Boreal forests cover large regions of the northern hemisphere, are shaped by wildfire, and play a significant role in global carbon cycling. Microorganisms mediate soil nutrient and carbon transformations in these systems, yet how fire alters microbially driven biogeochemical processes remains incompletely resolved. This study used controlled laboratory burns on intact soil cores from boreal forest to evaluate how fire affects soil bacterial and fungal community composition and microbial function, and how those changes relate to soil physicochemical responses.
The authors aimed to assess both resistance (the degree communities and soil properties remain unchanged immediately after burn) and resilience (ability to return toward pre-burn state over time) of microbial communities compared with soil pH and soil respiration. They also sought to link compositional shifts to functional potential by measuring glucose-specific carbon use efficiency (CUE) and by relating CUE to weighted mean predicted 16S rRNA gene copy number for bacteria and to fungal functional assignments from FUNGuild.
Intact soil cores were collected from boreal forest within Wood Buffalo National Park, Alberta, Canada. Two contrasting soil types were targeted: Histosols and the O horizons of Gleysols. Laboratory-controlled burns were applied to these intact cores and cores were incubated post-burn to monitor community and functional responses. The experimental design allowed comparison of burn effects across soil types that differ in horizon thickness and other properties.
Bacterial and fungal community composition was assessed following the laboratory burns. The study compared post-burn community composition to unburned controls to quantify changes and to calculate metrics of resistance and resilience. For bacterial communities, the researchers computed a weighted mean predicted 16S rRNA gene copy number, a trait associated with growth strategies that can shift after disturbance. Fungal taxa were assigned putative functional roles using FUNGuild to estimate relative abundances of symbiotrophic and saprotrophic groups.
Alongside community composition, the study measured soil pH and soil respiration to evaluate environmental and activity responses to burning. To infer potential impacts on microbial-mediated carbon cycling, the authors measured glucose-specific carbon use efficiency (CUE) in burned and unburned soils. CUE was used as an indicator of how efficiently microbial communities allocate assimilated carbon to biomass versus respiration.
Microbial community responses to burning differed by soil type. Both bacterial and fungal communities from Histosols exhibited higher resistance to burn treatments than communities from the O horizons of Gleysols. The authors suggest this pattern may stem from a greater direct impact of burning on the thinner Gleysol O horizons, where microbes are likely closer to the heat-affected surface and more vulnerable.
Both bacterial and fungal communities showed relatively low resilience after burning. The study found that resilience did not clearly increase with time since burn in the experimental timeframe, supporting prior observations that post-fire microbial recovery in boreal soils often occurs over years rather than months.
Soil pH and soil respiration were also compared for resistance and resilience alongside community metrics; the manuscript frames these comparisons to explore links between community composition shifts, the soil environment, and microbial activity.
Burning caused a decrease in CUE across treatments. Larger decreases in CUE were observed following longer, hotter laboratory burns. Decreases in CUE correlated with an increase in weighted mean predicted 16S rRNA gene copy number for bacterial communities, suggesting a shift toward bacterial taxa with traits associated with rapid growth and lower efficiency.
The observed correlation raises the possibility that weighted mean predicted 16S copy number could serve as a proxy for post-fire CUE in boreal forest soils. The authors caution that additional research is required to delimit how environmental conditions, available substrates, and time since fire influence this relationship. Fungal functional shifts estimated by FUNGuild were also examined to contextualize compositional changes, though the manuscript emphasizes the need for more work to robustly link fungal guild dynamics to CUE outcomes.
The study’s findings indicate that burn-induced shifts in microbial community composition are associated with altered microbial function in ways relevant to soil carbon cycling. Reduced CUE after burning implies a greater proportion of assimilated carbon is lost to respiration rather than incorporated into microbial biomass, potentially affecting soil carbon storage trajectories post-fire. Variation in resistance and resilience across soil types highlights that soil physical characteristics and horizon thickness mediate microbial vulnerability to fire and thus influence biogeochemical outcomes.
The authors note that the relationship between predicted 16S rRNA gene copy number and CUE requires further constraint across different environmental contexts, substrate availabilities, and timescales post-fire. The experiments were laboratory burns on intact cores, which allow controlled comparisons but may not capture all complexities of landscape-scale fires and longer-term ecological processes.
Data and code supporting the analyses are provided by the authors via a public repository linked in the preprint. The work was funded by the United States Department of Energy, and the preprint was posted on bioRxiv on September 3, 2026.
Controlled laboratory burns of intact boreal soil cores revealed persistent but variable effects on microbial community resistance, resilience, and function that depended on soil type. Histosols showed greater microbial resistance than the O horizons of Gleysols, while both bacteria and fungi exhibited limited resilience within the study timeframe. Burning reduced glucose-specific CUE, with stronger reductions following more intense burns, and these reductions correlated with increases in predicted bacterial 16S rRNA gene copy number. Together, these results suggest that post-fire shifts in microbial composition can meaningfully alter microbial carbon use and therefore influence soil carbon cycling, but additional research is needed to generalize these relationships across broader environmental conditions and longer recovery periods.