Stand age and fire return interval shape soil microbial communities in young, postfire lodgepole pine stands

As wildfire activity increases globally, understanding how soil microbial communities vary in young postfire forests and the effect of short fire-return intervals on these communities becomes increasingly important for anticipating postfire ecosystem function. We asked how soil bacterial and fungal communities varied during early postfire stand development in lodgepole pine (Pinus contorta var. latifolia) forests, and whether a short-interval (16 yr) reburn altered those trajectories. We sampled soils in 6-, 22-, and 34-year-old stands in Grand Teton National Park (Wyoming, USA) and used high-throughput sequencing to assess community composition, environmental drivers, and fire responsive taxa. Bacterial and fungal community composition shifted with stand age under typical fire return intervals. For fungi, Ascomycetes declined in relative abundance with stand age whereas Basidiomycetes increased, consistent with the expected increase in ectomycorrhizal fungi associated with lodgepole pine. Litter mass and soil nutrient properties explained 28.6% and 23.9% of variance in bacterial and fungal community composition, respectively. Following the short-interval stand-replacing fire, bacterial communities in 6-year-old stands closely resembled nearby stands (22-year-old) that did not reburn, whereas fungal communities more closely resembled nearby 6-year-old stands that burned in the historical long (> 100 yr) fire-return interval. These results suggest that microbial recovery during early stand development in postfire forests is shaped by changes in resource availability, and that short fire-return intervals may have greater effects on fungi than on bacteria.