Fire impacts tropical communities of soil fungi through changes to plant community composition, litter and soil chemistry

Fire has been predicted to be more severe and frequent in forests of the Australian Monsoon Tropics over the coming decades. The way in which groups of ecologically important soil fungi respond to disturbance caused by fire has not been studied in tropical forest ecosystems. Ectomycorrhizal (EM) fungi are important tree symbionts and saprotrophic fungi drive soil nutrient cycles. We analysed both publicly-available environmental DNA sequence data as well as soil chemistry data to test a hypothesis that fire events (1970 - 2017) in a contiguous tropical forest have altered the composition and diversity of EM and saprotrophic soil fungi. We tested this hypothesis by measuring community-level taxonomic composition, fungal diversity, species richness and evenness. We determined whether changes in fungal communities were associated with fire-altered soil chemical/physical properties, vegetation types, or the direct effect of fire. Soil fungi differed in abundance and community phylogenetic structure between forest sites that had experienced fire, and those sites dominated by unburned forest. Communities of EM fungi were structurally altered by fire at shallow soil horizons, as well as by vegetational changes between burned and unburned sites at deeper soil horizons. In contrast, fires influenced community composition of saprotrophic fungi by changing soil nutrient levels and altering litter composition. Pyrophilic, truffle-like EM fungi that rely on mycophagous mammals for dispersal were abundant at recently burned sites. We conclude that fire impacts EM fungi primarily by changing plant communities, whereas fire impacts saprotrophic fungi by reducing soil nutrient levels and altering litter composition.