Distinct assembly processes shape bacterial communities along unsaturated, groundwater fluctuated, and saturated zones.

The subsurface soil environment through the unsaturated (vadose) zone and saturated (below groundwater table) zone is one of the most active layers in the Earth's surface with biogeochemical interactions. Geochemical variables and geographic distance are key driving forces shaping the distribution of soil microbial communities, but our understandings are mainly limited to surface soil or shallow unsaturated zone (1-3 m beneath the ground). In this study, soil and sediment samples were collected from the unsaturated zone, through groundwater fluctuated zone, to saturated zone (up to 20 m) to unravel the assembly processes mediating vertical bacterial community succession across these three zones. Our results suggested both geochemical niches and bacterial diversity had different vertical patterns in each zone. With increased depth, pH increased and nutrient levels (C, N, P, K) and bacterial diversity declined in the unsaturated zone, and nutrients and bacterial diversity remained low levels after reaching the fluctuated and saturated zones. Nutrients were the key drivers shaping bacterial variation in the unsaturated zone, but limited nutrients and only 'depth' significantly explained the variations in the fluctuated zone and saturated zone, respectively. The co-occurrence network supported a more species co-existence pattern in the unsaturated zone than that in the other two zones. Due to the geochemical variations across three zones, the assembly of phylogenetically more clustered communities was observed through deterministic processes (e.g., 55% homogenizing selection) in the unsaturated zone, but the stochastic process (e.g., 50%-70% dispersal limitation) was more important in the fluctuated and saturated zones. These findings together suggested that the vertical distribution of soil bacterial community assembly was zone-specific and shaped by the degree of deterministic vs. stochastic processes. Our results provide a novel insight into the microbial community assembly across three different ecosystems in the Earth's critical zone and shed a light on subsurface biogeochemical processes.