A simple pyrocosm for studying soil microbial response to fire reveals a rapid, massive response by Pyronema species

We have designed a simple, inexpensive system for the studying the response of soil microbes to fire. This system allows one to create post-fire environments in soil in reproducible and realistic ways. Using it we show that the peak soil temperature achieved at a given depth occurs hours after the fire is out, lingers near peak temperature for a significant time, and is accurately predicted by the log of soil depth and the mass charcoal burned. Flash fuels that left no large coals were found to have a negligible soil heating effect. Coupling this system with Illumina MiSeq sequencing of the control and post-fire soil we show that we can stimulate a rapid, massive response by Pyronema, a well-known genus of postfire fungus, from uninoculated forest soil within two weeks of a test fire. This specific stimulation occurs in a background of many other fungal taxa that do not change significantly with the fire, although there is an overall reduction in richness and evenness. Extrapolating from the physical relationships we predict soil heating effects in wild fires are likely to be very patchy across the forest floor but the width of a survivable \"goldilocks zone\" will stay relatively constant across a range of fuel loads. We further predict that a necromass zone above it, which represents an open niche for pyrophilous microbes, increases in size rapidly with addition of fuel, and then remains nearly constant over a broad range of fuel loads. The simplicity of this experimental system, coupled with the availability of a set of sequenced, assembled and annotated genomes of pyrophilous fungi, offers a powerful tool for dissecting the ecology of post-fire microbial communities.