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Net ecosystem CO_2 exchange and its controlling factors during the growing season in an inter-tidal salt marsh in the Yellow River Estuary,China

Inter-tidal salt marsh plays a great important role in the global carbon cycle as a considerable potential capacity of the carbon sink. Compared to other types of wetlands,the inter-tidal salt marsh has a unique biogeochemical process under the combined action between freshwater and seawater. Therefore,there is considerable variability and uncertainty inits net ecosystem CO2exchange( NEE). However,few studies provide insights regarding the variability of NEE and its controlling factors in an inter-tidal salt marsh. Using the Eddy Covariance( EC) technique,we analyzed temporal variation in NEE and determined its control mechanisms coupled with meteorological and tidal inundation variables during the growing season( from April to October) of 2012 in an inter-tidal salt marsh in the Yellow River Estuary. The results showed that it was net CO2 absorption in the daytime and net CO2 release in the nighttime on a diurnal scale. The daily average NEE during the growing season was-0. 38 g CO2m-2d-1,with a maximum daily CO2 uptake rate of-3. 13 g CO2m-2d-1( June27) and a maximum release rate of 1. 47 g CO2m-2d-1( August 12). The monthly average NEE increased rapidly from May,and peaked in June,then decreasing gradually from July. The maximum monthly ecosystem respiration( Reco) was15. 16 g C /m2 in June when the hightest soil temperature was 27. 5 ℃. The monthly gross primary productivity( GPP)reached its peaking value( 25. 07 g C /m2) in July. During the growing season,NEE was mainly dominated by photosynthetic active radiation( PAR),soil temperature( Ts),soil water content( SWC) and tidal inundation. There was a rectangular hyperbolic relationship between the daytime net ecosystem CO2exchange( NEEdaytime) and PAR. The maximum ecosystem apparent quantum yield( α) and maximum photosynthesis rate( NEEsat) appeared in June(( 0.0086±0. 0019) μmol CO2μmol-1photons) and in May(( 4. 79±1. 52) μmol CO2m-2s-1),respectively. In addition,NEEdaytime also was positively correlated with Tsand SWC. During the growing season,NEEdaytimehad an exponential relationship with Ts. The mean value of Q10 was 1. 33,and it was positively related to SWC. During the typical sunny day of June 19 to June25,tidal inundation enhanced daytime net absorption of CO2 and nighttime CO2 release. As a result,tidal inundation increased the net ecosystem CO2 absorption by an average of 0. 76 g CO2m-2d-1. During the growing season,the inter-tidal salt marsh was an obvious CO2sink( 22. 28 g C /m2),with a cumulative emission of 96. 28 g C/m2 and a cumulative aborption of 118. 34 g C /m2.