Post-fire changes in soil microbial communities constrain heterotrophic respiration following wildfires

Background/Question/Methods

We currently lack a detailed understanding of changes in heterotrophic respiration following wildfires. This uncertainty hinders our ability to predict whether an ecosystem will be a net source of C to the atmosphere or a C sink following a fire. Studying soil microbial responses to fire is one promising approach to better understand post-fire patterns in heterotrophic respiration. We hypothesized that post-fire changes in soil microbial communities would constrain heterotrophic respiration following fires. We tested this hypothesis using a two-fold approach. First, we characterized soil fungal responses to wildfires in boreal forests using a fire chronosequence in interior Alaska with sites that burned in 2010, 2004, 1999, 1987, 1956, and 2 mature forest sites. We characterized fungal abundance at each site by measuring the length of fungal hyphae in soil. To understand changes in fungal community composition, we used pyrosequencing of 18S rDNA. To examine post-fire changes in decomposition rates at each site, we conducted a litterbag decomposition experiment using litter from aspen and black spruce trees. For our second approach, we conducted a global meta-analysis of published microbial responses to fire and, where possible, we also analyzed concurrent changes in soil CO₂ emissions following fires.

Results/Conclusions

Soil fungal communities in boreal forests were negatively affected by wildfires. Recently burned sites (1-12 years following fire) had significantly lower fungal abundance than older sites (P=0.0001). In addition, fungal community composition varied significantly with the time since fire (PerMANOVA: r²=0.231 P=0.001). The relative abundance of Basidiomycota significantly increased with stand age (r²=0.63, P=0.001) while Ascomycota relative abundance decreased significantly with stand age (r²=0.45, P=0.01). In support of our hypothesis, the percent mass loss of aspen and spruce litter increased significantly with stand age (Aspen: r² =0.69, P<0.001, Spruce: r²=0.61, P<0.001). Litter decomposing in older forest stands lost 11-18% more mass than litter decomposing at recently burned sites. These results are in agreement with the broader findings from our meta-analysis. Across all published studies, fire reduced microbial abundance by an average of 33.2% and fungal abundance by an average of 47.6%. Furthermore, post-fire changes in microbial abundance were significantly positively correlated with changes in soil CO₂ emissions (r=0.62, P=0.025). Overall, our results indicate that post-fire changes in microbial abundance and community composition may constrain heterotrophic respiration in post-fire soils. These data may be useful for refining models that predict the impact of wildfires on climate warming.