exceeded that of all other major cations combined. After the wildfire, the concentrations of all major cations increased in stream water sampled near the fire perimeter following the first storm of the season: K+ increased 12-fold, Na+ and Ca2+ increased 1.4-fold, and Mg2+ increased 1.6-fold. Our results suggested that the 12-fold increase in K+ in stream water resulted from K+ leached from ash in the fire scar. Both C and N were measured in the ash samples. The low N content of the ash indicated either high volatilization of N relative to C occurred, or burned material contained less N.more » « less
