Multiple plant growth-promoting bacteria enhance rice growth in non-grain-converted lands.

China faces a continuously growing food demand, while a large proportion of its cultivated land is increasingly being shifted to non-grain plantation, leading to widespread soil fertility degradation and microbial community imbalance in lands converted back to grain production. To address the critical research gap of targeted soil fertility restoration and yield enhancement in these marginal non-grain-converted cultivated lands using microbial-based strategies, this study aimed to screen and identify plant growth-promoting bacteria (PGPB) and systematically assess their effects on soil health and rice growth in non-grain-converted fields. Bacteria were isolated from the rhizosphere soils of non-grain-converted fields and identified through morphology and multilocus gene sequencing. Key plant growth-promoting (PGP) traits, including phosphate solubilization, nitrogen fixation, siderophore formation, and indole-3-acetic acid (IAA) production, were assessed. The effects of these strains on soil microbial communities and soil properties in converted rice fields were further evaluated through pot experiments and high-throughput sequencing. Among 589 isolated bacterial strains, eight were screened out with robust PGP traits, including phosphate solubilization capacity (solubilization zone diameter: 11.74-24.82 mm), siderophore production (orange zone diameter: 8.28-10.57 mm), IAA synthesis (25.61-96.22 μg/mL) and nitrogen fixation capacity. In vivo pot assays showed that three elite strains (LA-B511, YH-S3, and LA-B111) significantly promoted rice seedling growth, leading to increases in seedling height by 25.28%, 24.90%, and 18.86%; root length by 16.81%, 13.82%, and 21.95%; seedling dry weight by 20.81%, 38.55%, and 33.78%; and root dry weight by 27.17%, 25.74%, and 50.84%, respectively. Morphological and molecular analyses identified these three strains as Enterobacter hormaechei and Yokenella regensburgei. After 35 days of inoculation, soil available phosphorus (AP) content increased by 27.00%, 25.99%, and 16.65% compared to the non-inoculated control. Additionally, soil microbial communities were significantly reshaped, driven by changes in soil organic matter (SOM), soil pH, iron (Fe) content, total phosphorus (TP) and available phosphorus (AP). Overall, our results demonstrated that the screened PGPB strains can effectively improve rice seedling growth and soil health in non-grain-converted cultivated lands, providing a promising microbial-based amendment for early-stage rice establishment and sustainable grain production potential in these specific marginal non-grain-converted cultivated lands.