Forest fires alter soil fungal communities in boreal ecosystems: Implications for carbon cycling

Background/Question/Methods Forest fires are pervasive disturbances in boreal ecosystems and the frequency and intensity of boreal forest fires is predicted to increase under global change conditions. Understanding the microbial consequences of boreal forest fires is important given that these ecosystems contain large stocks of soil carbon. Since fungi regulate organic matter decomposition, fire-induced changes in soil fungal communities may have consequences for carbon cycling and storage in boreal ecosystems. We hypothesized that forest fires alter soil fungal communities, owing to fire-induced losses in aboveground vegetation and soil organic matter. To test our hypothesis, we examined fungal community structure in two boreal forest sites – a 1999 burn site and a mature forest site - using nucleotide analog addition, cloning, and DNA sequencing of 18S rDNA. In addition, to elucidate the relative importance of forest fires in structuring soil fungal communities, we sampled from ongoing nitrogen enrichment experiments at both forest sites. Finally, we aligned our DNA sequences with DNA sequences from a concurrent fungal resource study in this area to determine if substrate use by fungal taxa differed between the mature and recently burned forest.

Results/Conclusions The mature and burn forests were both dominated by basidiomycetes in the orders agaricales and russulales. The burn site also had a high occurrence of chytridiomycete fungi. At high levels of taxonomic resolution (99% and 97% sequence similarity cut-offs), we found no difference in fungal species richness and diversity between the mature and burn site. However, in support of our hypothesis, analyses with NMS and MRPP revealed that fungal communities differed between the mature and burn site (A=0.08, P<0.0001). Although the fungal communities differed, nitrogen addition plots from both forest sites grouped together in ordination space (A=0.02, P=0.02), indicating that nitrogen enrichment may have a convergent effect on soil fungal communities. The burn site contained fungi that degrade labile carbon compounds (i.e. arginine and glutamate). Conversely, the mature site was dominated by fungi that specialize in the decomposition of recalcitrant carbon (lignocellulose and protein-tannins) but also contained labile carbon users. Given the lower occurrence of recalcitrant carbon degraders in the post-fire soil, forest fires may slow carbon cycling in boreal ecosystems due to fire-induced changes in fungal community composition. Forthcoming analyses will determine the amount of time required for fungal communities to return to pre-fire composition.