| Summary: | Metal oxide nanoparticles (NPs) have been widely used in industrial and agricultural production and introduced into soils. The impact of these nanoparticles on soil nitrous oxide (N<sub>2</sub>O) emission is unclear. We conducted a microcosm experiment to investigate the effects of titanium oxide nanoparticles (TiO<sub>2</sub> NPs), copper oxide nanoparticles (CuO NPs), and aluminum oxide nanoparticles (Al<sub>2</sub>O<sub>3</sub> NPs) on soil N<sub>2</sub>O emissions and the abundance of functional genes related to N<sub>2</sub>O production/reduction. Compared to the soil without NPs addition, TiO<sub>2</sub> NPs applied to the soil produced no significant effect on N<sub>2</sub>O emissions. The denitrification process in the soil exposed to CuO NPs was inhibited by reducing the functional genes related to nitrite reductase (<i>nirK</i>) and increasing N<sub>2</sub>O reductase (<i>nosZ</i>), while CuO NPs added to the soil stimulated the cumulative N<sub>2</sub>O emissions by 92.7%. After the application of Al<sub>2</sub>O<sub>3</sub> NPs to the soil, the nitrification process was inhibited by inhibiting the functional genes of ammonia-oxidizing bacteria (AOB <i>amoA</i>), and soil N<sub>2</sub>O emission was reduced by 48.6%. Large-scale application of CuO NPs in agricultural soils may stimulate the N<sub>2</sub>O emissions resulting in potential environmental risks.
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