Wildfire smoke is typically viewed as an airborne pollutant and a major threat to human health.
But new research led by scientists at the University of Utah, Utah State University, and the Cary Institute of Ecosystem Studies shows that its effects do not end in the atmosphere.
When wildfire smoke and rainfall coincide, nutrients carried in smoke can be washed from the atmosphere and deposited across ecosystems.
The study found that these events can deliver substantial amounts of nitrogen, phosphorus and potassium, three nutrients essential to life, despite occurring on relatively few days each year.
The findings reveal an overlooked consequence of wildfire activity. In 2022, smoke-rain events occurred on roughly 5% of days at the study sites but accounted for around 20-30% of the annual rain-deposited nitrogen, phosphorus and potassium.
As wildfires become more frequent and smoke spreads across larger areas, these nutrient pulses could become increasingly important.
Tracking smoke from the sky to the ground
The research represents the first large-scale assessment of how wildfire smoke is transported into ecosystems through rainfall across hundreds of sites in the United States.
Researchers combined satellite observations of wildfire smoke with rainfall chemistry data from the National Atmospheric Deposition Program (NADP). The analysis covered 250 monitoring sites spanning 16 climate regions during 2014, 2020, and 2022.
They identified periods when smoke and rain occurred simultaneously, then assessed the amount of nitrogen, phosphorus and potassium deposited during those events.
The number of smoke-rain days increased sharply across the study period. Sites experienced an average of about four such days in 2014, rising to six in 2020, and approximately 22 in 2022.
The locations experiencing the greatest number of events also changed from year to year. The Upper Midwest consistently recorded the highest frequency, while the Northern Rockies, Southwest and Ohio Valley emerged as secondary hotspots in 2014, 2020, and 2022, respectively.
How wildfire smoke reaches ecosystems
Wildfires release a complex mixture of gases and particles as vegetation and other material burns. Some of this material remains suspended in the atmosphere, where it can travel significant distances from the original fire.
Rain provides a pathway for that material to return to the ground. As precipitation passes through a smoke plume, particles and compounds can be captured by water droplets and subsequently deposited onto land and into waterways.
This process, known as wet deposition, makes nutrients dissolved in the rain immediately accessible to organisms.
The effect is particularly significant because smoke can travel far beyond the area directly affected by a wildfire. The nutrients released by a fire may therefore be redistributed across ecosystems some distance from their original source.
However, smoke alone does not guarantee substantial deposition. Rainfall is the critical link that brings airborne material down to the surface, meaning the frequency and location of these events depend on the interaction between smoke plumes and weather patterns.
A nutrient pulse is not necessarily beneficial
The researchers caution against describing wildfire smoke as a form of natural fertiliser. The ecological consequences of additional nutrients depend heavily on the ecosystem receiving them.
In nutrient-poor environments, a sudden increase in nitrogen or phosphorus could alter biological activity. In lakes, additional nutrients may increase productivity and change the composition of aquatic communities.
Forest ecosystems can also respond differently depending on local conditions, with previous research indicating that smoke-derived deposition may produce both positive and negative effects.
The distinction is important because atmospheric nutrient deposition cannot simply be categorised as beneficial or harmful. Its consequences depend on the quantity deposited, the nutrients involved and the existing chemical and biological characteristics of the receiving ecosystem.
A growing consequence of more frequent fires
The study highlights a part of the wildfire cycle that has received considerably less attention than smoke exposure and air pollution.
With wildfire activity increasing in many regions, understanding where smoke travels and what it carries could become increasingly important for predicting changes in ecosystem chemistry.
More frequent smoky conditions could also mean more opportunities for rainfall to deliver fire-derived material to the landscape.
The researchers’ next step is to trace individual smoke plumes back to their sources and investigate how factors such as fire type influence the nutrients ultimately deposited by rainfall.
Comparing forest fires with agricultural burning could help determine whether different sources produce distinct chemical signatures.
Understanding what happens after wildfire smoke leaves the atmosphere could therefore provide a more complete picture of how fires reshape ecosystems, not only through burning and air pollution, but also through the redistribution of nutrients across the landscape.
