Background

Wildland fire is known to play an important role in aerosolizing microbes sourced from vegetation and soil communities, but little is known about their spatiotemporal patterns across environmental gradients in smoke plumes. During prescribed fires in Kansas tallgrass prairies, simultaneous deployment of three uncrewed aerial vehicles (UAVs) equipped with bioaerosol sampling and environmental sensor payloads enabled simultaneous measurements (~100 m intervals) across smoke plumes to better understand patterns of aerosolized microbes, relationships to their environment and sources, and their transport. Using data collected from nine smoke plume transects, we made comparisons to samples collected from ambient air to evaluate hypotheses about microbial composition and diversity as the smoke advected with prevailing winds.

Results

As distance from the flaming front increased, our data indicated changes in the composition of smoke-borne microbial assemblages that correlate to plume environmental gradients. The detectable contribution of terrestrial communities as sources of smoke bioaerosols decreased with deposition, smoke aging, and dilution with background air. Microbial composition of the smoke also differed based on the historical fire regimes of the burn units compared. Our results also demonstrate that “hazardous” Air Quality Index levels, based on particulate matter concentrations, corresponded to a fourfold increase in bacterial taxa richness, relating a novel microbial component to the metric most widely used to communicate with the public about smoke hazards.

Conclusions

Grassland fires demonstrated an attenuated smoke signal as smoke aged, traveled, and diffused, but across the ~300 m distance measured, the plume and its biological assemblage remained distinct from background air. Smoke assemblages were driven by fluctuation in relative humidity (RH), particulate matter (PM) concentration, and differences among the vegetation and soils of the source communities as influenced by historical fire frequency in the tallgrass prairie sites. The distinction between smoke and background air microbial composition and diversity was not as pronounced as in previous studies of higher intensity fires consuming greater fuel loads, yet richness and diversity increased with deteriorating air quality indices. This relationship shows that fire behavior producing more PM emits and transports a wider variety of terrestrial microbes from tallgrass prairies.