Experimentally determined traits shape bacterial community composition one and five years following wildfire

Abstract

Wildfires represent major ecological disturbances, burning 2–3% of Earth’s terrestrial area each year with sometimes drastic effects above- and belowground. Soil bacteria offer an ideal, yet understudied system within which to explore fundamental principles of fire ecology. To understand how wildfires restructure soil bacterial communities and alter their functioning, we sought to translate aboveground fire ecology to belowground systems by determining which microbial traits are important post-fire and whether changes in bacterial communities affect carbon cycling. We employed an uncommon approach to assigning bacterial traits, by first running three laboratory experiments to directly determine which microbes survive fires, grow quickly post-fire and/or thrive in the post-fire environment, while tracking CO2 emissions. We then quantified the abundance of taxa assigned to each trait in a large field dataset of soils one and five years after wildfires in the boreal forest of northern Canada. We found that fast-growing bacteria rapidly dominate post-fire soils but return to pre-burn relative abundances by five years post-fire. Although both fire survival and affinity for the post-fire environment were statistically significant predictors of post-fire community composition, neither are particularly influential. Our results from the incubation trials indicate that soil carbon fluxes post-wildfire are not likely limited by microbial communities, suggesting strong functional resilience. From these findings, we offer a traits-based framework of bacterial responses to wildfire.

Fire Impacts on the Soil Metabolome and Organic Matter Biodegradability

“…Thus, we anticipate that the physical displacement of CO 2 in our study was most likely restricted to the first few hours of incubation whereas the remaining CO 2 emissions were due to microbial respiration which can persist over weeks. For example, the biological mineralization of PyOM has been observed over the course of 57, 14, and 35 days in laboratory studies which are comparable time periods to the incubations conducted here. ,, Overall, the elevated WEOC content, enrichment of amino acids and organic acids, presence of saccharides, and detection of heterotrophic microbes in burned soil suggest that microbial mineralization of SOM contributed to the CO 2 emissions observed in these incubations.…”

“…The metabolism of SOM in complex burned soils observed in this study supports the laboratory-based studies that reported rapid biodegradation of PyOM, − , and our findings provide further evidence for the degradation pathways of aromatic compounds in burned soils. , Our characterization of organic acids, amino acids, and saccharides advances our collective understanding of what substrates are available for microbial metabolism in burned soils that can fuel postfire soil recovery. The loss of ectomycorrhizal fungi after burning (Figure F)which may have been impacted by soil disturbance and separation from plant hosts during soil sampling and pyrocosm assemblyand decreases in fungal diversity after burning (Figure B) have implications for nutrient transport through fungal networks and may constrain the recovery of ectomycorrhizal-obligate species such as lodgepole pine.…”

“…For example, the biological mineralization of PyOM has been observed over the course of 57, 14, and 35 days in laboratory studies which are comparable time periods to the incubations conducted here. 23,39,117 Overall, the elevated WEOC content, enrichment of amino acids and organic acids, presence of saccharides, and detection of heterotrophic microbes in burned soil suggest that microbial mineralization of SOM contributed to the CO 2 emissions observed in these incubations.…”

“…100,122 Considering that only ∼3.3% of total carbon in burned soil was released as CO 2 during the 50-day incubations, the readily biodegradable SOM pool in burned soil likely represents a relatively small fraction of the total SOM content. 117 Nevertheless, the soil CO 2 emission results support our first hypothesis that immediately after fire (i.e., the first 28 days), SOM in burned soil can be mineralized by microbes in a complex, burned soil environment.…”

“…Finally, this study demonstrated that pyrocosms can be used to evaluate postfire soil dynamics beyond analyzing shifts in fungal community composition as pioneered by Bruns et al 57 Environmental Implications. The metabolism of SOM in complex burned soils observed in this study supports the laboratory-based studies that reported rapid biodegradation of PyOM, [38][39][40][41]117 and our findings provide further evidence for the degradation pathways of aromatic compounds in burned soils. 12,39 Our characterization of organic acids, amino acids, and saccharides advances our collective understanding of what substrates are available for microbial metabolism in burned soils that can fuel postfire soil recovery.…”