Smart bioformulation of rhizosphere bacteria improves zinc uptake and rice (Oryza sativa) yield in zinc-deficient alluvial soil

Zinc (Zn) deficiency in both soil and grain remains a constraint to crop productivity and human nutrition across the alluvial soils of eastern India. The strong fixation of Zn in soil limits its availability, creating interest in microbial Zn solubilization, but the low survivability of inoculants in conventional solid carriers has restricted its use. In this backdrop, we assessed the survivability, and biofortification potential of three rice rhizosphere Zn-solubilizing bacterial (ZnSB) strains (B1: Burkholderia caribensis and B2 and B3: Bacillus subtilis ), formulated in liquid medium. Liquid formulations containing polyvinylpyrrolidone, glycerol, and phosphate buffer maintained high population (> 1 × 10 8 cfu/mL) even after nine months, whereas the microbial population showed rapid decline in solid carriers. To understand the effect of ZnSB, a pot experiment with rice was conducted with different treatments, including vermicompost (applied vs not applied), bacterial inoculation (no inoculation vs B1, B2, and B3), and Zn sources (ZnO, ZnS, ZnCO 3 , and Zn 3 (PO 4 ) 2 ) arranged in a three-factor factorial design. ZnSB inoculations significantly enhanced soil available Zn (1.7 folds) and Zn solubilization potential (2.7 folds) over the uninoculated control. Maximum concentration of Zn in rice grain (23.75–29.30 µg/g) and straw (19.10–21.40 µg/g) was recorded with the combined application of ZnSB, vermicompost, and ZnO. Principal component analysis identified B1 + ZnO + vermicompost as the most effective treatment for maximizing Zn availability and rice yield. Overall, the integration of liquid ZnSB bioformulation, ZnO, and organic amendment provides a sustainable solution for improving Zn biofortification and rice productivity in Zn-deficient alluvial soils of eastern India.