Different responses of sugarcane and rhizosphere soil microorganisms to single or  mixture application of PGPB

Background: Plant growth-promoting bacteria (PGPB) benefit plant growth and development via different direct and indirect mechanisms. However, our knowledge about rhizosphere soil response at different plant growth stages to diverse PGPB application in sugarcane is limited. In this study, four strains of bacteria genera ( Gluconacetobacter diazotrophicus PAL5, Streptomyces chartreusis WZS021, Bacillus spp . CA1, and Pseudomonas mosselii CN11) were inoculated into two sugarcane varieties (B8, ROC22) as single or mixture in a pot planting experiment. The effects of single or combined application of PGPB on nitrogen metabolism, agronomic traits, rhizosphere soil chemical and biological properties and microbial community were surveyed. Results: It was found that different treatments had different promotion ways for different sugarcane varieties and rhizosphere soils. PAL5 and CA1+CN11 significantly improved the nitrogen fixation efficiency of sugarcane, while WZS021 treatment enhanced phosphorus (available phosphorus and alkaline phosphatase). High-throughput sequencing (HTS) analysis revealed that Proteobacteria, Firmicutes, Chloroflexi, and Actinobacteria were the main microbial community phylum components. Correlation analysis indicates that phyla Proteobacteria and Bacteroidota played a key role in the nitrogen cycle of the soil-microbe-plant interaction system, while phylum Firmicutes had a crucial role in the phosphorus cycle. And we found that, In the varieties with weak bacterial species in the rhizosphere soil, the addition of the composite strain had the best effect, while in the varieties with rich bacterial species, the addition of the composite strain may have the exclusion phenomenon, which was not as good as the addition of the single dominant strain. Conclusions: The PGPB had excellent activities, such as nitrogen fixation, phosphorus and potassium solubilization, which could promote plant growth by decomposing soil nutrients. The inoculated strains can positively enrich the beneficial bacteria in sugarcane. However, there were variations in the quantities of these promoted properties in the treatments with different bacterial strains and sugarcane varieties. It was found that soil-disadvantaged and inoculum-specific bacteria were more favorable to plant development. The considerable variation in soil microbe provides a knowledge base and an experimental system for further mining and utilization of microbial strains.