Johnson, D. B. | bioRxiv Science

bioRxiv2022

An empirical approach to developing and testing a traits-based fire ecology framework for bacterial response to wildfires

Globally, wildfires represent major disturbances, burning millions of hectares annually. Wildfires can restructure soil microbial communities via changes in soil properties and microbial mortality. Fire-induced changes in bacterial communities may influence soil carbon cycling, and recovery to pre-burn community composition and function may take years. We investigated carbon cycling, soil properties, and the importance of three fire-adaptive strategies - fire survival, fast growth, and affinity for post-fire soil environmental conditions - in structuring soil bacterial communities following burns of varying temperatures in boreal forest soils. To identify taxa with each strategy, we simulated burns and incubated soils, tracking respiration and sequencing DNA and rRNA. We then quantified their abundances in the field following wildfires of varying burn severities. The importance of these strategies varies over time and with burn severity. Fire survival has a small but persistent effect on structuring burned soil communities. Fast growing bacteria rapidly colonize the post-fire soil but return to pre-burn relative abundances between one and five years post-fire. Taxa with an affinity for the post-fire environment thrive post-fire, but the effect of this strategy declines by five years post-fire, suggesting that other factors such as vegetation recovery or bacterial dispersal may influence community composition over decadal timescales. Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=109 SRC="FIGDIR/small/495025v1_ufig1.gif" ALT="Figure 1"> View larger version (53K): org.highwire.dtl.DTLVardef@f6cf22org.highwire.dtl.DTLVardef@198f5fdorg.highwire.dtl.DTLVardef@13de0f2org.highwire.dtl.DTLVardef@17b4712_HPS_FORMAT_FIGEXP M_FIG C_FIG

ecology↗

bioRxiv2022

Resilience in soil bacterial communities of the boreal forest from one to five years after wildfire across a severity gradient

Wildfires can represent a major disturbance to ecosystems, including soil microbial communities belowground. Furthermore, fire regimes are changing in many parts of the world, altering and often increasing fire severity, frequency, and size. The boreal forest and taiga plains ecoregions of northern Canada are characterized by naturally-occurring stand-replacing wildfires on a 40-350 year basis. We previously studied the effects of wildfire on soil microbial communities one year post-fire across 40 sites, spanning a range of burn severity. Here, we return to the same sites five years post-fire to test a series of hypotheses about the effects of fire on bacterial community composition. We ask the following questions: (1a) Do the fundamental factors structuring bacterial community composition remain the same five years post-fire? (1b) Do the effects of fire on bacterial community composition decrease between one and five years post-fire? (1c) Do shifts in bacterial community composition between one and five years post-fire suggest resilience? (2a) Does the importance of fast growth diminish between one and five years post-fire? (2b) Do short-term post-fire responders continue to dominate the community five years post-fire? We find the following: (1a) Five years post-fire, vegetation community, moisture regime, pH, total carbon, texture, and burned/unburned all remained significant predictors of bacterial community composition with similar predictive value (R2). (1b and 1c) Bacterial communities became more similar to unburned sites five years post-fire, across the range of severity, suggesting resilience, while general structure of co-occurrence networks remained similar one and five years post-fire. (2a) Fast growth potential, as estimated using predicted 16S rRNA copy numbers, was no longer significantly correlated with burn severity five years post-fire, indicating the importance of this trait for structuring bacterial community composition may be limited to relatively short timescales. (2b) Many taxa that were enriched in burned sites one year post-fire remained enriched five years post-fire, although the degree to which they were enriched generally decreased. Specific taxa of interest from the genera Massilia, Blastococcus, and Arthrobacter all remained significantly enriched, suggesting that they may have traits that allow them to continue to flourish in the post-fire environment, such as tolerance to increased pH or ability to degrade pyrogenic organic matter. This hypothesis-based work expands our understanding of the post-fire recovery of soil bacterial communities and raises new hypotheses to test in future studies.

ecology↗