The Response Mechanism Of Ecosystem CO₂ Exchange On Precipitation Distribution Over A Supra-tidal Wetland In The Yellow River Delta

Coastal wetlands are considered as significant sink for global C and contributors to global"blue carbon"resources,with high primary productivity,a low soil organic matter decomposition rate,a low CH₄ generation rate,and the ability to trap and bury significant amounts of allochthonus C.Global climate models project that changes in the amplitude and frequency of extreme meteorological events can result in seasonal redistribution of the precipitation,characterized by fewer and larger precipitation events.The unique characteristics of hydrologic regimes is dominated by the interaction of precipitation and a shallow,saline water table in the vertical direction over the supra-tidal wetland.The changes of precipitation distribution could affect ecosystem CO₂ exchange by influencing the transport of water and salt in soils.Using the eddy covariance technique,combining with micrometeorological observation,we analysed the dynamic characteristic of ecosystem CO₂ exchange and its influence factors,discussed the response mechanism of ecosystem CO₂ exchange on episodic flooding and seasonal distribution of precipitation in the supra-tidal wetland,and precipitation-induced changes in plant biomass drive interannual variability of the net ecosystem CO₂ exchange in a reclaimed coastal wetland.Diurnal patterns of net ecosystem CO₂ exchange（NEE）showed significant U-shape curve of the supra-tidal wetland.The courses of ecosystem CO₂ fluxes showed significant seasonal variations,with a net sink of CO₂ during the growing season and a net source of CO₂ for the remainder of the year.The difference of peak time,peak intensity and time duration of NEE might result from interannual variation of biological and non-biological conditions.At daily scales,PAR was the key controller of daytime NEE,and air temperature was the main factor of ninghttime NEE during the growing season.On the monthly scale,monthly NEE was significantly correlated with environmental variables（air temperature、leaf area index、net radiation and water content）.Stepwise multiple regression analyses found that T_air was the key controller of NEE on seasonal scale,accounting for 60%of the variations in NEE of the supra-tidal wetland.On the annual scale,R_n and T_air were the key controllers of NEE,accounting for 84%of the variations in NEE.Episodic flooding conditions reduced daytime uptake rate of CO₂ and the maximum rates of photosynthesis in the supra-tidal wetland.In addition,flooding clearly suppressed the nighttime CO₂ release（ecosystem respiration）from the supra-tidal wetland of the Yellow River Delta but increased its temperature sensitivity Q₁₀.On the one hand,during flooding events,the effective photosynthetic leaf area may be reduced as the shoots and leaves are partially or completely submerged.Meanwhile,to severe CO₂ limitation,the slow entry of CO₂ into leaves typically limits photosynthesis.On the one hand,when soil is inundated,the saturation of surface soils limits the diffusion of oxygen into the wetlands.Thus,lower O₂availability and inhibition of aerobic respiration lead to lower CO₂ emissions.In addition,due to slow diffusion rate of CO₂ in water,the diffusive boundary layer resistance can limit rates of CO₂ emission through the surface water.Therefore,wetland inundation generally is expected to decrease CO₂ emissions to the atmosphereThis study provides a dual effect of precipitation redistribution on ecosystem CO₂ exchange in the supra-tidal coastal wetland.The higher precipitation promoted net ecosystem CO₂ absorption due to the increased SWC and reduced salt stress during the fast growth stage.While the higher precipitation suppressed net ecosystem CO₂ uptake due to the increased waterlogged stress during the middle growth stage.These results illustrated that the precipitation distribution could modify the magnitude of NEE as well as its response to light and temperature in coastal wetlands.Therefore,understanding the responses of NEE to the precipitation redistribution is essential not only for predictive modeling of potential short-and long-term changes of carbon storage but also for predicting the possible impacts of climate change.On interannual scales,biomass was the dominant biological factor of interannual variations in NEE,and soil water content（SWC）was the dominant abiological factor of biomass of the supra-tidal wetland over the Yellow River Delta.Therefore,precipitation-induced changes in plant biomass drive interannual variability in the net ecosystem CO₂ exchange of a reclaimed coastal wetland.On annual scale,the supra-tidal wetland had stronger carbon-sink efficiency.The cumulative NEE was-248,-223,-164 and-247 g Cm^-2 for 2010,2011,2012 and2013,respectively;The cumulative gross primirary production（GPP）was 781 g Cm^-2,653 g Cm^-2,772 g Cm^-2 and 1004 g Cm^-2;The cumulative ecosystem respiration(R_eco)was 781 g Cm^-2,653 g Cm^-2,772 g Cm^-2 and 1004 g Cm^-2.