Natascha Kljun

Wildfires are a natural disturbance shaping boreal forests, but their effects on soil microbial communities and biogeochemical cycling remain unclear, particularly in Fennoscandia. We investigated the impact of fire severity and post-fire management on the soil microbial community two years after a boreal wildfire in Sweden. We quantified growth rates of bacteria, saprotrophic fungi (SF), ectomycorrhizal fungi (EMF), microbial respiration and carbon use efficiency (CUE) in an unburnt control site and areas affected by low- and high-severity fires, where trees were either left standing or salvage-logged. Wildfire and salvage-logging were associated with reduced microbial respiration and shifts in microbial guild structure from ectomycorrhizal to saprotroph dominated soils. Bacterial growth declined after fire and tree removal, and EMF growth was reduced by 20% and 59% following low- and high-severity fires, respectively. Salvage-logging live trees after a low-severity fire more than tripled the activity of SF groups in comparison to EMF. Although SF activity increased post-fire, there was no corresponding rise in soil organic matter (SOM) decomposition. Seasonal variation in CUE was linked to SOM content, with lower CUE in SOM-depleted soils. Despite the disturbance by fire and logging, the cumulative CUE remained unchanged in all treatments over the study period. These findings suggest that reduced carbon emissions and a shift in microbial guilds from mycorrhizal to saprotroph-dominated - may influence the long-term microbial contributions to soil carbon storage and forest recovery.