Priestia megaterium Inoculation Enhances the Stability of the Soil Bacterial Network and Promotes Cucumber Growth in a Newly Established Greenhouse

The rapid expansion of greenhouse agriculture demands sustainable strategies to maintain soil health and productivity from the outset. Priestia megaterium, a plant growth-promoting rhizobacterium (PGPR), has shown promise in improving plant growth and soil nutrient availability, but its efficacy in newly established greenhouse systems, where the soil microbiome is still developing, remains underexplored. This study evaluated the impact of P. megaterium inoculation on cucumber growth, soil nutrient bioavailability, and soil microbial communities in a greenhouse with only two years of operation. A two-year experiment was conducted with conventional fertilization as the control and P. megaterium inoculation (7.0 × 108 cfu mL−1) at different rates (37.5, 75, 150, and 300 L ha−1) and timings. Soil and plant nutrient content were measured, and microbial communities were analyzed through 16S rRNA sequencing and co-occurrence network analysis. Results showed that applying P. megaterium at 75 L ha−1 during seedling transplantation significantly increased soil available phosphorus (AP) by 11.64–26.48% and available potassium (AK) by 11.27–47.31% compared to the control, while enhancing cucumber yield by 6.71–9.28%. The inoculant also increased soil bacterial diversity, enriched beneficial genera such as Lysobacter, Pseudomonas, and Flavobacterium, and reduced the abundance of Xanthomonas. Furthermore, P. megaterium application promoted a more complex and stable bacterial network, with higher connectivity and modularity. These findings suggest that P. megaterium is a viable strategy for enhancing soil health and productivity in newly established greenhouse systems, offering an environmentally sustainable alternative to traditional fertilization methods.