Effect of Engineered Nanoparticles of Fe and Zn Oxides on Enzyme Activity and Bacterial Abundance in Soil at Ambient and Elevated Atmospheric CO2

Nanomaterials are finding widespread applications in industry, agriculture and environment due to their unique properties. But, before these materials are widely in use, their impact on soil ecology needs to be thoroughly investigated. In addition, how the nanoparticles behave in soil under elevated CO₂ is unknown. A pot culture experiment was conducted under controlled conditions in phytotron to study the effect of nanoparticles on microbial biomass and enzyme activity in soil under rice crop at ambient and elevated atmospheric CO₂. Manufactured nanoparticles of Fe and Zn (n-Fe₂O₃ and n-ZnO) were added to soil at concentrations of 100, 500 and 1,000 mg kg^?1. FeSO₄ (11.1 mg kg^?1), ZnSO₄ (5.33 mg kg^?1) and ‘control’ treatments were also included for comparison. Results showed highest bacterial population at 100 mg kg^?1 n-Fe₂O₃, which significantly decreased at 500 and 1,000 mg kg^?1 n-Fe₂O₃ or n-ZnO. Similarly the dehydrogenase activity is significantly higher with 100 mg kg^?1 n-Fe₂O₃ as compared to control and FeSO₄ or ZnSO₄. As compared to 100 mg kg^?1 concentration, there was twofold to fivefold decrease in fluorescein diacetate activity at 500 and 1,000 mg kg^?1 of nano-Fe₂O₃ treatment. At elevated CO₂, the microbial biomass and activities were higher at tillering than panicle initiation of rice crop over ambient CO₂ with 100 mg kg^?1 n-Fe₂O₃ and ZnO than other treatments. It is concluded that n-Fe₂O₃ and n-ZnO at 100 mg kg^?1 is effective to maintain various soil microbiological process but at higher concentrations (e.g., 500 and 1,000 mg kg^?1) negative impacts on soil ecology can be expected.