When Wildfire Smoke Mixes With Rain, It Can Deliver Nutrients to Ecosystems, Study Finds

Wildfire smoke is usually seen as an air pollutant that threatens human health. A new study finds that when smoke and rain coincide, nutrients released by burned landscapes rain back down onto ecosystems. Led by researchers at the University of Utah, Utah State University and the Cary Institute of Ecosystem Studies, the study shows smoke-rain events deliver notable nitrogen, phosphorus and potassium, nutrients essential to life, and could become an increasingly important but overlooked effect of wildfires as they grow more frequent. The findings appear in the journal Global Change Biology.

The team combined satellite-derived smoke observations with rainfall-chemistry data from the National Atmospheric Deposition Program, which has tracked rainwater chemistry since the 1970s, across 250 sites in 16 climate regions during 2014, 2020 and 2022. Smoke-rain days rose sharply over that period, averaging about four days a year in 2014, six in 2020 and about 22 in 2022, with the Upper Midwest the top hotspot in all three years.

When fire burns vegetation, it releases particles and gases into the atmosphere, and some later mix with rain. As rain falls through smoke, it washes the particles to the ground in a process called wet deposition. Once dissolved in water, the nutrients become bioavailable, meaning plants and other organisms can use them right away, said lead author Alexandra Ponette-González, an urban ecologist at the University of Utah. In 2022, a high-smoke year, smoke-rain days made up only about 5% of days but delivered roughly 20 to 30 percent of the year's rain-borne nitrogen, phosphorus and potassium.

The authors stressed the results do not show that wildfire smoke "fertilizes" ecosystems in a way that necessarily benefits growth. The effects depend on the ecosystem receiving the nutrients: in lakes, added nutrients can increase production and shift species composition, while past research on forests has found smoke deposition can both stimulate tree growth and decrease survival.