长江口潮间带湿地涨退潮期N_2O的排放通量研究

EXCHANGE FLUXES OF N_2O BETWEEN INTERTIDAL WETLANDS AND ATMOSPHERE DURING THE LOW AND HIGH TIDES IN YANGTZE ESTUARY

随着河口区接收上游人为氮排放量的增加,为这一区域氧化亚氮(N2O)的排放增加了很大不确定性。选择长江河口潮间带湿地为研究对象,分别采用原位静态箱法和静态顶空法,从2011年1月至12月对长江口沉积物-大气界面以及涨潮水-大气界面的N2O排放通量进行了为期一年的现场观测和研究。研究结果表明,沉积物-大气界面N2O通量有着显著的时空差异。N2O排放通量在日变化以及季节变化上都表现出明显的源汇转变,就年平均排放通量,光滩带沉积物-大气界面达到了5.99μgN2O/(m2·h),而海三棱藨草盐沼带与大气间N2O交换则十分微弱,为0.60μgN2O/(m2·h)。对长江口涨退潮期光滩和草滩上覆水体-大气界面N2O排放通量的研究表明,长江口涨退潮期在夏季和秋季,无论是光滩还是草滩均表现为大气N2O的稳定排放源,其中夏季平均25.3μgN2O/(m2·h),秋季平均排放通量为20.7μgN2O/(m2·h)。作为河口区上游排放氮素的直接接收者,和沉积物-大气界面N2O排放相比,长江口涨潮水-大气界面N2O排放稳定而又显著,是长江口N2O排放的主要贡献者,应成为这一区域N2O排放的关注热点。

As one of the most important greenhouse gases,nitrous oxide(N2O)has been recognized to contribute global warming by 25%.The Global nitrogen enrichment has resulted in increased N2O emission that greatly contributes to global warming and stratospheric ozone destruction. With the increase of upstream nitrogen importation,the uncertainties of N2O emissions also increased due to the complex nitrogen cycles in estuary area.Although some studies have already been carried out to identify the N2O emission fluxes in the Yangtze estuary,the N2O emission flux of water body during the tide movement remains unknown.From February to December2011,we chose Yangtze estuary intertidal zone(Scripus mariqueter salt marshes and bare tidal flat)as research area to study the natural N2O fluxes between the sediment and atmosphere and also between the tide water and atmosphere during the tide movement.The sediment-air and water-air N2O 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 N2O at the low tide when the sediments were exposed to the air.Considering both N2O emission and absorption,the annual average N2O fluxes of sediment-air interface in Scripus mariqueter salt marshes and bare tidal flat were 0.60 and 5.99μg N2O/(m2·h),respectively.N2O fluxes between the sediment and atmosphere showed large temporal variations.The transformation of N2O 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 N2O emission and absorption was probably caused by the low concentration of active inorganic nitrogen in the sediment,which brought many uncertainties to the N2O production ratio during the denitrification process.The N2O 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 N2O.The average air-sea fluxes of N2O in summer and autumn were 25.3and 20.7μgN2O/(m2·h),respectively.The water-air N2O fluxes didnt exhibit significant seasonal change and were also not influenced much by the ambient temperature.In terms of spatial variation,water-air N2O fluxes in Scripus mariqueter salt marshes were lower than that in bare tidal flat and the N2O fluxes were 19.66 and 25.88μg N2O/(m2·h),respectively.In addition,the differences of water-air N2O fluxes between the diurnal tide and night tide were also not obvious,which indicated that light condition exerted little impact on water-air N2O emission.The concrete water-air N2O fluxes of the diurnal tide and night tide were21.69 and 24.59μg N2O/(m2·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 N2O,thus the tide water should be paid more attention from the terms of the greenhouse gas emissions compared with the N2O fluxes from the sediment and atmosphere.