Previous research has indicated the impacts of PGPB on the growth of crops. PGPB treatment to maize [40] and wheat [41] seeds were found to deliver more nitrogen and enhanced the average yield compared to the untreated control. A combination of PGPB strains delivered more nitrogen for maize than a single-strain supplement and greatly boosted wheat output [42]. These results suggested that the applied bacterial strains were interacting with each other, and promote the nitrogen-fixation ability of the nitrogen-fixing bacteria. To understand the impacts of single and mixed PGPB application on sugarcane, the current study was performed. So, single (PAL5, WZS021) and mixed (CA1 + CN11, CN11 + WZS021) strains of bacteria were inoculated into two sugarcane varieties, B8 and ROC22. The results revealed that PGPB effectively increased the activities of nitrogen metabolism-related enzymes (GPT, GOT, NR, and GS) in sugarcane plants. The mixed strain CA1 + CN11 had the strongest effect on nitrogenase activity and chlorophyll activity, and significantly increased the dry weight of the underground part of the plant. It is speculated that the interaction effect of bacteria leads to the enhancement of nitrogen fixation capacity.Moreover, it was found that the single dominant strain PAL5 also had a strong effect on nitrogenase activity and nitrogen fixation of sugarcane, and the influence of above biomass was the most significant, so that the height of sugarcane was significantly higher than that of the control.The above results proved that the nitrogen-fixing ability of the growth-promoting bacteria can further promote plant growth by promoting nitrogen utilization and photosynthesis in sugarcane.
Inoculation of PGPB in the soil increased the concentration of mineral nitrogen as well as that of phosphorus and potassium in the soil, improved the nutrient utilisation rate, and promoted plant absorption and utilization of nitrogen, phosphorus and potassium by plants [43]. Soil nutrient detection in this study revealed that PAL5 had a large boosting influence on AN and AK in the rhizosphere soils of both sugarcane varieties, while the combination of CN11 + WZS021 and CA1 + CN11 was the second most important factor influencing these two elements. This might be connected to PAL5's very potent biological nitrogen fixing capabilities [44]. Sugarcane variety B8 had the highest AP content in response to PAL5, The 15N isotope dilution experiment also showed that PAL5 treatment significantly increased total nitrogen content in leaves and roots, thereby promoting the increase of aboveground biomass. Streptomyces chartreusis WZS021, participates in the soil phosphorus cycle, decomposing organic matter into available phosphorus, resulting in the accumulation of available phosphorus and available phosphorus components in soil. ROC22 had the highest AP in the WZS021 treatment. The results of CAT and AKP responses showed a similar pattern, with WZS021 being the most effective in variety B8, followed by the mixtures of strains. The most significant reaction response of CAT and AKP content in variety ROC22 was found still in PAL5 treatment, followed by the treatments with combination of CN11 + WZS021 strains. It is consisted that Actinomycetes spp. promote the release of soil insoluble phosphorus through microbial metabolic activities and participate in the soil phosphorus cycle [45–47]. Moreover, WZS021 and CN11 + WZS021 strains significantly affected plant height, indicating that the growthpromoting bacteria had excellent activities, such as nitrogen fixation, phosphorus and potassium solubilization, which could promote plant growth by decomposing soil nutrients. Soil nutrient discrepancies reflect variations in soil enzyme activity in response to various probiotic bacteria in different sugarcane varieties.
Based on 16S rRNA sequences, the microbial community composition in sugarcane rhizosphere soil was also investigated. The Chao index for ROC22 variety was higher than that for B8 variety at both seedling and elongation stages, indicating that the total number of bacteria species in the rhizosphere soil of ROC22 variety was higher than that of B8 and the soil microbial community varies at different growth stages of sugarcane and in different sugarcane varieties. A further investigation of its responses to different exogenous PGPB additions revealed that the bacterial microbial richness in sugarcane rhizosphere soils at seedling stage was greater in all the WZS021 treatments than in the other treatments. At elongation stage the microbial abundance in the rhizosphere soil of B8 in the CA1 + CN11 treatment was higher than in the other treatments. It was found that the WZS021 treatment had higher microbial abundance than the other treatments in ROC22 at the elongation stage. At the same time, there were differences in the response mechanism of different sugarcane varieties to the strain. 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.
