Root-Driven Filtering Overrides Biochar and Microbial Inoculants in Structuring Bacterial Assemblages of Seawater Rice Cultivation Ecosystem in a Saline–Alkali Soil

Saline–alkali soils significantly hinder agricultural productivity in China’s coastal areas. Although both plant growth-promoting rhizobacteria (PGPR) and biochar have individually demonstrated the capacity to boost crop yield and soil fertility, their synergistic effects on seawater rice and soil ecosystems remain uncertain. In this study, we examined the individual and interactive influences of lychee biochar (2.5% and 5% w/w) and PGPR inoculation on soil physicochemical properties and bacterial community assembly along a soil–root continuum, encompassing bulk soil, rhizosphere soil, rhizoplane, and root endosphere, in a controlled pot experiment with seawater rice. The application of biochar significantly altered soil pH, electrical conductivity, and nutrient availability in both bulk and rhizosphere soils, resulting in pronounced changes in bacterial community composition. The effects generated by biochar were partially mitigated when PGPR was co-applied. The relative abundances of Bacillota and Bacteroidota grew progressively from bulk soil to the root endosphere across all treatments, indicating a significant compartment-dependent selection. Co-occurrence network analysis and FAPROTAX-based functional predictions revealed several taxa and functions that were progressively enriched toward the root, including the halotolerant genera Exiguobacterium and Chryseobacterium, highlighting a significant host-mediated filtration process that functioned independently of the inoculated strains. Multivariate analyses further demonstrated that soil pH was the primary driver of bacterial community structure in bulk and rhizosphere soils, whereas plant-root selection dominated in the rhizoplane and endosphere. Overall, our results demonstrate that, within a seawater-rice and soil ecosystem, the selective influence of the host plant on root-associated microbiomes exceeds that of either biochar amendment or PGPR inoculation. This work improves our understanding of biochar–PGPR–plant interactions in saline–alkali soils and provides insight into sustainable strategies for enhancing rice production under salinity stress.