Ecosystem Carbon Flux And Its Influencing Mechanisms Over A Salt Marsh In The Jiuduansha Shoals

The coastal salt marsh wetland is one of the most productive ecosystems on the planet,in addition to its slow rate of organic matter degradation,it has become an effective blue carbon sink for mitigating global warming and has attracted the attention of the international community in recent years.In recent years,an increasing attention has been paid to the carbon sequestration potential of coastal wetland ecoystems.At present,studies on how tides effects on carbon dioxide(CO₂)exchange between coastal wetland ecosystems and the atmosphere were still limited.Therefore,elucidating the tidal effects on the CO₂ exchange process was significance for understanding the mechanisms and controlling factors of ecosystem CO₂ exchange in tidal wetlands and for researching the global carbon cycle and climate change.In this study,we measured the ecosystem CO₂ exchange?the bio-meteorological conditions and the tidal hydrologic in a Phragmites salt marsh of the Jiuduansha shoals in the Changjiang estuary during the period 2018-2019 using the eddy covariance(EC)method?bio-meteorological and tidal hydrologic observation systems,as well as the bio-meterological conditions of the Jiuduansha shoals in the Changjiang estuary during the period 2018-2019.Based on this,we investigated:1)the seasonal flux footprints under different wind directions and atmospheric conditions.2)the carbon sink ability and the dynamics of CO₂ exchanges.3)the relative importance of the environmental drivers on the ecosystem CO₂ exchange at hourly,diel,and multi-day timescales.4)the responses of ecosystem photosynthesis and respiration to tidal inundation,to finally illuminated the tidal effects on the mechanisms and controlling factors of ecosystem CO₂ exchange.The main conlusions are as follows:(1)The flux footprint measured with FSAM(Flux Source Area Model)adequately revealed the characteristics of surface flux in a Phragmites salt marsh of the Jiuduansha shoals.The diel variations of the flux footprints showed that,generally,the footprints in the nighttime were larger than those during the daytime.The distance between the location of maximum value of the flux footprint and the station in the nighttime was from 509.5 m to 921.5m and fluctuates greatly,while flux footprint in the daytime was relatively stable at about 440 m.The flux source area under the non-prevailing wind condition was larger than that under the prevailing wind condition.Moreover,the flux source area was much larger in stable stratification.The distance between the location of maximum value of the flux footprint and the station was also much larger in stable stratification.(2)The study area acted as a strong sink for atmospheric CO₂ during the period2018-2019.The cumulative net ecosystem CO₂exchange(NEE)were-869 and-550 g C m^-2 y^-1 for both 2018 and 2019,respectively.The cumulative(GPP)were 1790 and1377 g C m^-2 y^-1,and the cumulative ecosystem respiration(R_eco)were 921 and 827 g C m^-2 y^-1 for 2018 and 2019,respectively.The monthly averaged diel variation of NEE varied strongly among the seasons,but all showed a typical U-shape pattern,indicating that the CO₂ was fixed during the daytime,but released in the nighttime.Moreover,the net CO₂ uptake mainly occurred in the growing season.(3)The interactions between the CO₂exchange and the environmental variables(tidal water level,salinity,temperature and humidity,wind direction and speed,light)were characterized at hourly,diel,and multi-day timescales using information theory analysis showed that,photosynthetically active radiation(PAR)was a major controlling factor of CO₂ exchange(NEE),whereas air tempertuare(T_a)and the vapor pressure deficit(VPD)were the secondary influencing factors at diel and multi-day time scales,respectively.(4)We investigated the direct and indirect influence of tides on net ecosystem exchange(NEE)of CO₂.The mean nighttime NEE during spring tides was lower than that during neap tides,indicating suppressed ecosystem respiration(R_eco)under inundation,which were attributed to tidal water level(TWL),indicating TWL reduced sensitivity of R_eco to T_a.The response of the daytime NEE to tidal inundation was more variable,an enhanced NEE under tidal inundation was found both in the early vegetative stage and the peak vegetative stage,but a suppressed NEE during flowering and ripening stage,which were directly attributed to TWL and salinity while attributed to the influence of tidal activity did indirectly influence photosynthesis through its effect on air T_a and VPD.