Rhizosphere and Soil Depth Differentially Shape Microbial Community Composition and Assembly in Phragmites australis Salt-Marsh Soils.

Common reed (Phragmites australis) is a native dominant plant in many coastal wetlands. To determine how rhizosphere effects and soil depth shape microbial communities, we sampled the rhizosphere and three bulk-soil layers (0-15, 15-30, and 30-45 cm) in a monodominant common-reed stand in a coastal salt marsh. Soil physicochemical properties and bacterial and fungal α-diversity, community composition, and assembly processes were evaluated using one-way ANOVA, principal coordinates analysis (PCoA), permutational multivariate analysis of variance (PERMANOVA), neutral community models, and phylogenetic null models. Rhizosphere pH was lower than that of 0-15 cm bulk soil (mean 8.434 vs. 8.712) but remained alkaline; soil organic matter, total nitrogen, hydrolyzable nitrogen, and total phosphorus were greatest in the rhizosphere. Neither bacterial nor fungal richness or Shannon diversity differed significantly among compartments (p > 0.05); fungal Shannon means ranged from 2.083 to 3.077, with relatively higher Bacteroidota and lower Acidobacteriota abundance in the rhizosphere. Fungal composition did not differ significantly (pseudo-F = 0.609, R2 = 0.102, p = 0.9112), although Mucoromycota and Rozellomycota were relatively more abundant in the rhizosphere. Phylogenetic null models indicated predominantly deterministic bacterial assembly, with 10% dispersal limitation in the 15-30 cm layer. Fungal assembly was predominantly stochastic in bulk soils, whereas the rhizosphere was an exception: heterogeneous selection accounted for 60% of pairwise comparisons and median βNTI exceeded +2. The Mantel test identified only the association between total phosphorus and bacterial diversity as significant (0.01 < p < 0.05). These results show that rhizosphere filtering strongly structured bacterial composition and imposed deterministic selection on rhizosphere fungi.