This study quantifies the processes governing community assembly in subsurface microbial communities at the Hanford Site, Washington State. The researchers developed an analytical framework to estimate the relative influences of Selection, Drift, and Dispersal on community composition, as well as to identify abiotic factors that impose these processes. They found that Drift alone accounts for ~25% of spatial turnover in community composition, while Selection is the dominant force in deeper, finer-grained sediments (~60% of turnover), driven by an unmeasured spatially structured environmental variable. In shallower, coarser-grained sediments, Selection is weaker (~30% of turnover), influenced by hydrological factors. Low Dispersal can account for nearly 30% of turnover, primarily due to spatial isolation between sediment types. Highly permeable sediments are associated with high Dispersal, homogenizing community composition and accounting for over 20% of turnover. The framework distinguishes between Selection and Dispersal Limitation by reversing the direction of inference, using ecological patterns to identify environmental and spatial factors that impose these processes. It employs null models and phylogenetic turnover to quantify the influence of ecological processes. The study highlights the importance of considering both measured and unmeasured environmental variables in understanding microbial community assembly. The framework provides insights that cannot be achieved with traditional approaches, offering a more unified understanding of microbial communities. The results suggest that ecological processes such as Selection, Dispersal Limitation, and Drift play critical roles in shaping community composition, with environmental factors influencing these processes in complex ways. The study underscores the need for further research to refine these models and better understand the interplay between ecological processes and environmental variables in microbial communities.This study quantifies the processes governing community assembly in subsurface microbial communities at the Hanford Site, Washington State. The researchers developed an analytical framework to estimate the relative influences of Selection, Drift, and Dispersal on community composition, as well as to identify abiotic factors that impose these processes. They found that Drift alone accounts for ~25% of spatial turnover in community composition, while Selection is the dominant force in deeper, finer-grained sediments (~60% of turnover), driven by an unmeasured spatially structured environmental variable. In shallower, coarser-grained sediments, Selection is weaker (~30% of turnover), influenced by hydrological factors. Low Dispersal can account for nearly 30% of turnover, primarily due to spatial isolation between sediment types. Highly permeable sediments are associated with high Dispersal, homogenizing community composition and accounting for over 20% of turnover. The framework distinguishes between Selection and Dispersal Limitation by reversing the direction of inference, using ecological patterns to identify environmental and spatial factors that impose these processes. It employs null models and phylogenetic turnover to quantify the influence of ecological processes. The study highlights the importance of considering both measured and unmeasured environmental variables in understanding microbial community assembly. The framework provides insights that cannot be achieved with traditional approaches, offering a more unified understanding of microbial communities. The results suggest that ecological processes such as Selection, Dispersal Limitation, and Drift play critical roles in shaping community composition, with environmental factors influencing these processes in complex ways. The study underscores the need for further research to refine these models and better understand the interplay between ecological processes and environmental variables in microbial communities.