Bio-Organic Fertilizer with Bacillus velezensis Promoted Plant Growth by Regulating Soil Microbial Community Structure and C/N Cycle Function

This study investigated the effects of bio-organic fertilizer (BF) containing plant growth-promoting bacterium <i>Bacillus velezensis</i> SS-20 on soil properties, microbial community structure, and C/N cycle functional genes. The results showed that compared with chemical fertilizer (CF) and deactivated bio-organic fertilizer (BFD) treatments, BF significantly improved soil physicochemical properties. Soil pH, organic matter, total nitrogen, total potassium, and total phosphorus content under BF treatment were increased by 14.8%, 56.5%, 48.2%, 38.8%, and 58.4%, respectively, compared to the control; soil urease and sucrase activities increased by 3.5 and 2.4 times those of CF treatment, respectively. Meanwhile, BF increased pakchoi yield by 11.2% (vs. CF). BF treatment enhanced the relative abundance of beneficial bacteria <i>Actinomycetota</i> by 28.4% compared with the BFD treatment and raised that of fungi <i>Ascomycota</i> to 1.9 times that of the control. At the genus level, BF significantly enriched biocontrol-relevant genus <i>Pseudogymnoascus</i>, whose abundance reached three times that of CF treatment, while the abundance of potentially harmful genus <i>Penicillium</i> decreased by 82%. BF also led to a high degree of synergy in carbon and nitrogen cycles. Functional gene analysis indicated that BF down-regulated multiple carbon-degrading genes, increased organic nitrogen metabolism genes by 5.3%, and reduced denitrification genes by 13.3%. Overall, bio-organic fertilizer optimized the soil microenvironment, regulated the microbial community structure, and improved C/N use efficiency and plant growth by introducing functional microorganisms and organic matter.