Microbes are vital in driving mineral weathering and soil biogeochemical processes either directly or indirectly through their metabolic activities. Both composition and function of microbial communities are known to respond to gradients in environmental variables, which can be expected to change across soil horizons and consequently give rise to changes in weathering rates with depth. However, most studies of microbial community composition and function in soils to date have focused only on the upper 0-10 cm of soil, neglecting deeper horizons, where organic carbon and oxygen concentrations decrease and mineralogy shifts towards that of primary minerals. To identify environmental drivers of soil microbial community composition to depth, we collected soil samples from the surface to bedrock at five field sites spanning North America and Australia, all developed from shales. Geochemical, mineralogical, and physical properties of the soils were analysed, and microbial community composition was evaluated through DNA extraction and 16S rRNA amplicon sequencing.
Across all sites and depths, total clay concentration and the mineralogical composition of these clays emerged as significant environmental drivers of microbial community composition. Relative abundances of taxa known to exhibit primarily aerobic and/or heterotrophic metabolisms (Actinobacteria, Acidobacteria, Planctomycetes, Verrucomicrobia) tended to decrease with depth; whereas relative abundances of taxa known to exhibit primarily fermentative or anaerobic metabolisms (Firmicutes, Bacteroidetes, Crenarchaeota) tended to increase with depth. Environmental drivers produced distinct microbial community compositions between the North American and Australian sites despite their similar parent material. SIMPER analysis identified Acidobacteria, Actinobacteria, and Bacteroidetes as accounting for a combined 25% of average community dissimilarity between the North American and Australian sites. The clear depth-related changes in microbial community composition in our study emphasise the need to understand microbial community composition and function in subsurface soil horizons and hence improve quantification of their roles in weathering and nutrient cycling processes.