Bacteria-soil-plant linkages underlie the mosaic structure of the soil bacterial communities in near-natural stands of Białowieża Primeval Forest.

Primary temperate forests serve as a natural framework for studying linkages between vegetation, soil properties and microbial communities under minimal human disturbance. Here, we characterize how soil bacterial communities and functional potential vary across five dominant forest types of the Białowieża National Park, representing a natural mosaic of vegetation and edaphic conditions. Using full-length 16S rRNA Oxford Nanopore sequencing, functional profiling via BIOLOG EcoPlates, and applying multivariate analyses, we detected clear differences in bacterial composition and carbon-substrate utilization profile among forest types. Distance-based redundancy analysis (dbRDA) identified soil pH as the primary abiotic gradient shaping bacterial communities, while RLQ, fourth-corner and multiblock sPLS analyses consistently supported bacteria-soil-vegetation linkages. Three consistent ecological clusters emerged across the forest mosaic. Coniferous forests with acidophilic bacterial assemblages linked to strongly acidic soils and ericaceous understoreys, broadleaf forests with bacterial genera associated with moderately acidic, nutrient-depleted soils and shade-tolerant vegetation, and alder forests characterized by richer, more metabolically active microbial communities occurring in less acidic soils with tall-herb understoreys. Mixed forests displayed broad internal variability, reflecting their wide range of vegetation and soil conditions. Overall, environmental filtering structures distinct bacterial communities of this primary temperate forest, providing a valuable baseline for future plant-soil-microbiome studies.