Diurnal variations of carbon dioxide,methane,and nitrous oxide fluxes from invasive Spartina alterniflora dominated coastal wetland in northern Jiangsu Province

The invasions of the alien species such as Spartina alterniflora along the northern Jiangsu coastlines have posed a threat to biodiversity and the ecosystem function.Yet,limited attention has been given to their potential influence on greenhouse gas(GHG) emissions,including the diurnal variations of GHG fluxes that are fundamental in estimating the carbon and nitrogen budget.In this study,we examined the diurnal variation in fluxes of carbon dioxide(CO),methane(CH),and nitrous oxide(N2O) from a S.alterniflora intertidal flat in June,October,and December of 2013 and April of 2014 representing the summer,autumn,winter,and spring seasons,respectively.We found that the average CHfluxes on the diurnal scale were positive during the growing season while negative otherwise.The tidal flat of S.alterniflora acted as a source of CHin summer(June) and a combination of source and sink in other seasons.We observed higher diurnal variations in the COand NO fluxes during the growing season(1 536.5 mg COmhand 25.6 μg NO mh) compared with those measured in the non-growing season(379.1 mg COmhand 16.5 μg NO mh).The mean fluxes of CHwere higher at night than that in the daytime during all the seasons but October.The diurnal variation in the fluxes of COin June and NO in December fluctuated more than that in October and April.However,two peak curves in October and April were observed for the diurnal changes in COand NO fluxes(prominent peaks were found in the morning of October and in the afternoon of April,respectively).The highest diurnal variation in the NO fluxes took place at 15:00(86.4 μg NO mh) in June with an unimodal distribution.Water logging in October increased the emission of CO(especially at nighttime),yet decreased NO and CHemissions to a different degree on the daily scale because of the restrained diffusion rates of the gases.The seasonal and diurnal variations of CHand COfluxes did not correlate to the air and soil temperatures,whereas the seasonal and diurnal variation of the fluxes of NO in June exhibited a significant correlation with air temperature.When NO and CHfluxes were converted to CO-e equivalents,the emissions of NO had a remarkable potential to impact the global warming.The mean daily flux(MF) and total daily flux(TDF) were higher in the growing season,nevertheless,the MF and TDF of COwere higher in October and those of CHand NO were higher in June.In spite of the difference in the optimal sampling times throughout the observation period,our results obtained have implications for sampling and scaling strategies in estimating the GHG fluxes in coastal saline wetlands.