Soil viral communities shifted significantly after wildfire in chaparral and woodland habitats.

Increased wildfire activity warrants more research into fire-driven biotic changes in soil, including soil viral communities, given the roles of soil microbes in organic matter decomposition, nutrient cycling, and post-fire recovery. Leveraging viral size-fraction metagenomes (viromes), here we studied viral community responses to wildfire in woodland and chaparral soils at five timepoints over 1 year following the California LNU Complex wildfire. We also compared post-fire samples to unburned controls at the final three timepoints and leveraged published viromes from the same sites nine months before the fire as pre-burn controls. Viral community composition differed significantly in burned samples compared to controls from both habitats, as did soil chemistry and prokaryotic communities (16S rRNA gene amplicons). Viromic DNA yields (a proxy for viral particle abundances) indicated initial viral biomass reductions due to the fire, but a return to baseline abundances (indistinguishable from controls) within five months. Fire-associated habitat filtering was further indicated by a comparison to the PIGEON viral "species" (viral operational taxonomic unit (vOTU)) reference database, with vOTUs from a burned conifer forest representing 19%-31% of PIGEON vOTUs detected in the burned habitats but only 0.6%-6% in controls. Together, these results indicate significant changes in soil viral communities due to wildfire, attributable at least in part to concomitant changes in their prokaryotic host communities and soil physicochemistry.