Does Cold Wildfire Smoke Contribute to Water Repellent Soils in Burned Areas?

Does Cold Wildfire Smoke Contribute to Water Repellent Soils in Burned Areas?

Does Cold Wildfire Smoke Contribute to Water Repellent Soils in Burned Areas?

May 25, 2021
RENO, NEV.

By Kelsey Fitzgerald

Soil Science
Wildfires
Hydrology

Above: After a wildfire, soils in burned areas often become water repellent, leading to increased erosion and flooding after rainfall events. The hillside shown here burned in California’s Loyalton Fire during August 2020.

Credit: Kelsey Fitzgerald/DRI.

A new DRI pilot study finds severe water repellency in sand samples after treatment with both hot and cold smoke.

After a wildfire, soils in burned areas often become water repellent, leading to increased erosion and flooding after rainfall events – a phenomenon that many scientists have attributed to smoke and heat-induced changes in soil chemistry. But this post-fire water repellency may also be caused by wildfire smoke in the absence of heat, according to a new paper from the Desert Research Institute (DRI) in Nevada.

In this pilot study (exploratory research that takes place before a larger-scale study), an interdisciplinary team of scientists led by DRI Associate Research Professor of Atmospheric Science Vera Samburova, Ph.D., exposed samples of clean sand to smoke from burning Jeffrey pine needles and branches in DRI’s combustion chamber, then analyzed the time it took for water droplets placed on the sand surface to be absorbed – a measure of water repellency.

Natasha Sushenko processes samples in the Environmental Microbiology Lab at the Desert Research Institute during a COVID-19 wastewater monitoring study.

A new pilot study by an interdisciplinary team from DRI exposed samples of clean sand to smoke from burning Jeffrey pine needles and branches, then analyzed the time it took for water droplets placed on the sand surface to be absorbed — a measure of water repellency. After exposure to smoke, water droplets sometimes remained on the sand surface for more than 50 minutes without soaking in.

Credit: Vera Samburova/DRI.

The full text of the paper, Effect of Biomass-Burning Emissions on Soil Water Repellent: A Pilot Laboratory Study, is available from Fire: https://www.mdpi.com/2571-6255/4/2/24

The pilot study investigated the effects of smoke and heat on water repellency of the sand and was the first study to also incorporate an analysis of cold smoke. In the experiments, sand was used in place of soil because it could be cleaned thoroughly and analyzed accurately, and Jeffrey pine for a fuel source because it represents a common wildland fire fuel in the Western U.S.

Before exposure to Jeffrey pine smoke, water droplets placed on the surface of the sand samples were quickly absorbed. But after exposure to smoke, the sand samples showed severe-to-extreme water repellency, in some cases retaining water droplets on the sand surface for more than 50 minutes without soaking in. It made little difference whether or not samples had been exposed to heat and smoke, or just cold smoke.

“The classic explanation for fire-induced water repellency is that it is caused as smoke diffuses under rather hot conditions and settles down into the soils, but our work shows that the smoke does not have to be hot to turn the sand hydrophobic — simply the presence of the chemical substances in the smoke is enough,” Samburova said. “This is something we really need to look deeper into because soil water repellency leads to increases in flooding, erosion, and surface runoff.”

Above, left: Jeffrey pine needles and sticks were used as a fuel source in the new DRI study because Jeffrey pine represents a common wildland fire fuel in the Western U.S.

Credit: Vera Samburova/DRI.

Above, right: Jeffrey pine needles and branches burn inside of the combustion chamber at DRI during a new study that investigated the effects of smoke and heat on water repellent of sand samples.

Credit: Vera Samburova/DRI.

This study built on previously published work by former DRI postdoctoral researcher Rose Shillito, Ph.D., (currently with the U.S. Army Corps of Engineers), Markus Berli, Ph.D., of DRI, and Teamrat Ghezzehei, Ph.D., of University of California, Merced, in which the researchers developed an analytical model for relating soil water repellency to infiltration of water.

