Root anatomical traits modulate the assembly and nitrogen‐transformation potential of root‐associated microbiomes in a temperate steppe

Summary Plants have evolved numerous belowground strategies to capture nutrients. While root functional differentiation between absorption and transportation has been inferred from order‐based traits, the role of microbiomes in mediating this differentiation remains unclear. We measured traits (anatomical, chemical, and morphological) of lower order (absorption‐dominated) and higher order (transportation‐dominated) roots of 37 herbaceous species (13 monocots and 24 dicots) in a temperate grassland. Furthermore, we employed high‐throughput quantitative polymerase chain reaction and 16S rRNA gene sequencing to investigate bacterial nitrogen‐transformation gene abundance, diversity, and community assembly along the soil‐root continuum (rhizosphere, rhizoplane, and endosphere) and analyzed their relationships with root traits. Monocot roots exhibited greater bacterial diversity and nitrogen‐transformation gene abundances than dicots. Within dicots, lower order roots showed higher bacterial diversity and nitrogen‐transformation gene abundances than higher order roots, a pattern not observed in monocots. Lower order roots, characterized by higher cortex proportion, facilitated the enrichment of diverse bacteria and recruitment of nitrogen‐transformation microorganisms. These patterns were associated with a decrease in homogeneous selection from lower order to higher order roots. This study reveals the mechanisms of functional differences among herbaceous root orders from a microbiome perspective, offering further insights into how root–microbe interactions underpin nitrogen‐transformation potential in terrestrial ecosystems.