PS 22-5 - Precipitation patterns in drylands drive NOx emissions from AO communities

Background/Question/Methods

Dryland ecosystems are hotspots for the emission of nitric oxide (NO), a reactive air pollutant and indirect greenhouse gas. In these dry and aerobic systems, the oxidation of ammonia by nitrifiers has been shown to control NO emissions when dry soils wet up. However, it remains unclear what fraction of the NO is produced by different nitrifying communities such as ammonia-oxidizing archaea (AOA) and bacteria (AOB), and how their contribution may vary as a function of past precipitation: How may past droughts or wetter conditions affect soil NO emissions and the ammonia-oxidizing communities that produce it? To answer this question, we manipulated plots in a Pinyon-Juniper dryland to either exclude or increase precipitation during the summer and winter seasons. We also selectively inhibited AOA and AOB communities in lab microcosms to assess their contribution to NO emissions over 48-hours. We hypothesized that AOA would contribute most to NO emissions in response to the legacies of past dry conditions because of their extremophile nature, but that the proportion of NO emissions derived from AOB would increase in plots receiving more precipitation.

Results/Conclusions

NO emissions were highest from plots where precipitation was excluded. Cumulative NO emissions over the 48-hour incubation were nearly three times higher in the summer precipitation exclusion plots (70.0 ± 29.8 ng N-NO g-1) compared to plots watered in the winter (25.2 ± 14.2 ng N-NO g-1; p = 0.04). In these precipitation exclusion plots, 77% of the total NO flux was produced by AOB, whereas in winter plots with added precipitation, AOB produced only 36% of the total NO flux. In contrast to our hypothesis, dry conditions did not favor NO emissions from AOA. Rather, AOB controlled NO emissions, presumably because past drought events increased AOB access to ammonia; soil ammonium concentrations were three times higher in precipitation exclusion plots relative to the control plots. Our results suggest that the history of past precipitation events can affect NO emissions by altering the contribution of NO derived from AOB relative to AOA.