“Our earlier paper focused on how fire changes the properties of soils, from a hydrology perspective,” Berli explained. “In our current study, we were interested in learning more about the chemistry behind the process of how soils come to be hydrophobic. We’re bringing together geochemistry and organic geochemistry with soil physics and hydrology to understand the impact of fire-induced water repellency on hydrology.”

The project team is now working on a larger proposal to further investigate questions touched on by this study about the roles of heat and smoke in fire-induced water repellency. Among other things, they would like to know how long soil water repellency lasts after a fire, and gain a better understanding of the detailed processes and mechanisms through which cold smoke affects the soil.

In her free time, Natasha enjoys hiking and being outside in beautiful areas like the Desolation Wilderness in California.

DRI’s combustion chamber, pictured here, is a specialized facility that has been designed and built for the open combustion of solid fuels under controlled conditions. In this experiment, it was used to expose samples of clean sand to Jeffrey pine smoke. 

Credit: Kelsey Fitzgerald/DRI.

Gaining a thorough understanding of the process that leads to fire-induced soil water repellency is important because land managers need this information in order to accurately predict where soils are likely to be hydrophobic after a fire, Berli explained.

“We still don’t really understand the processes that lead to this fire-induced soil water repellency,” Berli said. “Depending on what we find, the measures to predict fire-induced water repellency might be different, and this can have a significant impact on how we can predict and prevent flooding or debris flows that happen after a fire.”

“This study was one big step forward, but it highlights the importance of future research on how fires affect soil, because wildfires are affecting thousands and thousands of square kilometers of land each year in the Western U.S., ” Samburova added. “Some of our future goals are to find out how exactly this soil water repellent happens, where it happens and how long it lasts.”

Additional Information:

This study was made possible with support from DRI and the National Science Foundation. Study authors included Vera Samburova, Ph.D., Rose Shillito, Ph.D. (currently with U.S. Army Corps of Engineers), Markus Berli, Ph.D., Andrey Khlystov, Ph.D., and Hans Moosmüller, Ph.D., all from DRI.

The full text of the paper, Effect of Biomass-Burning Emissions on Soil Water Repellency: A Pilot Laboratory Study, is available from Fire: https://www.mdpi.com/2571-6255/4/2/24

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About the Desert Research Institute
The Desert Research Institute (DRI) is a recognized world leader in basic and applied interdisciplinary research. Committed to scientific excellence and integrity, DRI faculty, students, and staff have developed scientific knowledge and innovative technologies in research projects around the globe. Since 1959, DRI’s research has advanced scientific knowledge, supported Nevada’s diversifying economy, provided science-based educational opportunities, and informed policy makers, business leaders, and community members. With campuses in Reno and Las Vegas, DRI serves as the non-profit research arm of the Nevada System of Higher Education. For more information, please visit www.dri.edu

Q & A with AGU Presenter Rose Shillito

Q & A with AGU Presenter Rose Shillito

Rose Shillito is a hydrologist and graduate student researcher working with Markus Berli, Ph.D., associate research professor of environmental science. Rose has worked at DRI since 2011, and she plans to defend her doctoral dissertation at UNLV and earn her Ph.D. in geosciences this fall.

DRI: In a couple of sentences, what is the ‘plain English’ summary of what are you presenting at AGU? 

Rose Shillito: Fire can cause soils to become water repellent—water will not spontaneously enter the soil. We have developed a physically-based model to understand and predict the effect of soil water repellency on infiltration, thus on the potential for post-fire flooding and erosion.

(Rose was recently featured by the UNLV Foundation for her work on post-fire water repellency. Read the article here: https://www.unlv.edu/news/article/trickle-down-effect.)

DRI: What are you most looking forward to at AGU this year? What do you hope to learn, or who do you hope to connect with? 

