Relative and absolute abundances of bacteria and fungi in soil at the Achenkirch soil warming experiment

Long-term soil warming may alter microbial community structure and functioning in forest soils, thereby affecting carbon and nutrient cycling processes. We examined the effects of >14 years of soil warming (+4°C during snow-free seasons) on the fungal biomass marker ergosterol, and on fungal and bacterial communities in a spruce dominated mountain forest in the Austrian Alps. Soil warming decreased ergosterol, and the ergosterol-to-microbial biomass carbon (MBC) ratio at 0-10 and 10-20 cm soil depth, with a stronger decline in ergosterol, indicating a higher sensitivity of fungi than bacteria to long-term warming. Warming also shifted the fungal community at both soil depths, favoring Boletus luridus, an ectomycorrhizal (ECM) fungus, which emerged as the dominant OTU in warmed plots. Its abundance was positively associated with fine root biomass, root tip density, and soil respiration in warmed plots but negatively with ergosterol and MBC in control plots at 010 cm depth. The dominance of ECM over saprotrophic fungi (SAP) under warming at topsoil likely resulted from increased fine root production and enhanced competition for substrates and nutrients. Bacterial abundance and community composition remained mostly unaffected at both depths, likely due to their greater resilience to elevated temperatures and their high taxonomic diversity. Our findings therefore suggest that long-term warming primarily affects fungal community composition and functional traits, thereby enhancing the contribution of ECM with fine roots to the carbon cycle in the calcareous forest soil.