Factors controlling nitrous oxide at the microbial community and estuarine scale

This paper examines the effect of oxygen on nitrous oxide (N2O) concentrations in estuarine waters. N2O has been measured year-round in the Schelde estuary, a high-nitrogen, low-oxygen macrotidal system. N2O concentrations were above atmospheric equilibrium levels indicating that this estuary represents a source to the atmosphere. The distribution of N2O showed consistent and systematic relationships with distribution patterns of ammonium, oxygen, nitrite and nitrification activities. A controlled laboratory experiment with a natural bacterial community from the Schelde estuary revealed maximum N2O production to occur at oxygen concentrations of about 5 muM. This production was inhibited by acetylene, a nitrification inhibitor. Maximum N2O concentration in the field occurred at oxygen concentrations below 35 muM. The difference in the oxygen concentration that results in maximum N2O may have arisen because low-oxygen environments present in the estuary were destroyed by stirring in our laboratory experiment. It appears that low oxygen concentrations in estuarine water trigger enhanced N2O production if ammonium is present in sufficient amounts. This conclusion is further illustrated by data from the Thames, Loire and Gironde estuaries.