RS: At AGU, I like to get an overview of research in my specific topic, but also get a general overview of research directions and methods in my field (hydrology). I get a chance to connect with colleagues and make new connections with other researchers.

DRI: The theme of this year’s meeting is “What Science Stands For.” From your perspective, what does science stand for? 

RS: Currently, to me, science is about answering questions.

Meet Rose at her AGU poster session, “Effective Infiltration Measurements for Fire-Affected Water-Repellent Soils,” happening Tuesday, December 11th during the afternoon session. (Session H23L-2548 in the program.)

This Q&A is part of a series of profiles of DRI scientists who will be participating in the 2018 AGU Fall Meeting, to be held in Washington DC during the week of December 10th. Learn more about this annual meeting of 24,000 scientists from a wide range of disciplines here: https://fallmeeting.agu.org/2018/.

Low-severity wildfires impact soils more than previously believed

Low-severity wildfires impact soils more than previously believed

Above: In semi-arid ecosystems such as the Humboldt-Toiyabe National Forest near Las Vegas, which burned as part of the Carpenter 1 fire during July and August 2013, fuel is limited and fires tend to be short lived and low in peak temperature. New research shows that these fires are more harmful to soils than they initially appear. This photo was taken on January 6, 2015 – approximately 18 months after the wildfire. Credit: Teamrat Ghezzehei, UCM.


New research shows negative effects of fire on soil structure and organic matter

Las Vegas, NV (August 28, 2018): Low-severity wildland fires and prescribed burns have long been presumed by scientists and resource managers to be harmless to soils, but this may not be the case, new research shows.

According to two new studies by a team from the University of California, Merced (UCM) and the Desert Research Institute (DRI), low-severity burns – in which fire moves quickly and soil temperature does not exceed 250oC (482oF) – cause damage to soil structure and organic matter in ways that are not immediately apparent after a fire.

“When you have a high-severity fire, you burn off the organic matter from the soil and the impact is immediate,” said Teamrat Ghezzehei, Ph.D., principal investigator of the two studies and Associate Professor of Environmental Soil Physics at UCM. “In a low-severity fire, the organic matter doesn’t burn off, and there is no visible destruction right away. But the burning weakens the soil structure, and unless you come back at a later time and carefully look at the soil, you wouldn’t notice the damage.”

DRI researcher Markus Berli, Ph.D., Associate Research Professor of Environmental Science, became interested in studying this phenomenon while visiting a burned area near Ely, Nev. in 2009, where he made the unexpected observation that a prescribed, low-severity fire had resulted in soil structure damage in the burned area. He and several colleagues from DRI conducted a follow-up study on another controlled burn in the area, and found that soil structure that appeared to be fine immediately after a fire but deteriorated over the weeks and months that followed. Berli then teamed up with Ghezzehei and a team from UCM that included graduate student Mathew Jian, and Associate Professor Asmeret Asefaw Berhe, Ph.D., to further investigate.

Researcher examines soils in a burned area near Las Vegas.

Researcher Markus Berli from the Desert Research Institute examines the soils at a burned area in the Humboldt-Toiyabe National Forest near Las Vegas on January 6, 2015, approximately 18 months after the area burned in the Carpenter 1 fire of 2013. Credit: Teamrat Ghezzehei, UCM.

Soil consists of large and small mineral particles (gravel, sand, silt, and clay) which are bound together by organic matter, water and other materials to form aggregates. When soil aggregates are exposed to severe fires, the organic matter burns, altering the physical structure of the soil and increasing the risk of erosion in burned areas. In low-severity burn areas where organic matter doesn’t experience significant losses, the team wondered if the soil structure was being degraded by another process, such as by the boiling of water held within soil aggregates?

In a study published in AGU Geophysical Research Letters in May 2018, the UCM-DRI team investigated this question, using soil samples from an unburned forest area in Mariposa County, Calif. and from unburned shrubland in Clark County, Nev. to analyze the impacts of low-severity fires on soil structure. They heated soil aggregates to temperatures that simulated the conditions of a low-severity fire (175oC/347oF) over a 15-minute period, then looked for changes in the soil’s internal pore pressure and tensile strength (the force required to pull the aggregate apart).

