Effects of Modified Chicken Manure Biochar-Based Bacillus velezensis Agent on Vegetable Growth, Saline–Alkali Soil Properties, and Microbial Communities

Soil salinization limits sustainable agriculture. This study used a citric acid–FeSO4 method to create modified chicken manure biochar (MCMB) as a carrier for the plant growth-promoting Bacillus velezensis SS-20, forming a biochar-based microbial agent (JS). A pot experiment included four treatments: CK (control), SS (SS-20 only), JF (MCMB only), and JS (biochar-based agent). The JS treatment significantly improved pakchoi plant height and aboveground fresh weight by 26.33% and 76.62%, respectively, compared to CK. It also lowered soil pH, electrical conductivity (EC), and salt content, while enhancing soil catalase activity by 8.83%. The ameliorative effects of SS and JF were less pronounced than those of JS. The contribution of biochar and SS-20 to the improvement of the soil quality index suggests additive effects with minimal interaction. Redundancy analysis identified EC and total potassium as the key environmental variables influencing microbial community structure at the phylum and genus levels, respectively. JS influenced the relative abundances of candidate lineages, such as Myxococcota, AG11, and Gemmatirosa, as well as functional genes associated with energy metabolism and cell motility. In summary, JS enhanced soil properties, modulated differentially abundant taxa and their metabolic potential, and consequently facilitated pakchoi production in saline–alkali soil. This study offers practical insights for the ecological management of saline–alkali farmland.