Decoupled responses of mycorrhizal fungal communities and function to recurrent wildfire

Understanding the effects of fire on ecosystem function is critical for both above- and belowground processes. Mycorrhizal fungi play essential roles belowground, yet most studies rely on DNA-based methods that capture community composition but not functional attributes. Here, we assessed mycorrhizal fungal community responses alongside functional traits of biomass, and hyphal chemistry using in-growth mesh bags. We studied 12 dry sclerophyll forest sites in the Sydney Basin, spanning gradients of historical fire frequency and fire severity from the most recent fire (the 2019/20 Black Summer fires). We evaluated the effects of fire regime and soil nutrient availability using high-throughput DNA sequencing, joint species distribution modelling, and direct measurements of biomass and hyphal elemental composition. Mycorrhizal community composition was associated with fire frequency and severity, but did not correspond to measured functional changes. In contrast, nutrient availability, particularly soil orthophosphate, had limited effects on community composition, but influenced fungal function, increasing biomass and altering hyphal stoichiometry. Our study establishes critical post-fire baselines for these traits and suggests that fire regimes select for particular mycorrhizal fungal communities, but that responses are decoupled between community composition and function three years post-fire. This highlights the resilience of mycorrhizal function under varying fire regimes. • Fire regimes shape mycorrhizal community composition but not measured functions. • Mycorrhizal fungal biomass and hyphal elemental composition are not affected by fire. • Instead, soil orthophosphate influences fungal biomass and hyphal elemental composition. • We provide baseline data for post-fire mycorrhizal fungal traits in Australia.