Stanford, Colo. State lead review of fire research

The huge, long-lasting wildfires that have become increasingly common in recent years can cause changes in soil chemistry that affect water contamination, air quality and plant growth.

These changes, however, are poorly monitored and rarely factor into post-fire recovery efforts or risk assessments, according to a CSU-led article published May 14 in Nature Reviews Earth & Environment.

The literature review, initiated and led by Thomas Borch, a soil chemist and professor in CSU’s Department of Soil and Crop Sciences, found that better techniques are needed to monitor fire-induced changes in soil chemistry. This enhanced monitoring could inform decisions on how to treat drinking water sourced from burned areas, support reforestation, and protect workers against toxins during post-fire cleanup, rebuilding or revegetation.

The work was conducted in collaboration with researchers at Stanford University. CSU doctoral student Jacob VanderRoest and postdoctoral researcher Holly Roth also contributed to the review article.

“A better understanding of the molecular mechanisms in soil can help explain, for instance, why drinking water from a forest fire-impacted watershed is suddenly more toxic, or why a forest is not coming back,” said Borch, a senior author of the study.

“In our study, we mesh organic and inorganic chemistry together, whereas a lot of fire research will typically just consider one subject area,” said Claudia Avila, a soil biogeochemist who co-led the study with Alandra Lopez while both researchers were postdoctoral scholars in the lab of Stanford Doerr School of Sustainability Professor Scott Fendorf.

Climate and ecosystem impacts

The review highlights evidence from recent studies suggesting wildfires may release more planet-warming carbon dioxide into the atmosphere than anticipated. Charcoal-like remnants of burned wood and other organic materials, known as black carbon, may not trap carbon dioxide for as long as scientists had hoped.

“Carbon that’s gone through forest fires and becomes black carbon can actually turn more readily into carbon dioxide by microbes than previously thought,” said Fendorf, the Terry Huffington Professor at Stanford.

“From a climate perspective, we still have a poor understanding of how much of the carbon that is left after a fire has the potential to be transformed into greenhouse gasses, such as carbon dioxide,” said Borch, who worked in Fendorf’s lab as a postdoctoral fellow 20 years ago.

Wildfires can have many benefits for ecosystems, the authors note. For example, some fires can increase the nitrogen content in soil organic matter and augment the water solubility of soil organic carbon, setting the stage for regrowth. However, recovery depends on the presence of other chemicals. For instance, certain types of organic molecules formed in soil during fires, called karrikins, are needed for many seeds to germinate. If the local soil chemistry and fire conditions do not produce enough of these karrikins, revegetation may be stunted.

Other research included in the new review has shown that wildfires can double the soil concentration of a group of toxic chemicals known as polycyclic aromatic hydrocarbons, which can induce chemical reactions that inhibit revegetation. These molecular-scale effects could well explain the mystery of vast areas where trees have struggled to reestablish after wildfires in the Rocky Mountains, Borch said.

Wildfires can also alter the chemical properties of inorganic materials such as metals within soils. Fire can change the metals into dangerous forms that readily move through the environment, ending up in the air or nearby water, the authors explained. The collaborators from Stanford University had previously documented high levels of a hazardous form of the metal chromium at wildfire sites resulting from heat-induced transformation of naturally occurring, benign forms of chromium. At sites where extremely hot, long-lasting fires cooked soils to high temperatures for extended periods, chromium persisted for many months until the next large rain event.

Other research on chromium indicates that after lower-intensity fires, remnant plant and animal tissue in soil can allow the toxic form of chromium to return to its inert form. Taken together, these studies illustrate the broader reality that wildfire impacts on soil chemistry depend on the intricate nature of the fire and landscape, including fire duration and temperature.

Predicting and mitigating risks

Broader surveillance and modeling could inform wildfire management decisions and strategies for protecting lives, property and natural resources. Avila offers an example of how this approach to informed stewardship could help prevent the leaching of metals into drinking water supplies.

“By identifying an area that has a high potential for, say, chromium release, we can call for prescribed burns that are lower intensity and reduce the potential for high-intensity, toxin-releasing fires,” said Avila, who is now an assistant professor of environmental and ocean sciences at the University of San Diego.

“If we can grasp the complexity of the intertwined processes that are happening both on the organic and the inorganic side, then that helps give us the ability to predict outcomes for different fire, landscape and geological conditions,” Fendorf said.

Source: Colorado State University