Bacterial N2O production following O2 exposure

Background/Question/Methods

Bacterial-mediated denitrification plays a significant role in mitigating the effects of excessive nitrogen (N) loading from agricultural and wastewater sources, yet incomplete denitrification results in production of nitrous oxide (N₂O), a potent greenhouse gas. Oxygen (O₂) directly inhibits denitrification, influences N₂O production, and modifies bacterial community composition. Previous work on O₂-mediated bacterial N₂O production has focused on the response of bacterial isolates in culture to O₂ or changes in biogenic N₂O emissions from terrestrial or marine ecosystems. The purpose of this study was to examine how freshwater wetland bacterial community composition and N₂O emissions are affected by, and recover from, O₂exposures of varying durations. 

Microcosms were constructed with wetland sediments. Treated microcosms received nutrient amendments (5 mg N-NO₃^- L^-1) and were subjected to 0-, 12-, 24-, or 48-hour (h) periods of O₂ exposure before being returned to anaerobic conditions to allow for recovery. Controls received no nitrate amendment and were continuously anaerobic. Headspace samples were collected every 6 h after initial O₂ exposure to quantify N₂O production. Denitrification rates were estimated using the acetylene block method. Terminal-restriction fragment length polymorphism (T-RFLP) analyses of 16S rDNA gene sequences were used to compare pre- and post-disturbance profiles of the bacterial community.

Results/Conclusions

Estimates of denitrification rates in treated microcosms were significantly greater than that of the control (p<0.05) indicating that nutrient availability was a limiting factor in denitrification in the control microcosms. Increases in mean N₂O production were seen within 6 h after O₂ evacuation for treatments undergoing 12- and 24-h periods of O₂ exposure. There was an overall significant positive correlation (p<0.05) between N₂O emissions and time since treatments began for the control (r²= 0.47), 12 h (r²= 0.47), and 24 h (r²=0.65) O₂ exposure treatments. These results suggest that O₂ can influence N₂O production and that O₂ disturbance, along with nutrient availability, might limit recovery to pre-disturbance emission rates. Potentially the length of the period of oxygen exposure will have a negative impact on rates of recovery when anaerobic conditions are restored.