Contrasting Warming and Ozone Effects on Denitrifiers Dominate Soil N 2 O Emissions

Nitrous oxide (N O) in the atmosphere is a major greenhouse gas and reacts with volatile organic compounds to create ozone (an air pollutant) in the troposphere. Climate change factors such as warming and elevated ozone (eO ) affect N O fluxes, but the direction and magnitude of these effects are uncertain and the underlying mechanisms remain unclear. We examined the impact of simulated warming (control + 3.6 °C) and eO (control + 45 ppb) on soil N O fluxes in a soybean agroecosystem. Results obtained showed that warming significantly increased soil labile C, microbial biomass, and soil N mineralization, but eO reduced these parameters. Warming enhanced N O-producing denitrifers ( nirS- and nirK-type), corresponding to increases in both the rate and sum of N O emissions. In contrast, eO significantly reduced both N O-producing and N O-consuming ( nosZ-type) denitrifiers but had no impact on N O emissions. Further, eO offsets the effects of warming on soil labile C, microbial biomass, and the population size of denitrifiers but still increased N O emissions, indicating a direct effect of temperature on N O emissions. Together, these findings suggest that warming may promote N O production through increasing both the abundance and activities of N O-producing microbes, positively feeding back to the ongoing climate change.