Enrichment is not necessarily recruitment in root microbiome assembly

Rhizosphere enrichment is often interpreted as evidence that plants recruit beneficial microorganisms from soil, but enrichment and recruitment describe different levels of inference. Enrichment is a compositional pattern, whereas root association is a broader outcome produced by distinct processes through which microorganisms reach, attach to, colonize, and persist on roots. Within this outcome, recruitment in a narrower, directional sense denotes cases in which host-derived cues promote microbial attraction to, and/or specific attachment on, roots. Recruitment in this sense is independent of benefit because recruited microbes may be beneficial, neutral, or harmful. This distinction is often blurred when differential abundance alone is used to infer host-driven recruitment. Recruitment claims should instead be evaluated through microbial traits and host physiological, immune, and metabolic states that jointly determine root association. Motile bacteria with flagella, chemotaxis systems, and extracellular polysaccharides can respond directionally to root-derived cues and stabilize attachment on root surfaces. Many root-associated fungi lack comparable active motility, and their enrichment more often reflects contact opportunity, surface compatibility, and host immune permissiveness. Recent work further shows that enrichment can be uncoupled from plant benefit, because host physiological states may create permissive niches, whereas strain-level competition determines functional outcomes. We therefore propose a framework that separates microbe-driven attraction and attachment from host-mediated accommodation and persistence. Rhizosphere enrichment is thus an observable outcome, not a default signature of recruitment or benefit. Precise recruitment language requires trait-based, spatial, temporal, and functional evidence linking microbial presence to the mechanisms that generate and maintain root association.