The effect of zinc oxide nanoparticle-organic manure amended ultisol and loam soils on plant growth response and rhizosphere bacterial community of peanut (Arachis hypogaea) was evaluated using standard methods under greenhouse conditions. Results indicate germination rates ranged between 30 and 100% in the amended soils compared to 50 and 70% in the controls. ZnO nanoparticles exerted concentration-dependent and varying effects on the plant root and shoot lengths, weights, nodules and pod formation in the two soil types. Heterotrophic bacterial counts ranged from 7.21 ± 0.51 to 7.38 ± 0.5 Log10CFUg-1 in the amended ultisol and 6.99 ± 0.55 Log10CFUg-1 in the control with a log reduction to 6.70 ± 0.39 Log10CFUg-1 in 500 mgkg⁻¹ ZnO spiked soil. Counts in the amended loam soil ranged between 6.59 ± 0.48 and 7.22 ± 0.41 Log10CFUg-1 relative to 6.80 ± 0.58 Log10CFUg-1 in the control. ZnO induced concentration-dependent effect on oxygen uptake rate relative to the controls. The organisms were members of the genera Lactobacillus, Pseudomonas, Bacillus, Rhizobium, Xanthobacter, Enterobacter, Citrobacter, Nitrosomonas and Agromyces. ZnO nanoparticle exerted concentration-dependent stimulatory and inhibitory effects on the plant growth response, oxygen uptake rate and induced temporal shifts in soil microbial abundance. It is challenging to generalize a consistent response of the plant or microorganisms because ZnO nanoparticles interacted with A. hypogaea and soil bacterial community in ways that differ in the ultisol and loam soil.

We evaluated the effect of 4000 mg zinc oxide (ZnO, 99%, 30 nm) nanoparticle on the physicochemical and microbiological properties of organic manure amended ultisol and loam soil cultivated with Arachis hypogaea using standard methods. The results indicate varying effects on the physicochemical properties in relation to the soil type. The pH of the control ultisol at 7.85 ± 0.17 and 8.3 ± 0.12 in the amended ultisol whereas, the control loam was 7.15 ± 0.17 and 7.41 ± 0.11 in the amended soil indicating 1.06- and 1.04-times higher difference than the controls respectively.  Phosphorus concentration at 57.82 ± 0.54%, 50.81 ± 0.22% and 55.97 ± 0.04%, 59.97 ± 0.02% was 1.14 times lower in the ZnO amended ultisol and 1.07 times higher in amended loam soil compared to the respective controls. The organic matter content in the control and amended ultisol was 2.28 ± 0.32% and 0.91 ± 0.02%, 3.68 ± 0.36% and 0.36 ± 0.02% in the control and amended loam soil. The concentration of nitrate in the control ultisol was 0.05 ± 0.01% and 0.03 ± 0.01% in the amended soil. The nitrate in the control loam soil was 0.08 ± 0.01% relative to 0.02 ± 0.01% in the treated soil and these differences were significant at p = 0.05. The concentration of nutritive salts was reduced and in contrast iron, copper, exchangeable acids, exchange capacity, clay and silt increased in the amended soils. Further to this, heterotrophic ammonia and nitrate-oxidizing bacterial population were inhibited in the amended soils and denitrifying organisms were stimulated. The organisms were members of the genera Pseudomonas, Xanthobacter, Enterobacter, Bacillus, Lactobacillus, Citrobacter, Nitrosomonas, Agromyces and Rhizobium. ZnO nanoparticles altered the soil physicochemical properties which exacerbated the negative effect on microbial abundance and varied with the soil type.