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

Estuaries are expected to contribute large nitrous oxide (NO) emissions, however the environmental controls and implications of NO emissions have not been well understood. Here we investigated water NO 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 NO concentrations and fluxes were in a range of 15.8-84.9 nmol L and 0.66-22.2 µg m h, respectively. These studied estuaries were oversaturated in NO, with the saturations of 118-615%. Water dissolved NO concentrations, saturations and fluxes increased significantly as nitrogen concentrations increase, whereas they did not differ significantly between the wet and dry seasons. Water NO emissions, however, were also lower in the estuaries characterized by large discharge and water flow. NO saturations and fluxes were determined directly by water nitrogen and oxygen concentrations and more indirectly by water temperature and velocity. The δN-NO and site preference-NO 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 NO emissions, respectively. Indirect NO emission factors were 0.08-0.14% and decreased with increasing total nitrogen concentrations. It is estimated that water NO emissions in CO equiv could offset approximately 4.9% of average CO sink of China estuaries. Therefore, these results suggest that nitrogen concentrations and hydrologic characteristics together modify NO emissions and that estuaries may be the important contributors to NO emissions.