Salinity gradients shape rhizosphere bacterial diversity and assembly of wheat in saline-alkali rice-wheat rotation soils.

The saline-alkali soils of the Yellow River Delta constitute an important reserve of arable land resources in China, and the rice-wheat cropping system has been widely recognized as an effective approach for improving soil quality and increasing crop yields in this region. However, region-specific field investigations evaluating the effects of salinity on wheat rhizosphere bacterial communities within this system remain limited. To fill this research gap, we conducted a field salinity gradient experiment with low-salinity (LST, 1.5 g/kg), medium-salinity (MST, 3.0 g/kg), and high-salinity (HST, 5.0 g/kg) treatments to explore how salt levels alter soil physicochemical traits and rhizosphere bacterial assemblages. We found rising salinity degraded soil fertility: compared with LST, MST and HST decreased soil pH by 0.73 and 0.70 units, organic matter by 35.4% and 68.5%, ammonium nitrogen by 54.2% and 46.5%, and sucrase activity by 82.6% and 72.7%. Available phosphorus reached the lowest level under MST, decreasing by 41.8% and 40.3% vs. LST and HST, while nitrate nitrogen peaked at HST with increments of 20.8% and 22.0% relative to LST and MST. OTU analysis showed continuous transitional succession of microbiota along the salinity gradient. Elevated salinity boosted the relative abundance of dominant phyla including Proteobacteria and Bacteroidota, as well as genera Thiobacillus and Confluentibacter, while suppressing the abundance of Actinobacteriota and its representative genus Nocardioides. At the genus level, unclassified bacterial taxa dominated all treatments and increased in relative abundance with rising salinity, representing valuable untapped microbial resources in saline-alkali wheat rhizosphere soils. Redundancy analysis verified ammonium nitrogen strongly correlated with community composition, and LEfSe distinguished Flavobacterium, Sulfurimonas, and Pseudomonas as characteristic high-salinity-enriched taxa.