Abstract:

As one of the most important greenhouse gases, nitrous oxide (N2O) has been recognized to contribute global warming by 25%. The Global nitrogen enrichment has resulted in increased N2O emission that greatly contributes to global warming and stratospheric ozone destruction. With the increase of upstream nitrogen importation, the uncertainties of N2O emissions also increased due to the complex nitrogen cycles in estuary area. Although some studies have already been carried out to identify the N2O emission fluxes in the Yangtze estuary, the N2O emission flux of water body during the tide movement remains unknown. From February to December 2011, we chose Yangtze estuary intertidal zone (Scripus mariqueter salt marshes and bare tidal flat) as research area to study the natural N2O fluxes between the sediment and atmosphere and also between the tide water and atmosphere during the tide movement. The sedimentair and waterair N2O fluxes were respectively measured using the static closed chamber and static headspace techniques. The study results indicated that both Scripus mariqueter salt marshes and bare tidal flat acted as net sources of atmospheric N2O at the low tide when the sediments were exposed to the air. Considering both N2O emission and absorption, the annual average N2O fluxes of sedimentair interface in Scripus mariqueter salt marshes and bare tidal flat were 060 and 599 μg N2O/(m2〖DK〗·h), respectively. N2O fluxes between the sediment and atmosphere showed large temporal variations. The transformation of N2O sink and source in both Scripus mariqueter salt marshes and bare flat appeared in different time scales including daily and monthly time scale. The fluctuation of N2O emission and absorption was probably caused by the low concentration of active inorganic nitrogen in the sediment, which brought many uncertainties to the N2O production ratio during the denitrification process. The N2O fluxes between the waterair interfaces during the processes of tide rising and ebbing showed that both Scripus mariqueter salt marshes and the bare tidal flat acted as the steady and significant emission sources of the atmospheric N2O. The average airsea fluxes of N2O in summer and autumn were 253 and 207 μgN2O/(m2〖DK〗·h), respectively. The waterair N2O fluxes didnt exhibit significant seasonal change and were also not influenced much by the ambient temperature. In terms of spatial variation, waterair N2O fluxes in Scripus mariqueter salt marshes were lower than that in bare tidal flat and the N2O fluxes were 1966 and 2588 μg N2O/(m2〖DK〗·h), respectively. In addition, the differences of waterair N2O fluxes between the diurnal tide and night tide were also not obvious, which indicated that light condition exerted little impact on waterair N2O emission. The concrete waterair N2O fluxes of the diurnal tide and night tide were 2169 and 2459 μg N2O/(m2〖DK〗·h), respectively. As the main receiver of upstream imported nitrogen, the water body in the Yangtze estuary during the tide movement acted as the more significant emission source of atmospheric N2O, thus the tide water should be paid more attention from the terms of the greenhouse gas emissions compared with the N2O fluxes from the sediment and atmosphere.〖