Root Microbiome Assembly Underpins the Ecological Role of Emergent Macrophytes in a Pilot-Scale Constructed Wetland

Constructed wetlands (CWs) depend on plant-microbe associations for pollutant transformation, yet the processes linking macrophyte identity to rhizosphere microbiome assembly remain insufficiently resolved. We compared bacterial communities associated with C. articulatus (Ca) and T. geniculata (Tg) roots with an unplanted treatment (Up) in pilot-scale CWs on the Colombian Caribbean coast. Bacterial communities were characterized by 16S rRNA gene amplicon sequencing. Co-occurrence networks, taxon-environment correlations, and the phylogenetic bin-based null model iCAMP were used to assess community structure, potential associations and assembly mechanisms. CaS supported significantly greater bacterial diversity than TgS and Up, while each macrophyte recruited a distinct community. A shared core microbiome dominated by Rhodocyclaceae, Sphingomonadaceae, Chitinophagaceae, Pirellulaceae, and Xanthomonadaceae comprised taxa with putative roles in nutrient and organic matter transformations. Several bacterial families were significantly associated with PO₄^3-, NH₄⁺, and dissolved oxygen. Assembly was predominantly stochastic, with dispersal limitation (45.2%) and ecological drift (33.2%) exceeding deterministic selection (16.5%). Nevertheless, planted treatments displayed higher network modularity and connectivity and inferred functional redundancy than Up, consistent with niche differentiation. These findings suggest that species-specific macrophyte configurations are associated with differences in root-associated microbial community composition, while assembly and network analyses provide exploratory insights into community organization. The limited replication precludes causal attribution to plant identity or conclusions regarding treatment performance. Future studies integrating controlled planting densities, increased replication, meta-omics, and root exudate characterization are needed to clarify plant-microbiome relationships and evaluate their potential relevance to constructed wetland functioning.