Sources and sinks of N2O in the subtropical Jiulong River Estuary, Southeast China

Nitrous oxide (N 2 O) is one of the most important greenhouse gases and contributes to the depletion of ozone in the stratosphere. Estuaries are areas of intensive biological production and associated N 2 O emissions through both denitrification and nitrification processes. The spatial and temporal variations of N 2 O in the Jiulong River Estuary, a subtropical estuary, were explored to evaluate sources and sinks of N 2 O in this area. The estuary was found to be a strong source of N 2 O, its saturation in the surface water ranged from 113 to 2926% relative to the ambient atmospheric concentrations, showing great temporal and spatial variations and was influenced by multiple factors such as the concentration of dissolved inorganic nitrogen (DIN, i.e., NO 3 − , NH 4 + , and NO 2 − ), salinity and dissolved oxygen. N 2 O concentrations were at a high level in upper estuary but reduced to the lower parts of the estuary. Groundwater input could be another contributor to N 2 O in the estuary. Almost all N 2 O within the estuary was released into the atmosphere rather than being transported to the bay. The N 2 O flux in the estuary (mean 597 μmol/m 2 /d) was at the higher end of the range observed in estuaries worldwide due to the very high DIN loads in the Jiulong River Estuary. Our data indicate that the N 2 O saturation in the estuary continues to increase, although the DIN inputs began to decline in 2011, which might be relate to the improved environmental conditions with increased oxygen concentrations. N 2 O production pathways have changed from predominantly denitrification in the past toward significant production from nitrification in the present. Further investigation is needed to better understand the behavior of N 2 O in the Jiulong River Estuary.