Effect of Zinc-Solubilizing Rhizobacteria on Zinc Accumulation, Microbial Activity, and Growth of Cassava

Zinc (Zn) is an essential micronutrient for plant growth, but its limited bioavailability in soils affects cassava productivity. Zinc-solubilizing rhizobacteria (ZSR) can enhance Zn availability, promoting better plant uptake and growth. This study evaluates the effectiveness of Pseudomonas and Bacillus spp. in improving Zn solubilization, microbial respiration, and Zn accumulation in cassava (Manihot esculenta). The experiment was conducted at the National Root Crops Research Institute (NRCRI), Umudike, Nigeria, under screen house conditions. A completely randomized design was used with eight treatments and three replications. Pseudomonas (P), Bacillus (B), ZnSO₄ only, Pseudomonas + Bacillus (P+B), Pseudomonas + Zn (P+Z), Bacillus + Zn (B+Z), Pseudomonas + Bacillus + Zn (P+B+Z), and a control. Bacterial isolates were screened for Zn solubilization ability, and their growth was monitored in different Zn sources and times. CO₂ respiration rates were measured to assess microbial activity, while Zn accumulation in cassava roots, stems, and leaves was determined at harvest using atomic absorption spectrophotometry. Results showed that Pseudomonas sp and Bacillus sp showed higher solubilization efficiency during the screening with 300% and 280% respectively. The ZnSO4 treatment exhibited the highest turbidity bacterial growth rate (OD600) of 2.5nm and 2.0nm under pseudomonas and bacillus sp and P+B+Z treatment showed a significant increase in CO₂ respiration level at 8MAP (52.6mg/kg), indicating enhanced microbial activity. Zinc accumulation followed the trend: P+B+Z > P+Z > B+Z > P+B > Zn only > Control, with the highest Zn concentrations of 3.93 mg/kg and plant height of 150.13cm. In conclusion, the combined application of Pseudomonas and Bacillus (ZSR) with ZnSO₄ significantly improved Zn solubilization, microbial activity, and Zn uptake in cassava. These findings suggest that ZSB-based biofertilization could enhance cassava nutrition and productivity. Field validation is recommended to optimize bacterial formulations for large-scale applications.