Nitrous oxide emissions from subtropical estuaries: Insights for environmental controls and implications

Estuaries are expected to contribute large nitrous oxide (N₂O) emissions, however the environmental controls and implications of N₂O emissions have not been well understood. Here we investigated water N₂O concentrations, fluxes and sources in wet and dry seasons for 2019-2020 in five subtropical estuaries spanning hydrologic characteristics and nitrogen concentrations gradient. Water dissolved N₂O concentrations and fluxes were in a range of 15.8-84.9 nmol L^-1 and 0.66-22.2 µg m^-2 h^-1, respectively. These studied estuaries were oversaturated in N₂O, with the saturations of 118-615%. Water dissolved N₂O concentrations, saturations and fluxes increased significantly as nitrogen concentrations increase, whereas they did not differ significantly between the wet and dry seasons. Water N₂O emissions, however, were also lower in the estuaries characterized by large discharge and water flow. N₂O saturations and fluxes were determined directly by water nitrogen and oxygen concentrations and more indirectly by water temperature and velocity. The δ¹⁵N-N₂O and site preference-N₂O varied respectively from 2.86 to 11.31‰ and from 1.58 to 11.72‰, which overlapped the values between nitrification and denitrification. Nitrification and denitrification were responsible for 18.7-38.1% and 61.9-81.3% of N₂O emissions, respectively. Indirect N₂O emission factors were 0.08-0.14% and decreased with increasing total nitrogen concentrations. It is estimated that water N₂O emissions in CO₂ equiv could offset approximately 4.9% of average CO₂ sink of China estuaries. Therefore, these results suggest that nitrogen concentrations and hydrologic characteristics together modify N₂O emissions and that estuaries may be the important contributors to N₂O emissions.