Environmetally persistent free radicals in wildfire charcoals

Wildfires produce large amounts of pyrogenic carbon / charcoal, which in this study was found to be an important source of environmentally persistent free radicals, precursors to toxic reactive oxygen species. These findings open up a new facet to post-fire ecology for further investigation.

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The seed of this transdisciplinary collaboration was planted in 2019 during a conversation between myself and wildfire researchers Cristina Santín and Stefan Doerr. Following an invited talk I gave at Swansea University, we had an intense discussion about the environmental effects of wildfire charcoal in post-fire environments, which remain a subject poorly investigated. Globally fires produce about 256 megatons of pyrogenic carbon or charcoal each year. Wildfire charcoals have some similarities with biochar, an engineered pyrogenic carbon produced under highly controlled conditions and studied mainly in the context of soil improvement in agriculture, climate mitigation, and contaminant remediation, a field I have worked in before. Discussing similarities and differences between wildfire charcoal and biochar, we started to identify questions that have received some attention in the biochar community but have not yet been investigated for the arguably more widely spread and reactive wildfire charcoals.

In biochar and soot, surface bound free electrons referred to as environmentally persistent free radicals (EPFR) have recently been identified as important precursors of toxic reactive oxygen species. When conceptualizing this study, we hypothesized, that we could also find EPFR in wildfire charcoals collected from post-fire landscapes. To test this hypothesis, I reached out to Marc Pignitter, an analytical chemist specialized in the detection of free electrons via a method called electron spin resonance spectroscopy at the University of Vienna. Once he was on board, I got in touch with my colleagues at Swansea to identify samples they may collect within the scope of other projects that we could analyse. Together with Nathalie Tepe and Thilo Hofmann from my “home base”, the environmental geosciences division (EDGE) in the Centre for Microbiology and Environmental Systems Science (CMESS) at University of Vienna, we analysed 60 samples from 10 wildfires, that included crown as well as surface fires in forest, shrubland and grassland spanning different boreal, temperate, subtropical and tropical climates. Thereby we were able to reflect with our samples, at least to some extent, the large variety in ecosystems that can be affected by wildfires around the world.

Co-author Cristina Santín sampling wildfire charcoal after an experimental crown fire a boreal forest of Canada.

In our study just published in Communications Earth & Environment we measured EPFR concentrations that were orders of magnitude above background concentrations in soils, confirming our hypothesis that EPFR are ubiquitous in wildfire charcoals. We also found that concentrations increased with the degree of carbonization and were highest for wood-derived wildfire charcoals. We expected that EPFR concentrations would quickly decrease in the field after a fire, as previous literature suggests that most EPFR react with water to form reactive oxygen species within a few hours. However, we found that EPFR remained stable for an unexpectedly long time of at least 5 years in one sample series collected at different times since fire. We suggest that wildfire charcoal is an important global source of environmentally persistent free radicals, and therefore potentially of harmful reactive oxygen species.

The implications of our findings as of now are difficult to foresee, but we believe that EPFR and reactive oxygen species produced thereof may be an important factor that needs consideration when assessing post-fire landscapes and ecosystem functioning therein. For example, EPFR may cause an increase in oxidative stress to living organisms for prolonged periods of time, especially in ecosystems where fires are rare (e.g. the Amazon or the Arctic tundra). With global warming leading to larger and more severe fires in many parts of the world, understanding fully their implications on affected ecosystems is urgent. With this study we hope to inspire the post-fire ecology community to include an additional facet in their assessment of post-fire landscapes.

Gabriel Sigmund

Junior Group Leader, University of Vienna, Centre for Microbiology and Environmental Systems Science

Gabriels research focus lays on the effects of pyrogenic carbon on the natural environment.