Ecological niche and plantation composition differentially shape bacterial community composition and assembly across the root-soil continuum in Populus simonii plantations

Mixed plantations are widely established to restore degraded sandy landscapes, yet bacterial community patterns across roots and surrounding soils remain poorly understood. This study examined bacterial diversity, composition, inferred assembly patterns, environmental associations, and predicted functional potential in contrasting Populus simonii plantations. We analysed 60 composite samples from root (R), rhizosphere (RS), and non-rhizosphere soil (NRS) in four stands in the Kubuqi Desert, northern China. These comprised a P. simonii monoculture and mixtures with Salix psammophila , Caragana korshinskii , and Pinus sylvestris var. Mongolica , with five plots sampled per stand. Root communities exhibited lower richness and diversity and distinct taxonomic composition compared with the two soil compartments. PERMANOVA confirmed significant effects of both ecological niche ( F = 24.995, p = 0.001) than stand type ( F = 2.649, p = 0.005) on community composition, with niche showing the stronger association. Null-model classifications varied among sample groups. Homogeneous selection was the most frequently assigned category in most groups, whereas dispersal limitation accounted for a larger proportion of comparisons in the sampled P. simonii and C. korshinskii mixture. Co-occurrence patterns also varied across ecological niches and plantation types, with the rhizosphere showing the highest connectivity and mixed plantations generally exhibiting greater connectivity than the monoculture. Soil water content was strongly associated with bacterial community variation ( r² =0.815, p = 0.0005). Predicted KEGG and COG profiles varied among niche and plantation combinations, but no consistent functional response to mixed planting was observed. Together, these results reveal that bacterial taxonomic composition and community assembly responded to different ecological dimensions, ecological niche was more strongly associated with compositional differentiation, whereas assembly processes varied more clearly among the four sampled stands. This decoupling highlights the multidimensional organization of bacterial communities across the root-soil continuum in sandy plantations.