Microbial allies: shaping growth, physiology, and rhizosphere dynamics of onion ( <i>Allium cepa</i> L.)

The present study investigates the dual impact of microbial biofertilizers on the phenotypic performance and rhizosphere microbiome composition in an onion crop. A pot experiment was conducted with seven treatments of microbial inoculants, such as Azotobacter, Azospirillum, <i>Piriformospora indica</i>, phosphate solubilizing bacteria (PSB), and control treatments with and without chemical fertilizers. The growth, physiological, and biochemical traits of onion were assessed alongside rhizospheric soil microbiome profiling using 16S rRNA metagenomic sequencing. Significant enhancement in plant height, leaf number, leaf area, chlorophyll content, photosynthetic rate, and antioxidant enzyme activity with low leaf temperature was observed in plants inoculated with Azotobacter and Azospirillum. Notably, the Azotobacter treatment yielded a significant enhancement in the bulb phenol content. Rhizosphere metagenomic analysis revealed 17 dominant phyla, with <i>Actinobacteria</i> (25.3%), <i>Proteobacteria</i> (22.2%), <i>Firmicutes</i> (12.8%), and <i>Chloroflexi</i> (11.02%) comprising over 70% of the total microbiome. Alpha and beta diversity metrics indicated that microbial inoculation, especially with Azospirillum and PSB, enriched the soil microbial community structure. Distinct clustering and correlations with specific microbial taxa such as <i>Candidatus Nitrososphaera</i> and <i>Pseudomonas</i> were observed in response to individual biofertilizer treatments. This study highlights the potential of biofertilizers not only in enhancing onion growth and development but also in modulating beneficial rhizosphere microbial communities. Integrating biofertilizers into onion production systems could reduce the dependency on chemical fertilizers and promote sustainable crop management.