Vertical Co-Evolution of Bacterial Communities and Sedimentary Geochemistry Across a Deep Regolith Profile in the Black Soil Critical Zone of Northeast China

Black soils are globally important for agricultural productivity, yet most studies of soil microbial communities are confined to the plow layer, leaving the deep regolith–bedrock interface largely unexplored. Here, we investigated bacterial diversity, community assembly, and co-occurrence networks along a 34-m regolith profile from the mollic epipedon to unweathered bedrock in the Songnen Plain, Northeast China. Bacterial alpha diversity exhibited a depth-dependent pattern with a secondary peak at 7.2–7.6 m, coinciding with buried organic carbon derived from paleolacustrine sedimentation. Community composition shifted significantly with depth, with Acinetobacter and Achromobacter enriching in deep layers as indicators of bio-weathering processes. Heterogeneous selection dominated community assembly, reinforced by strong dispersal limitation along the sedimentary gradient. Random forest and structural equation modeling identified depth as the primary driver, with pH, chemical weathering, and silt content mediating indirect effects. Network analysis revealed Chloroflexi as keystone taxa, with modules linked to methanogenic and nitrification functions in anaerobic deep strata. These findings demonstrate that sedimentary geochemical gradients and bacterial communities have co-evolved throughout the Black Soil Critical Zone, providing a mechanistic framework for whole-profile soil health management in one of the world’s most productive agricultural regions.