The Short Exposure Effect of CeO2 Nanoparticles on Microbial Metabolism in Agricultural Soil

With the development of nanotechnology, the soil ecosystem is becoming increasingly exposed to engineered nanoparticles (NPs). Few studies thus far have reported the susceptibility of soil microorganisms to NPs. The aim of this work is to understand the effects of CeO₂ NPs on microorganism activity and to evaluate the nanotoxicity in agricultural ecosystems. We incubated soil samples contaminated with CeO₂ NPs at concentrations of 0, 0.5, 5, 50 mg kg^-1 for 30 days. We quantified the toxic effect of CeO₂ NPs on soil metabolism by combining microcalorimetry with specific enzymatic tests (urease, catalase and fluorescein diacetate hydrolase, FDA) and azobacter counting. The thermodynamic parameters obtained from the powertime curves show reductions of total heat output, Q_total, and peak heat output, P_max, with increasing CeO₂ NP concentration. This finding reveals that CeO₂ NPs are toxic to the metabolism of microbial populations in soil. Similar tests with urease, catalase and FDA, exhibited similar negative concentration relationships, thus providing further evidence to support the microcalorimetric results. Furthermore, the soil diazotroph group is particularly sensitive to CeO₂ NPs concentrations of 5 and 50 mg kg^-1, indicating disturbance of N cycling. In conclusion, these results indicate that CeO₂ NPs become bioavailable for microorganisms in soil, thereby exerting toxic effects on metabolism activity and the azobacter group.