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1. Reduction in hydraulic conductivity for high burn severity soils begins after the first rainfall event: Results from laboratory-scale rainfall simulation experiments

   https://doi.org/10.17221/26/2026-SWR  

   Frimpong, Nana_Afua Gyau; Huerta, Jacob; Webb, Ryan (June 2026, Soil and Water Research)

   Wildfires can greatly impact the hydraulic properties of soil. This study aims to utilize laboratory experiments to simulate burned soil conditions under a range of slope angles and rainfall intensities to address the research question: How does the presence of ash after the first rainfall event impact the hydraulic properties of burned soil in complex terrain? Sandy loam soils for this study were sourced from a mixed conifer forested area in the San Juan Mountains of southern Colorado, USA. Measurements of soil hydraulic properties in experimental microplots were taken (1) before burning, (2) after burning, and (3) 24 hours after a rainfall simulation. A total of 15 experimental microplots with ash applied were run through rainfall simulations at slope angles ranging from 10° to 30°. Results found ash eroded had no significant relation to slope angle, but there was a significant reduction in field saturated hydraulic conductivity of samples with ash after rainfall simulations. To build on the findings presented here, future research should conduct field-based studies across various ecosystems and soil types to observe post-fire soil changes over time. 

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2. Assessment of the Effects of the 2021 Caldor Megafire on Soil Physical Properties, Eastern Sierra Nevada, USA

   https://doi.org/10.3390/fire6020066  

   Sion, Brad; Samburova, Vera; Berli, Markus; Baish, Christopher; Bustarde, Janelle; Houseman, Sally (February 2023, Fire)

   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. 

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3. Wildfire and Extreme Rainfall Reduce Soil Carbon and Nitrogen Pools in a Semiarid Shrubland Ecosystem

   https://doi.org/10.1029/2025JG009183  

   Lipson, D A; Alvarez, A G; Brouwer, J N; Cleland, E E; Lively, J B; Parsons, D S; Rivas, Fortunato Vincente; Smith, K L; Varon, E L; Xu, X (February 2026, Journal of Geophysical Research: Biogeosciences)

   Abstract Due to a combination of anthropogenic causes, semi‐arid shrublands like those in California and other Mediterranean‐type ecosystems are increasingly subject to wildfires and variable rainfall, including drought and atmospheric rivers. Fire changes soil structure, leaving soils vulnerable to erosion and loss of organic matter (OM), especially when followed by intense rain events. However, interactions of fire and extreme rainfall are challenging to study due to their unpredictability and the complex interacting factors such as fire intensity, vegetation structure, topography, and soil type. This study took advantage of a wildfire, natural variation in rainfall, and a controlled rain‐shelter experiment to examine the interactions of fire and rain on soil structure and OM storage. Fire reduced organic carbon and nitrogen associated with particulate organic matter and water stable aggregates, and changed soil hydrology, slowing infiltration of water to deeper layers. The combination of rain shelters and the extreme rain fall year (2023) allowed us to evaluate threshold effects of rain in post‐fire soils: in 2023, there was a loss of total soil OM from burned relative to unburned plots; rain shelter effects on soil organic matter pools were only detectable in 2023; and the interactive effects of fire and rain shelters in this year reduced infiltration of rain into soil in burned areas exposed to full rain conditions. These results suggest a feedback mechanism in which fire causes loss of soil OM, damages soil structure, and decreases rainfall infiltration, increasing erosion through increased runoff and slowing recovery of soil structure. 

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4. A Case Study of Soil Moisture and Infiltration after an Urban Fire

   https://doi.org/10.3390/fire3020022  

   Alkin, Quinn; Kinoshita, Alicia M. (June 2020, Fire)

   null (Ed.)

   There is an increased risk of future fire disturbances due to climate change and anthropogenic activity. These disturbances can impact soil moisture content and infiltration, which are important antecedent conditions for predicting rainfall–runoff processes in semi-arid regions. Yet these conditions are not well documented. This case study provides critical field measurements and information, which are needed to improve our understanding of mechanisms such as precipitation and temperature that lead to the variability of soil properties and processes in urban and burned landscapes. In June 2018, a fire burned a portion of the riparian zone in Alvarado Creek, an urban tributary of the San Diego River in California, United States. This fire provided an opportunity to observe soil moisture content and infiltration for one year after the fire. Three transects (one burned and two unburned) were monitored periodically to evaluate the complex spatial and temporal dynamics of soil moisture and infiltration patterns. Average dry season soil moisture content was less than five percent volume water content (%VWC) for all transects, and the burned transect exhibited the lowest %VWC during the wet season. Infiltration rates displayed a high degree of spatial and temporal variability. However, the location with the highest burn severity had the lowest average infiltration rate. The observed differences between the burned and unburned transects indicate that the fire altered hydrologic processes of the landscape and reduced the ability of the soil to retain water during the wet season. This research provides the first high-resolution soil moisture and infiltration field analysis of an urban fire-disturbed stream in southern California and a method to characterize post-fire hydrologic conditions for rainfall–runoff processes. 

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5. How interactions between wildfire and seasonal soil moisture fluxes drive nitrogen cycling in Northern Sierra Nevada forests

   https://doi.org/10.1071/WF21064  

   Brady, Mary K.; Hanan, Erin J.; Dickinson, Matthew B.; Miesel, Jessica R.; Wade, Laura; Greenberg, Jonathan (January 2022, International Journal of Wildland Fire)

   As wildfires become larger and more severe across western North America, it grows increasingly important to understand how they will affect the biogeochemical processes influencing ecosystem recovery. Soil nitrogen (N) cycling is a key process constraining recovery rates. In addition to its direct responses to fire, N cycling can also respond to other post-fire transformations, including increases or decreases in microbial biomass, soil moisture, and pH. To examine the short-term effects of wildfire on belowground processes in the northern Sierra Nevada, we collected soil samples along a gradient from unburned to high fire severity over 10  months following a wildfire. This included immediate pre- and post-fire sampling for many variables at most sites. While season and soil moisture did not substantially alter pH, microbial biomass, net N mineralisation, and nitrification in unburned locations, they interacted with burn severity in complex ways to constrain N cycling after fire. In areas that burned, pH increased (at least initially) after fire, and there were non-monotonic changes in microbial biomass. Net N mineralisation also had variable responses to wetting in burned locations. These changes suggest burn severity and precipitation patterns can interact to alter N cycling rates following fire. 

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