Shifts in precipitation patterns pose a significant threat to tallgrass prairies by altering soil microbial communities essential for plant resilience. Andropogon gerardii, a key species in tallgrass prairies, spans the North American precipitation gradient. However, it remains unclear to what extent the rhizosphere is affected due to the plant host-environmental interaction. To assess how environmental and host factors shape rhizobiome, we conducted a biogeographical survey of A. gerardii across 25 sites within its historical range, and characterized microbial communities from the rhizosphere and adjacent local soil samples using 16S amplicon sequencing. We showed that while geographic location largely structured both rhizosphere and soil communities, regional precipitation emerged as a primary driver of microbial community assembly. We observed distinct microbial divides across the dry, mesic, and wet regions of the North American \"arid-humid divide.\" Importantly, we found the first compelling large-scale evidence that regional precipitation profoundly influenced rhizobiome assembly patterns. In the aridest regions, rhizosphere microbial communities exhibited significantly more predicted stochasticity than the local soil, and had microbial families related to host-benefiting functions. Our study highly suggested enhanced plant host selection for specific microbial variants in response to abiotic stress.