Differential impacts of fall versus spring prescribed burns on microbial biomass, richness, and composition in young mixed conifer forests

Prescribed burns aim to restore ecological processes and mitigate high-severity wildfire risks. Historically, California wildfires occur in summer or fall, but due to limited burning opportunities, prescribed burns occur in fall and spring. Yet, whether the burn season affects ecological outcomes is largely unknown. Here, we test prescribed burn season impacts on soil burn severity, bacterial and fungal abundance, richness, and composition as assessed with 16S and 18S qPCR and 16S and ITS2 Illumina MiSeq. We implemented a Before-After-Control-Impact design with 9 California montane mixed conifer forest stands (4 fall, 4 spring, 1 unburned control). We assessed resilience by collecting 6 sub-samples per stand at 6 time points ranging from pre-fire to 3 days, 1 and 6 months, and 1 and 2 years after fall and spring burns. Fall burns significantly reduced bacterial and fungal abundance and richness, whereas spring burns did not. Indeed, at 24 days after fall burns compared to pre-fire, bacterial and fungal richness were reduced by 24-30%, with richness of ectomycorrhizal fungi reduced by 45% and saprobic fungi by 28%. After the fall burn, fungal richness recovered within a year and fungal abundance within 6 months, whereas bacterial abundance and richness recovered in 2 years. Bacterial and fungal communities experienced compositional turnovers after both burns, with the emergence of several genera of "fire-loving" pyrophilous bacteria (Massilia, Paenibacillus) and fungi (Geminibasidium, Pyronema, Neurospora). Our findings suggest that, although seasonal differences were evident, both fall and spring prescribed burns mitigate wildfire impacts while promoting the succession of pyrophilous microbes.