The hierarchy of root branching order determines bacterial composition, microbial carrying capacity and microbial filtering.

Fine roots vary dramatically in their functions, which range from resource absorption to within-plant resource transport. These differences should alter resource availability to root-associated microorganisms, yet most root microbiome studies involve fine root homogenization. We hypothesized that microbial filtering would be greatest in the most distal roots. To test this, we sampled roots of six temperate tree species from a 23-year-old common garden planting, separating by branching order. Rhizoplane bacterial composition was characterized with 16S rRNA gene sequencing, while bacterial abundance was determined on a subset of trees through flow cytometry. Root order strongly impacted composition across tree species, with absorptive lower order roots exerting the greatest selective pressure. Microbial carrying capacity was higher in absorptive roots in two of three tested tree species. This study indicates lower order roots as the main point of microbial interaction with fine roots, suggesting that root homogenization could mask microbial recruitment signatures.