During the experiment, they observed that pore pressure within the soil aggregates rose to a peak as water boiled and vaporized, then dropped as the bonds in the soil aggregates broke and vapor escaped. Tensile strength measurements showed that the wetter soil aggregates had been weakened more than drier soil samples during this process.

“Our results show that the heat produced by low-severity fires is actually enough to do damage to soil structure, and that the damage is worse if the soils are wet,” Berli explained. “This is important information for resource managers because it implies that prescribed burns and other fires that occur during wetter times of year may be more harmful to soils than fires that occur during dry times.”Next, the research team wondered what the impact of this structural degradation was on the organic matter that the soil structure normally protects. Soil organic matter consists primarily of microbes and decomposing plant tissue, and contributes to the overall stability and water-holding capacity of soils.

In a second study that was published in Frontiers in Environmental Science in late July, the UCM-DRI research team conducted simulated burn experiments to weaken the structure of the soil aggregates, and tested the soils for changes in quality and quantity of several types of organic matter over a 70-day period.

They found that heating of soils led to the release of organic carbon into the atmosphere as CO2 during the weeks and months after the fire, and again found that the highest levels of degradation occurred in soils that were moist. This loss of organic carbon is important for several reasons, Ghezzehei explained.

“The loss of organic matter from soil to the atmosphere directly contributes to climate change, because that carbon is released as CO2,” Ghezzehei said. “Organic matter that is lost due to fires is also the most important reserve of nutrients for soil micro-organisms, and it is the glue that holds soil aggregates together. Once you lose the structure, there are a lot of other things that happen. For example, infiltration becomes slower, you get more runoff, you have erosion.”

Researcher collects soil samples in burned area near Las Vegas.

Researcher Rose Shillito from DRI collects soil samples in a burned area in the Humboldt-Toiyabe National Forest near Las Vegas on January 6, 2015, approximately 18 months after the area burned in the Carpenter 1 fire of 2013. Credit: Teamrat Ghezzehei, UCM.

Although the research team’s findings showed several detrimental effects of fire on soils, low-severity wildfires and prescribed burns are known to benefit ecosystems in other ways — recycling nutrients back into the soil and getting rid of overgrown vegetation, for example. It is not yet clear whether the negative impacts on soil associated with these low-severity fires outweigh the positives, Berli says, but the team hopes that their research results will help to inform land managers as they manage wildfires and plan prescribed burns.

“There is very little fuel in arid and semi-arid areas, and thus fires tend to be short lived and relatively low in peak temperature,” Ghezzehei said. “In contrast to the hot fires and that burn for days and weeks that we see in the news, these seem to be benign and we usually treat them as such. Our work shows that low-severity fires are not as harmless as they may appear.”

The study, “Soil Structural Degradation During Low‐Severity Burns,” was published on May 31, 2018 in the journal AGU Geophysical Research Letters and is available here: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018GL078053.

The study, “Vulnerability of Physically Protected Soil Organic Carbon to Loss Under Low Severity Fires,” was published July 19, 2018 in the journal Frontiers in Environmental Science, and is available here: https://www.frontiersin.org/articles/10.3389/fenvs.2018.00066/full.

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The Desert Research Institute (DRI) is a recognized world leader in basic and applied interdisciplinary research. Committed to scientific excellence and integrity, DRI faculty, students, and staff have developed scientific knowledge and innovative technologies in research projects around the globe. Since 1959, DRI’s research has advanced scientific knowledge, supported Nevada’s diversifying economy, provided science-based educational opportunities, and informed policy makers, business leaders, and community members. With campuses in Reno and Las Vegas, DRI serves as the non-profit research arm of the Nevada System of Higher Education. For more information, please visit  www.dri.edu.