Effect of Zinc Oxide Nanoparticles on Nitrous Oxide Emissions in Agricultural Soil

Zinc oxide nanoparticles (ZnO NPs) are widely used and exposed to the soil environment, but their effect on soil nitrous oxide (N₂O) emissions remains unclear. In this study, a microcosm experiment was conducted to explore the effects of different ZnO NPs concentrations (0, 100, 500, and 1000 mg kg⁻¹) on N₂O emissions and associated functional genes related to N₂O amendment with carbon (C) or nitrogen (N) substrates. Partial least squares path modeling (PLS-PM) was used to explore possible pathways controlling N₂O emissions induced by ZnO NPs. In the treatment without C or N substrates, 100 and 500 mg kg⁻¹ ZnO NPs did not affect N₂O production, but 1000 mg kg⁻¹ ZnO NPs stimulated N₂O production. Interestingly, compared with the soils without ZnO NPs, the total N₂O emissions in the presence of different ZnO NPs concentrations increased by 2.36–4.85-, 1.51–1.62-, and 6.28–8.35-fold following C, N and both C & N substrate amendments, respectively. Moreover, ZnO NPs increased the functional genes of ammonia-oxidizing bacteria (AOB <i>amoA</i>) and nitrite reductase (<i>nirS</i>) and led to the exhaustion of nitrate but reduced the gene copies of ammonia-oxidizing archaea (AOA <i>amoA</i>). In addition, the redundancy analysis results showed that the AOB <i>amoA</i> and <i>nirS</i> genes were positively correlated with total N₂O emissions, and the PLS-PM results showed that ZnO NPs indirectly affected N₂O emissions by influencing soil nitrate content, nitrifiers and denitrifiers. Overall, our results showed that ZnO NPs increase N₂O emissions by increasing nitrification (AOB <i>amoA</i>) and denitrification (<i>nirS</i>), and we highlight that the exposure of ZnO NPs in agricultural fields probably results in a high risk of N₂O emissions when coupled with C and N substrate amendments, contributing to global climate warming.