Functional Screening of Zinc-Solubilizing Bacteria Isolated from the Rhizosphere Soil for In-vitro Zinc Carbonate Solubilization Potential

Abstract

Zinc (Zn) deficiency is a major constraint in crop production, particularly in calcareous and alkaline soils where zinc predominantly exists in insoluble forms such as zinc carbonate (Zn CO₃). To address this challenge, zinc-solubilizing bacteria (ZSB) offer an eco-friendly and sustainable strategy for enhancing zinc bioavailability in soil. This study aimed to isolate, screen and evaluate the zinc carbonate solubilization potential of bacterial strains obtained from the rhizosphere of pulse crops including mung bean, urd, pigeon pea, lentil and chickpea. A total of 10 bacterial isolates were recovered using serial dilution and plating techniques. Morphological and biochemical characterization (catalase and oxidase tests) were performed to identify functionally diverse ZSB strains. The qualitative assessment of zinc solubilization on Tris minimal agar medium containing 0.1% ZnCO₃ revealed the formation of clear halo zones around colonies. Among the isolates, AJP-6 and AJL-8 exhibited the highest solubilization indices (4.33 and 4.12, respectively) and solubilization efficiencies (333.34% and 312.50%, respectively), indicating robust solubilizing potential. Quantitative broth assays conducted over 5, 10 and 15-day intervals demonstrated progressive zinc carbonate solubilization and a concurrent decrease in pH, signifying organic acid production. Notably, isolate AJP-6 recorded the highest solubilized zinc concentration of 172.35 mg L⁻¹ on the 15th day, followed by AJL-8 (169.73 mg L⁻¹), while the control showed no significant solubilization. The study confirms that native ZSB isolates, particularly AJP-6 and AJL-8, possess significant capabilities to mobilize insoluble ZnCO₃ by acidifying their environment, thereby making zinc more accessible to plants. These findings highlight the potential of using these isolates as biofertilizers to improve zinc nutrition in crops and promote sustainable agricultural practices. Further field trials and consortia formulation may optimize their performance under real soil conditions.