Spatial distribution of microbial biomass and residues across soil aggregate fractions at different elevations in the Central Austrian Alps

The distribution of microbial biomass and residues and their role in carbon (C) dynamics within soil aggregate fractions in the Alps are still poorly explored. We assessed the distribution of bacterial and fungal residues and their impact on the contribution of the microbial community to C dynamics within soil aggregate size fractions at different elevations. We sampled three sites located at different elevations (1300, 1600 and 2100 m a.s.l.) and subject to different temperature and precipitation regimes in the Central Austrian Alps. Fungal biomass and residues were higher in macro-aggregate fractions compared to micro-aggregates, as indicated by the ratios ergosterol/microbial biomass C and fungal C/bacterial C, suggesting a combined but discrete contribution of fungi and bacterial residues to soil aggregate formation. A greater contribution of microbial residues to soil organic C (SOC) in macro-aggregates demonstrates the importance of microbial residues for the soil organic matter pool's capacity to function as an active nutrient sink and source reservoir. The mid- and low-elevation spruce sites were dominated by fungi and higher aggregate stability, whereas the high-elevation grassland site exhibited a stronger bacterial community as indicated by fungal C/bacterial C ratio. In addition, greater proportions of fungal and bacterial residue in SOC at mid-elevation spruce and high-elevation grassland sites respectively, indicate differential role of fungi and bacteria in SOC sequestration. However, the observed changes in microbial community and residue contribution were mainly driven by variations in soil pH, C/N ratio and fine root biomass. Our findings indicate that fungi played a key role in the internal cycling of SOC and aggregate stability at the mid-elevation spruce site, while bacterial residues were successively activated and contributed increasingly to SOC sequestration at the high-elevation grassland site.