Host species identity outweighs life form in shaping the root endophytic bacterial communities of 10 Cymbidium species grown under uniform conditions

Research on orchid microbiomes has long been dominated by mycorrhizal fungi, whereas the community structure of orchid-associated bacteria remains poorly understood, particularly across host life forms. We compared the root endophytic bacterial communities of 10 Cymbidium species (six epiphytic and four terrestrial), all cultivated in the same greenhouse. Communities were profiled by 16S rRNA gene amplicon sequencing, with six biological replicates per species. After rarefaction to 29,628 sequences per sample, we obtained 129,922 amplicon sequence variants (ASVs) assigned to 33 phyla and 779 genera. Proteobacteria dominated the communities (mean relative abundance 83.4%), followed by Actinobacteriota and Acidobacteriota; the most abundant genera were Pseudomonas, Sphingomonas , the Burkholderia–Caballeronia–Paraburkholderia complex, Bradyrhizobium and Reyranella . Only seven ASVs were shared by all 10 species (each detected in at least one replicate sample of every species), and only one ASV occurred in all 60 samples, indicating an extremely small interspecific core microbiome. No alpha-diversity index differed between life forms. Among host species, however, the Shannon, Simpson, and Pielou evenness indices differed significantly among species. Single-factor (marginal) permutational multivariate analysis indicated a hierarchy of host effects: life form explained only 6.9% of community variation, infrageneric section 35.6%, and host species 59.9%, with significant differentiation even among species within the same section ( R 2 ≈ 0.51); because life form, infrageneric section and host species are partially confounded in this design, these values describe associations rather than fully independent effects. Random forest classification independently corroborated this hierarchy. Community composition correctly assigned all 60 samples to their host species, whereas life-form prediction did not generalize across species. Under leave-one-species-out cross-validation, life-form accuracy fell to 50.0%, below the 60% majority-class baseline. LEfSe analysis identified 279 biomarker taxa spanning all 10 species. Predicted functional profiles were dominated by biosynthesis pathways (69.8%) and were largely stable between life forms. The neutral community model explained little of the variation in ASV occurrence frequency ( R 2 = 0.035). This indicates that neutral dispersal alone cannot account for community assembly. The normalized stochasticity ratio averaged 0.576, only slightly above the 0.5 threshold for stochastic dominance, pointing to predominantly deterministic assembly with a secondary stochastic contribution. Within the limits of this partially confounded design, these results indicate that, with environmental heterogeneity minimized, host species identity is a stronger correlate of root endophytic bacterial community differentiation than life form in Cymbidium , providing a basis for the species-specific utilization of beneficial endophytes in orchid cultivation and conservation.