Isolation and Screening of Zinc Solubilising Bacteria from Pulse Rhizosphere under Calcareous Soil Conditions

Zinc (Zn) deficiency is a widespread micronutrient disorder in Indian agricultural soils, particularly in calcareous regions where high pH and carbonate levels precipitate zinc into insoluble forms, limiting plant availability. This study aimed to isolate and evaluate efficient zinc solubilising bacteria (ZSB) from the rhizosphere soils of five major pulse crops-Mung bean (Vigna radiata), Urd bean (Vigna mungo), pigeon pea (Cajanus cajan), lentil (Lens culinaris) and chickpea (Cicer arietinum)-cultivated in calcareous soils of Bihar, India. Ten bacterial isolates (RJM-1 to RJC-10) were obtained via serial dilution plating and screened qualitatively on Tris minimal agar supplemented with 0.1% zinc oxide (ZnO). Zinc solubilization potential was assessed using halo zone diameter, solubilization index (SI), solubilization efficiency (SE) and Khandeparkar’s selection ratio. Quantitative assays in broth culture measured pH reduction and soluble zinc concentration at 5, 10 and 15 days of incubation. Results revealed significant variability among isolates in their ZnO solubilising capabilities. Isolate RJP-6 exhibited the highest performance with a halo zone diameter of 27 mm, SI of 4.37, SE of 337.55% and Khandeparkar’s ratio of 3.37, followed closely by RJL-8 with 26 mm, SI of 4.25 and SE of 325.00%. Both isolates showed substantial acidification of the broth, with RJP-6 lowering the pH to 5.53 and RJL-8 to 5.59 at 15 days, indicating strong organic acid production. These findings underscore the potential of RJP-6 and RJL-8 as efficient bioinoculants for enhancing zinc availability in pulse-based cropping systems, offering an eco-friendly and cost-effective alternative to chemical zinc fertilisers. Future research should validate the efficacy of RJP-6 and RJL-8 through multi-location trials and molecular characterisation to optimise their use as bioinoculants in pulse-based systems.