Effects Of Tides On Ecosystem Exchange Of CO₂ And CH₄ Over A Salt Marsh In The Yellow River Delta,China

Salt marshes between terrestrial and coastal ecosystems have been considered to be as some of most efficient sink due to high carbon deposition rate and lower CH₄ emission rate.Tidal activity may affect methane emission by controlling its production,oxidation and transport.As a unique hydrological characteristic,tides can strongly affect carbon balance in a salt marsh despite their short duration.Carbon sequestration potential of salt marsh plays an important role in tackling sea level rise and other climate change induced.Using the eddy covariance technique,we measured the net ecosystem CO₂ exchange?NEE?,methane fluxes?CH₄?and its environmental factors and tidal change over a salt marsh in the Yellow River Delta.It aimed 1)to investigate dynamics of NEE and CH₄ emission in growing season;2)to selective analysis the effect of drying and wetting cycles induced by tides on NEE;3)to illuminate that tidal action drives episodic and high-magnitude emissions of CH₄ from the wetland;4)to quantify the carbon sink ability of the salt marsh during growing season.Diurnal patterns of NEE among growing season were very similar in U-shaped curve of unimodal but varied substantially in amplitude.The maximum of the averaged CO₂ uptake and ecosystem respiration were 2.19 g CO₂ m^-2 d^-1 and 6.17 g CO₂ m^-2 d^-1.Tidal hydrology drives the release of CH₄ in the high tidal flat and middle tidal flat of the Yellow River delta.The daily average CH₄ was 0.063mg·m^-2·h^-11 during the growing season.The tidal process promoted the daytime CO₂ uptake,but it didn't clearly affect the nighttime CO₂ release.Tidal inundation was a major factor influencing daytime NEE.The diurnal change of NEE showed a distinct U-shaped curve on both drought and wet stages,but not with substantial variation in its amplitude during the drought stage.The drying and wetting cycles enhanced the absorption of daytime CO₂.Under drought stage,the mean of the maximum photosynthetic rate(A_max),apparent quantum yield???and ecosystem respiration(R_eco)were higher than those in wet stage.In addition,the drying and wetting cycles suppressed the nighttime CO₂ release from the salt marsh but increased its temperature sensitivity.The mean methane fluxes before flooding,flooding and ebbing stages and after ebbing were-0.40 nmol·m^-2·s^-1,0.02 nmol·m^-2·s^-11 and 1.78 nmol·m^-2·s^-1.Methane fluxes during after ebb were clearly higher than that tidal fluctuation and before flood.There was significant difference of methane emission between before flooding and after ebbing?P<0.05?.The large amount of CH₄ emission after falling tide can not be maintained for a long time,and then it gradually converted from emissions to absorption.Drought induced lags in methane flux recovery that depend on drought timing and severity before flooding.During the growing season,the salt marsh was an obvious CO₂ sink and CH₄source.It was a CO₂ sink of 16.6 g C m^-2 in 2016.Approximately 111.1 g C m^-22 was assimilated by GPP,and 96.6 g C m^-22 was released by ecosystem respiration.The total emission flux of CH₄ was 8.53 mg·m^-2·month^-1,which was a weak emission source.Therefore,we can predict these resources in anticipation of climate change.