Modelling The Diurnal Variations Of Methane Emissions From The Cyperus Malaccensis And Spartina Alterniflora Tidal Marshes In The Minjiang River Estuary

Tidal marshes are important sources of atmospheric methane;methane emissions from tidal marshes generally exhibit significant diurnal variations.However,the research conclusions of the diurnal variations in tidal marshes were inconsistent,and the processes and mechanisms of the diurnal variations are still poorly explored.On the other hand,model in simulating methane emission variations from wetlands on the diurnal scale has not been reported.In order to establish a model in simulating diurnal variations of methane emissions from tidal marshes,and to explore the patterns and the mechanisms of diurnal methane flux variations in tidal marshes,this paper simulated the diurnal variations of methane emissions from the Cyperus malaccensis and Spartina alterniflora tidal marshes in the Minjiang River Estuary;the simulation was based on the modifications to the seasonal-scale model—the Walter model.The modified model considered the effects of sediment temperature,pH,redox potential?Eh?,salinity,as well as tide condition and atmospheric relative humidity?RH?on the methane flux.The modified model was applied in 7 research days.The simulated and observed results suggested that,the season and vegetation may influence the overall magnitude of methane fluxes on the research days;the fluxes were higher in summer and in S.alterniflora marsh.The weather,tide condition?including the inundation depth and inundation period?,and the season may influence the diurnal variation patterns,induced complexity and uncertainty in diurnal methane flux variations in the C.malaccensis and S.alterniflora marshes in the Minjiang River Estuary.The diurnal variation patterns can be summarized as follows:1)In fine days,methane fluxes were generally higher during the day and lower at night.However,the hours of the maximum fluxes might be different because of the effect of season and the interference of inundation periods;the maximum flux could emerge in the morning,at noon,in the afternoon,or at dusk.Moreover,in wet days,methane fluxes might be lower during the day and higher at night.2)Methane fluxes in inundation periods were significantly lower than those in non-inundation periods when the inundation was deep and the inundation period was short.However,in the case of shallow inundation and long inundation periods,differences between the fluxes in inundation and non-inundation periods might be insignificant;the fluxes in inundation periods might even significantly higher than those in non-inundation periods.The simulated results indicated that,the methane production rates in the C.malaccensis and S.alterniflora marshes were significantly higher in inundation periods;the diurnal variations of methane oxidation rates were regulated by the capacity of the plant-mediated transport;the methane concentrations in the upper layers were generally higher from early hours to the morning.Moreover,the simulated methane fluxes through3 transport pathways suggested that,plant-mediated transport and ebullition were the dominant pathways for methane emissions from the C.malaccensis and S.alterniflora tidal marshes in the Minjiang River Estuary;in contrast,the diffusive fluxes were extremely low.The average proportions of plant-mediated transport,ebullition,and diffusion during the 7 days were 82.4%,17.3%and 0.3%,respectively.The plant-mediated flux and ebullitive flux exhibited complex and uncertain diurnal variations,while the diffusive flux had no significant diurnal variations.Based on the analysis of model inputs and outputs,this paper concluded that the diurnal variation patterns of methane emissions from the C.malaccensis and S.alterniflora tidal marshes in the Minjiang River Estuary were functions of various factors.The sediment temperature,atmospheric RH,and tide condition had direct impacts on the variation patterns.The sediment pH,Eh,and salinity regulated the diurnal variations by influencing the methane production;thus,these factors regulated the variation patterns indirectly.The model sensitivity tests suggested that,the maximum potential methane production rate?R₀?was an important parameter in regulating methane fluxes;moreover,variations in sediment temperature,pH,Eh,salinity,and atmospheric RH could also induce significant changes in methane fluxes.The test of the overall performance of the modified model showed that,the slope,intercept,and determination coefficient of the liner regression of modeled verus observed results were 0.94,1.06,and 0.81,respectively;the root mean-square error,mean deviation,and model efficiency of the modified model were 52.4%,9.5%,and 0.78,respectively.The result suggested that the modified model could precisely describe the diurnal variations of methane emissions from the C.malaccensis and S.alterniflora tidal marshes in the Minjiang River Estuary.The performances of the modified model on each day and on the overall were much better than those of the original model,indicating that the modifications had significantly improved the performance of Walter model on the diurnal scale.