Sources of Nitrogen Used for Denitrification and Nitrous Oxide Formation in Sediments of the Hypernutrified Colne, the Nutrified Humber, and the Oligotrophic Conwy Estuaries, United Kingdom

Rates of denitrification and nitrous oxide (N2O) formation, and the sources of N2and N2O, were examined by the isotope-pairing technique in three U.K. estuaries (Colne, Humber, Conwy), which ranged from extremely nutrified to oligotrophic. Nitrous oxide concentrations were supersaturated throughout the year with annual averages of 603% in the Colne, 158% in the Humber, and 133% in the Conwy, indicating that the estuaries were sources of atmospheric N2O. Denitrification and N2O formation were from benthic processes, and not from water-column processes. Generally, both denitrification and N2O formation decreased down the estuary as nitrate concentrations lowered. The proportion of N2and N2O derived from nitrate in the overlying water column (Dw) also decreased with nitrate concentration, while that from benthic coupled nitrification-denitrification (Dn) increased. Scaled to the total estuary area, in the hypernutrified Colne, water-column nitrate was the main source of N2and N2O; in the moderately nutrified Humber, Dwand Dncontributed approximately equally, while in the oligotrophic Conwy, coupled nitrification-denitrification (Dn) was the main source of N2and N2O. For the first time, the formation of N2O from either the nitrification or denitrification steps of Dnwas also determined. In the Colne, that from the nitrification step predominated at the top of the estuary but decreased down the estuary, while in the Humber that from denitrification dominated at the top and decreased down the estuary. In the oligotrophic Conwy, there were approximately equal contributions.