Nitrous oxide in the Schelde estuary: production by nitrification and emission to the atmosphere

Abstract

Concentrations of nitrous oxide (N2O), oxygen, nitrate, and ammonium, as well as nitrification activity were determined along the salinity gradient of the Schelde Estuary, Northwest Europe, in October 1993, March 1994, and July 1996. The entire estuary was always supersaturated with N2O. Concentrations ranged from 338 nmol dm−3 (31 times supersaturated) in the O2-poor upper estuary, down to about 10 nmol dm−3 (slight supersaturation) at the mouth of the estuary. Nearly all N2O was lost to the atmosphere within the estuary rather than being transported to the open sea. The mean annual emission from the Schelde estuary to the atmosphere was estimated to be 2.8×108 g. Per unit area, this flux is large when compared to data published for other estuaries. Nitrification in the water column was the main source of N2O. The oxygen gradient along the estuary controlled the location and intensity of nitrification. In the upper estuary, the maximum nitrification activity was 6.4 μmol N dm−3 h−1. The N2O yield, associated with nitrification, varied spatially and seasonally. Between 0.1% and 0.4% of the oxidised NH4+ was converted to N2O. The total amount of N2O in the estuary appeared to be controlled by the NH4+ concentration of the river water which entered the estuary.