Wildfire Smoke in Rain Can Deliver Nutrient Bursts to Ecosystems, Study Finds
Wildfire smoke is often discussed as an airborne pollutant and a threat to human health. But a new study reveals that when smoke and rain coincide, nutrients released by burned landscapes can rain back down, delivering large bursts of nitrogen, phosphorus, and potassium across ecosystems in the United States.
The study, published in Global Change Biology on Tuesday, was led by the University of Utah and involved researchers from Utah State University and the Cary Institute of Ecosystem Studies. The team reviewed data from 2014, 2020, and 2022 collected at 250 climate sites across 16 regions, including information from the National Atmospheric Deposition Program, which tracks the chemical composition of rainwater. Satellite observations of smoke and national rainfall data were also used to determine the frequency of smoke-rain events and the nutrients left behind.
According to the study, rain-smoke days accounted for 20% to 30% of the annual rain-deposited nitrogen, phosphorus, and potassium, despite making up only 5% of days. The researchers found more types of rain-smoke events in 2022 than in 2014, a change they suggest could be tied to the growing frequency and magnitude of extreme wildfire events.
Regional hot spots for these events shifted over the study period, with the Upper Midwest emerging as a persistent hot spot. However, the study found only modest increases in smoke-affected wet deposition within that region.
Lead author Alexandra Ponette-González, an urban ecologist at the University of Utah and the Natural History Museum of Utah, emphasized the significance of the findings. "When rain hits an ecosystem, the nutrients are bioavailable — organisms, whether in lake or on land, can immediately access them because they're dissolved in water," she said in a statement. "This study puts a number on how much wildfire can contribute to nutrient delivery and raises important questions for what that means for ecosystem health."
Ponette-González also noted the broader perspective: "It's important to remember that what goes up must come down. There's so much focus on what goes up and how that affects human health. We're interested in everything that falls out of the atmosphere and lands on ecosystems, and what that means for our environment."
The study's authors caution that it remains unclear whether these nutrient pulses are beneficial or harmful. While they could fertilize ecosystems, they might also contribute to problems such as algal blooms in water bodies. Coauthor Janice Brahney, a watershed scientist at Utah State University, said, "We can't characterize atmospheric deposition as simply good or bad."
The findings are described as a starting point for further research into the ecological consequences of wildfire smoke.
The study was prompted in part by an event five years ago, when federal meteorologists were baffled as a cold front failed to push wildfire smoke out of Utah, instead creating a smoky rain that fell over most of the state's northern half. That incident raised questions about what exactly is in the rain when it collides with wildfire smoke — questions this study begins to answer.