Proteobacteria, Firmicutes, Actinobacteria, Bacteroidetes, Chloroflexi, Gemmatimonadetes, Planctomycetes, and Nitrospirae phylum-level distribution were shown to predominate in sugarcane [22, 48], This is consistent with the present study. Phyla Actinobacteria, Firmicutes, and Proteobacteria were also members of the leading phyla in the sugarcane rhizosphere soil microbial community, whereas Bacteroidota was less represented. Y Gu, J Wang, W Cai, G Li, Y Mei and S Yang [49] found that phylum Aspergillus, Actinobacteria phylum and Firmicutes phylum were the dominant endophytic bacteria in sugarcane exposed to different treatments. The Bray-Curtis algorithm revealed that Bacillus was the leading genus in all samples, with a substantial increase in abundance in all rhizosphere soils treated with PGPB, which were characterised after the CA1 + CN11 treatment. Furthermore, following PGPB inoculation treatment, the abundance of Streptomyces, Sphingomonas, Arthrobacter, and Nocardioides increased significantly in the B8 rhizosphere soil. As a result, the soil supports the survival of several microbe species for self-regulation. The disadvantaged microbial community, on the other hand, has a poor capacity to compete for its own nutrients, causes minimal change in the soil environment, and performs outstanding activities such as nitrogen fixation, phosphorus and potassium solubilization, all of which contribute to the soil nutrient environment.
However, after elongation, sugarcane soil diversity was similar for BT3 vs BT1 treatments. Chao index showed that the diversity index of complex strains in B8 was higher, and RT2 and RT3 treatments in ROC22 were farther apart from the other treatments. It showed the difference between the complex strains and single strain. Therefore, it is speculated that the inoculated strains can positively enrich the beneficial bacteria in sugarcane. Application of bacteria that were disadvantaged and functionally specialised to the soil environment was more favorable to plant development, so it seemed that the microbiota had a more pronounced influence compared to the dominant population.
Variation in the composition and diversity of soil microbial communities is closely related to soil physicochemical properties [50, 51]. Inoculation of PGPB increased soil nutrients, enzyme activities and improved soil microbial communities [52]. S Yang, J Xiao, T Liang, W He and H Tan [50] found that different fertilizer treatments improved sugarcane soil biological properties and soil bacterial diversity. Spearman's correlation analysis showed that The nitrogen-fixing Bacillus species promote plant growth directly via nitrogen fixation, phosphate solubilization and production of phytohormones and indirectly through the production of antibiotics, hydrolytic enzymes and siderophores [53, 54]. Bacillus subtilis fixes nitrogen and promotes the growth and development of crops [55]. It can not only promote the growth of crop plant height but also significantly increase root activity, net photosynthetic rate and yield per plant. RDA analysis indicated that AP, AK, AKP, UE and CAT were all significantly positively correlated with Firmicutes and BRC1. Consistent with the results, higher levels of AP, AK, AKP, UE and CAT were detected in the T2 treatment. This may be the reason why the addition of WZS021 alone had a weaker effect on biomass, SPAD, and AN in sugarcane than the addition of PAL5 and CN11 + CA1 since WZS021 tends to act on the phosphorus cycle, while PAL5 and CA1 tend to enhance the nitrogen cycle. H Minjie [56] showed that soil effective phosphorus content and AKP activities were closely related to the microbial abundance in soil. This meant that an interaction exists between the level of soil fertility and the bacterial community [51]. In addition, the results of functional prediction of bacterial flora showed that the bacteria were involved in four metabolism pathways, including global and overview maps, carbohydrate metabolism, amino acid metabolism and energy metabolism in the rhizosphere soil of sugarcane at seedling and elongation stages. It implied the importance of microbial metabolism in soil.