Bioefficacy of Sulfur-Oxidizing Bacteria and Sulfur Concentrations in Enhancing Onion (Allium cepa L.) Growth Parameters

WO isolates of sulfur oxidizing bacteria (SOB) were isolated from the rhizosphere of onion plants (Allium cepa L.), Assiut Governorate, Egypt. These two isolates SOB12 and SOB15 were identified using 16S rRNA gene sequencing as Bacillus spizizenii strain B566 and Priestia aryabhattai strain B567, respectively. Aimed to evaluate the effect of two sulfur-oxidizing bacterial in combination with different sulfur concentrations, on the growth, nutrient uptake, and biochemical composition of onion (Allium cepa L.) plants. Thirteen treatments were applied in pot experiments using sulfur doses at 50%, 100%, and 150% of the recommended rate, either alone or combined with bacterial inoculants. Both bacterial strains positively influenced plant performance, with SOB15 generally outperforming SOB12 and sulfur alone across most measured parameters. However, the best overall results were observed in treatments combining SOB12 and SOB15 individually or in combination with 50% of the recommended sulfur dose. These treatments significantly enhanced plant height, leaf number, bulb weight, photosynthetic pigments, nutrient uptake (N, P, K, S), and biochemical constituents such as carbohydrates, amino acids, phenolics, and pyruvic acid. Notably, at 50% sulfur, co-inoculation (SOB12+SOB15) significantly increased bulb dry weight by 127% relative to 50% sulfur without inoculation; at 100% sulfur, SOB15 increased nitrogen uptake by 45% and bulb fresh weight by 65% versus sulfur alone; and at 150% sulfur, SOB15 raised total carbohydrate concentration by 146% compared with sulfur alone. These findings highlight the potential of SOB12 and SOB15, particularly P. aryabhattai (SOB15), as effective biofertilizers for promoting sustainable onion cultivation under sulfur-limited conditions.