Smoke-Affected Rain Fertilizes Terrestrial and Aquatic Ecosystems

Wildland fire smoke and ash are pervasive and a significant source of particulate matter to the atmosphere. In addition to effects on air quality, wildland smoke and ash deposit to ecosystems via dry (particulate) and wet (precipitation) deposition. Here we combine remotely sensed smoke polygon data with rainfall and weekly wet deposition data from > 250 U.S. National Atmospheric Deposition Program (NADP) sites to quantify wildland smoke influence on wet ion deposition during 2014, 2020, and 2022. Combined records of daily smoke and rain indicate that sites experienced 21 smoke-rain days (when smoke and rainfall co-occur), on average, in 2022 compared to 4 days in 2014 and 6 days in 2020. The number of smoke-rain days and their spatial distribution were driven more by variation in smoke than rain occurrence. Across 16 climate regions, the Upper Midwest was a persistent hotspot for smoke-rain days in all 3 years. Mixed effects models estimating site-level impacts of smoke-rain on a weekly basis showed that smoke-rain days had significant positive effects on wet deposition of all nine ions evaluated. Orthophosphate (PO₄ ^3-, 46%), potassium (K⁺, 41%), and ammonium (NH₄ ⁺, 36%) deposition were most enhanced compared to non-smoke-rain days. The magnitude of these effects varied by region. With each additional smoke-rain day, weekly wet NH₄ ⁺ deposition increased by 100%-220% in Alaska & the Arctic, Northwest, and West regions, whereas PO₄ ^3- deposition increased ~100% in the Northeast. Based on modeled estimates, smoke-rain days contributed ~20%-30% of annual wet NH₄ ⁺, PO₄ ^3-, K⁺, and Ca²⁺ deposition in 2022. Further, molar N:P ratios of wet deposition were higher at 76% of NADP sites during weeks with versus without smoke-rain. Our results demonstrate that smoke-affected wet deposition can deliver biologically important nutrients to ecosystems, which may be leading to fire-fertilizing of terrestrial and aquatic ecosystems.