Plant influence shapes Andropogon gerardii rhizobiome assembly strategies in response to decreasing precipitation.

Predicted changes in precipitation threaten tallgrass prairies by altering microbial communities essential for plant resilience. Andropogon gerardii, a dominant tallgrass, spans the North American precipitation gradient. However, it remains unclear to what extent the rhizosphere microbiomes (rhizobiomes) are influenced by the plant-host-environmental interaction. To assess how environmental and host influences shape the rhizobiome, we surveyed A. gerardii populations across 25 remnant prairie sites within its native range in the United States, characterizing the microbiomes in the rhizosphere and soils using 16S amplicon sequencing. We demonstrated that while geographic location was the primary driver of community structure of both rhizosphere and soil communities, regional precipitation emerged as an influential determinant of the microbial community assembly. We observed distinct microbial divides across the dry and wet regions of the North American "arid-humid divide." Importantly, we found compelling large-scale evidence that regional precipitation has a profound influence on rhizobiome assembly. In the most arid regions, rhizosphere microbial communities exhibited significantly greater compositional convergence than local soil communities and harbored distinct, enriched sub-populations of taxa previously associated with host-benefiting functions. Our study aims to lay the groundwork for future investigations into the functional pathways through which limited precipitation shapes rhizobiome assembly, and how these effects are further impacted by host influence and local adaptation.IMPORTANCEIn this study, we conducted a biogeographical survey of the native tallgrass species Andropogon gerardii across 25 remnant prairie sites spanning the contiguous United States. Using 16S rRNA amplicon sequencing, we analyzed microbial communities from both local soils and plant-associated rhizobiomes. Our results demonstrate that regional precipitation is an influential driver of microbial community assembly, with clear compositional shifts observed across the 100th meridian, the North American "arid-humid divide." Critically, in the most arid locations, rhizobiomes converged toward a more homogeneous community structure, with both βNTI and RCBray indicating more deterministic assembly, consistent with intensified selective filtering under limited precipitation. These patterns establish a framework for future work to dissect the specific host and microbial mechanisms that enforce deterministic community assembly under precipitation stress.