Isolation and Screening of Zinc Solubilizing Bacteria from Pulses Rhizosphere and their Effect on In-vitro Solubilization of Zinc Oxide

Zinc is an essential micronutrient required for various physiological and biochemical functions in plants, yet its availability in soils is often limited due to its presence in insoluble forms. Zinc deficiency in agricultural soils especially in pulse-growing regions of India, poses a significant constraint to crop productivity and nutritional quality. This study aimed to isolate and characterize zinc solubilizing bacteria (ZSB) from the rhizosphere of major pulse crops-Mung bean (Vigna radiata), Urd bean (Vigna mungo), pigeon pea (Cajanus cajan), lentil (Lens culinaris) and chickpea (Cicer arietinum)-to evaluate their potential for zinc oxide (ZnO) solubilization under in vitro conditions. Ten bacterial isolates were obtained through serial dilution and plating methods from rhizospheric soils of pulse crops collected from different agricultural locations in Bihar. The isolates were subjected to morphological and biochemical characterization, including catalase and oxidase activity tests. Their zinc solubilization ability was first screened qualitatively using Tris minimal agar medium amended with 0.1% ZnO and solubilization was assessed based on halo zone formation, solubilization index (SI), solubilization efficiency (SE) and Khandeparkar’s selection ratio. Quantitative assessment was carried out in broth culture, where pH changes and zinc concentrations were measured at 5, 10 and 15 days after incubation. Zinc concentration in the broth was estimated using Atomic Absorption Spectrophotometry (AAS). Among the ten isolates, AJP-6 and AJL-8 demonstrated superior zinc solubilization performance. AJP-6 exhibited the highest halo zone diameter (27 mm), SE (337.55%) and significant ZnO solubilization in broth (249.75 mg/L at 15 days), accompanied by a substantial reduction in medium pH (5.53). AJL-8 closely followed with 234.23 mg/L ZnO solubilized and a pH of 5.59 at the same interval. These results indicate active organic acid production and high metabolic efficiency in mobilizing insoluble Zn. The findings suggest that native ZSB strains such as AJP-6 and AJL-8 have significant potential as bioinoculants for zinc-deficient soils, especially in pulse-based cropping systems. Their application may reduce dependence on chemical fertilizers, enhance micronutrient availability and support sustainable agricultural practices. Further in vivo trials are warranted to validate their efficacy under field conditions.