Brine-induced soil gradients drive microbial community assembly and ecological partitioning

Brine (produced water) releases from oil and gas infrastructure alter soil physicochemical properties, disrupt vegetation, and affect microbial communities vital for soil function. We evaluated long-term brine disturbance effects on soil microbiota at a 25-hectare boreal site in northern Alberta, Canada. Soils, including both topsoil (A horizon) and subsoil (B horizon) layers, were sampled along four 300 m transects spanning undisturbed forest to brine-impacted areas. Soil gradients intensified toward brine-impacted zones, with electrical conductivity (EC) increasing from 0.1 to 40 dS m^-1, sodium adsorption ratio (SAR) from 0.1 to 41, and pH from 4 to 8. Microbial diversity declined with rising EC, SAR, and pH, however, even in very strongly saline soils (EC > 16 dS m^-1), Shannon diversity indices remained above 7. Approximately one-third of the microbial genera shifted in abundance along these gradients, with salinity-adapted taxa enriched and key oligotrophic groups declining. We identified tipping points in salinity, sodicity, and pH gradients—at EC 1.9 and 4.2 dS m^-1, SAR 3.5 and 6.4, and pH ∼5.5—coinciding with major shifts in community composition. Habitat partitioning was evident, with 27% and 39% of taxa specialized to unimpacted and brine-impacted soils, respectively, while 20% were generalists. Network analysis revealed denser community assembly but reduced robustness in brine-impacted soils, indicating greater vulnerability to environmental perturbations. These findings highlight how soil microbiota reflect both the detrimental effects of brine disturbance and adaptive responses, underscoring their value as bioindicators for soil health assessment in salt-impacted landscapes.