Wildfire smoke and rain linked to nutrient pulses across US monitoring sites

The University of Utah reported on September 22 that rain falling beneath wildfire smoke was associated with larger deposits of several nutrients across US monitoring sites. The underlying study, published in August, estimates what reached the ground in rainwater; it does not show whether the added material helped or harmed the ecosystems receiving it.

The findings matter because smoke can move material released by a fire beyond the burned landscape. In the study's national models, one additional day when smoke and rain coincided within a week was associated with higher wet deposition of each of nine measured ions. The estimated increase depended on the substance, ranging from 6% to 46%.

What the monitors recorded

Researchers combined satellite maps of smoke with rainfall records and weekly rainwater chemistry measurements from more than 250 US National Atmospheric Deposition Program sites. They examined 2014, 2020 and 2022. A smoke-rain day counted when a mapped plume, regardless of its density, overlapped a monitoring site on a day when rain fell there.

The team then compared daily counts of those conditions with the substances collected in weekly samples. That approach can identify an association across sites and weeks, but a weekly sample cannot isolate the chemistry of one particular shower. The researchers' estimates describe dissolved material deposited in rain, rather than nutrients taken up by plants or changes measured in lakes.

Smoke-rain exposure also differed across the years studied. Sites averaged four such days in 2014, six in 2020 and 21 in 2022, with the Upper Midwest a recurring hotspot. For 2022, the authors estimated that smoke-rain accounted for roughly 20% to 30% of annual wet deposition of several nutrients, including nearly one-third of orthophosphate and potassium and as much as 20% of ammonium.

For orthophosphate specifically, the modeled annual amount attributed to smoke-rain was about 0.036 kilograms per hectare in 2022. The corresponding estimates were 0.006 in 2014 and 0.015 in 2020. These are modeled contributions to deposition in the selected years, not a measured trend across all years or a forecast for present-day fires.

Why an added nutrient is not automatically a benefit

The chemistry of a nutrient pulse may matter as much as its size. At 76% of monitored sites, deposition during smoke-rain weeks had a higher nitrogen-to-phosphorus molar ratio than during weeks without smoke-rain. The authors say the ecological implications depend on which nutrient limits growth in a particular ecosystem. Their analysis did not measure the biological response.

A separate Cornell University-led study offers a narrower observation from a central New York watershed exposed to Canadian wildfire smoke in summer 2023. Over two months of stream and rainwater sampling, researchers found dissolved phosphorus concentrations in the stream at two to three times their pre-smoke level. They identified rain after the smoke as a potentially important part of that response.

The Cornell researchers cautioned that one watershed cannot establish how widely such a response occurs. An earlier US Forest Service study in central Idaho likewise found that rain falling through smoke during a fire had nutrient concentrations 20 to 70 times those in ordinary rain. Yet it estimated those inputs amounted to only 1% to 4% of annual nutrient gains to trees in an adjacent watershed.

Together, those earlier studies show why a detectable change in rain or stream chemistry does not, by itself, settle its ecological importance. The Utah-led study adds estimates across many monitoring sites, but it does not establish that its modeled nutrient deposits improved or damaged ecosystems nationwide.

Limits on the estimates

The modeled association varied by region. The paper estimated roughly 104% more weekly orthophosphate deposition per additional smoke-rain day in the Northeast. Some western estimates were larger, but the authors reported greater uncertainty for several western regional results. The national figures should therefore not be read as the expected change at any individual site.

Smoke detection is another constraint. The study used daytime visible satellite imagery; clouds can hide smoke, haze can resemble it in some regions, and atmospheric layering can complicate identification. The analysis also excluded dry deposition, which the authors say occurs on many more smoke days than deposition in rain.

The observations end in 2022 because later data were unavailable when the project began. University of Utah engineer and coauthor Heather Holmes said understanding how smoke plumes, atmospheric dynamics and precipitation interact is important for explaining nutrient transport and deposition. Whether the estimated pulses change the health or productivity of particular ecosystems remains an open question.