Release Kinetics and 16S rRNA Gene Sequencing of Zinc Solubilizing Bacteria Screened from Zinc Deficient Paddy Soils of Northeast Region of India

Abstract

Low solubility of insoluble zinc (Zn) complexes and low fertilizer Zn use efficiency owing to a range of soil conditions results in Zn deficiency in paddy soils, which also limits rice productivity. In the present study, prospective Zn-solubilizing bacteria (ZSB) were screened from Zn-deficient paddy soils of northeast (NE) India to assess their ability to solubilize insoluble Zn compounds. Thirteen isolates of ZSB were retrieved following the qualitative assay for clear halo zone formation and Zn solubilization index (ZSI) in modified Pikovskaya’s agar plates containing ZnO (0.12%) and Zn3(PO4)2.2H2O (0.22%) separately. Quantitative release of diethylene triamine penta-acetic acid-extracted (DTPA)-Zn, pH change, and Zn release kinetics were evaluated for the isolates in liquid medium using ZnO following 40 days of incubation. 16S rRNA gene amplification and sequence analysis were used to identify ZSB isolates with a high capacity to solubilize ZnO. The isolates were identified as Bacillus subtilis, Pseudomonas putida, Pseudomonas aeruginosa, Paraburkholderia tropica and Bacillus circulans. The isolate Bacillus subtilis ZSBK95 showed the largest clear halo zone (38 mm), with a ZSI of 4.22 and a cumulative release of DTPA-Zn (205.56 μg mL-1 in the medium containing ZnO. Similarly, the higher initial mineralization rate (7.75 μg mL-1 day-1) was observed for Bacillus subtilis ZSBK95. A significant (p<0.05) drop in pH (5.07- 6.10) from the initial value (pH 7.0) was observed in all the ZSB isolates during the incubation period, along with corresponding release of DTPA-Zn, which was reflected as a strong negative correlation (-0.838, n=56, p≤0.01) between pH drop and Zn solubilization. The results indicated that the five identified ZSB isolates have high potential for application as biofertilizers in Zn-deficient paddy soils.