Contrasting Warming and Ozone Effects on Denitrifiers Dominate Soil N2O Emissions

Nitrous oxide (N2O) 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(3)) affect N2O 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 degrees C) and eO(3) (control + 45 ppb) on soil N2O fluxes in a soybean agroecosystem. Results obtained showed that warming significantly increased soil labile C, microbial biomass, and soil N mineralization, but eO(3) reduced these parameters. Warming enhanced N2O-producing denitrifers (nirS- and nirK-type), corresponding to increases in both the rate and sum of N2O emissions. In contrast, eO(3) significantly reduced both N2O-producing and N2O-consuming (nosZ-type) denitrifiers but had no impact on N2O emissions. Further, eO(3) offsets the effects of warming on soil labile C, microbial biomass, and the population size of denitrifiers but still increased N2O emissions, indicating a direct effect of temperature on N2O emissions. Together, these findings suggest that warming may promote N2O production through increasing both the abundance and activities of N2O-producing microbes, positively feeding back to the ongoing climate change.