Nitrous oxide production from nitrification and denitrification in marine sediment at low oxygen concentrations

Abstract

Measurements of N2O production (release of free N2O), nitrification and denitrification were made simultaneously in NH4Cl- and KNO3-amended suspensions of marine sediment. An open flow system was designed for the application of low partial pressures of O2 (0-10 kPa [kilopascals]) to the sediment. The overall rate of N2O production increased dramatically at the lowest O2 tensions (0-0.2 kPa) and had a maximum at complete anoxia. The specific rates of N2O production from nitrification (N2On) and from denitrification (N2Od) were determined after separation of the processes with inhibitors. Within the range of 0-0.2 kPa O2, the rate of N2On production showed an apparent maximum of 0.1 kPa O2 where the production accounted for 25% of the total activity of nitrification (NH4+ oxidation). The rate of N2Od production continued to increase as the O2 fell to O. The proportion of N2Od to the total N2Od plus N2 produced from denitrification increased at the higher O2 tensions and reached the maximum of about 50% at 5 kPa O2. Except for a narrow range between 0.1-0.2 kPa O2, denitrification was the main source of N2O at 0-10 kPa O2.