Factors controlling nitrous oxide at the microbial community and estuarine scale

Abstract

This paper examines the effect of oxygen on nitrous oxide (N 2 O) concentrations in estuarine waters. N 2 O has been measured year-round in the Schelde estuary, a high-nitrogen, lowoxygen macrotidal system. N 2 O concentrations were above atmospheric equilibrium levels indicating that this estuary represents a source to the atmosphere. The distribution of N 2 O 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 N 2 O production to occur at oxygen concentrations of about 5 M. This production was inhibited by acetylene, a nitrification inhibitor. Maximum N 2 O concentration in the field occurred at oxygen concentrations below 35 M. The difference in the oxygen concentration that results in maximum N 2 O 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 N 2 O production if ammonium is present in sufficient amounts. This conclusion is further illustrated by data from the Thames, Loire and Gironde estuaries.