Net Ecosystem CO₂ Exchange And Its Influencing Mechanisms In A Tidal Salt Marsh In The Yellow River Delta,China

Salt marsh is one of the three coastal blue carbon ecosystems.It plays a critical role in reducing and mitigating climate change.To investigate the characteristics and effect factors of net ecosystem CO₂ exchange（NEE）and its major components,gross primary production（GPP）and ecosystem respiration(R_eco),measurements of CO₂fluxes were performed in a tidal salt marsh in the Yellow River Delta using the micrometeorological eddy covariance（EC）technique during the growing season of2011,2012,2013,2014 and 2015.The detail contents of this dissertation are listed as follows:（1）The daily variation,seasonal variation and annual variation of CO₂ fluxes of a salt marsh in the Yellow River Delta were analyzed by continuous observation on the NEE of the tidal salt marsh ecosystem.Our results showed that the diurnal patterns of NEE among different months were very similar in shape but varied substantially in amplitude between the five years.（2）Daytime NEE values were strongly correlated with photosynthetically active radiation（PAR）.During the growing season of 2011,2012 and 2013,PAR could be responsible for 37%,43%and 40%variation of half-hourly NEE,respectively.Nighttime NEE values were mainly affected by air temperature（T_a）and soil water content（SWC）.During the growing season of 2011,2012 and 2013,air temperature could be responsible for 27%,18%and 11%variation of ecosystem respiration,respectively.The temperature sensitivity coefficients(Q₁₀)of R_ecoco were estimated to be 1.69 in 2011,1.61 in 2012,and 1.60 in 2013.Soil pH at the depth of 0-10 cm and10-20 cm showed significant linear correlation with GPP and NEE.（3）Combined with the enhanced vegetation index（EVI）,we revealed the influence mechanism of biological factor on the CO₂ fluxes of the tidal salt marsh in the Yellow River Delta.EVI could quickly and intuitively reflects the growth of vegetation in a large area.Enhanced vegetation index（EVI）showed linear positive relationship with 8-day average R_eco,GPP and NEE in the three years.EVI could indicate the changes in GPP,R_ecoco and NEE in a certain extent.The relationship between aboveground biomass and GPP,R_ecoco and NEE could be well express by quadratic equation.（4）We analyzed the light response curves of daytime NEE in sunny and cloudy days under different soil water conditions in the growing season in a tidal salt marsh in the Yellow River Delta,China.Our results showed that the relationship between daytime NEE and PAR showed a statistically significant cubic regression curve in sunny days under extreme dry condition and in long-term flooding days.Net ecosystem CO₂ absorption in cloudy days was higher than that in sunny days under extreme dry condition in the salt marsh.Under wet condition,higher light energy maximum conversion rate in cloudy days but larger capacity for net CO₂ uptake in sunny days.Parameters of the Michaelis-Menten model estimated in saturation days were larger than that in extreme dry days under cloudy condition.Overall,the combined effect of both soil water condition and available solar radiation changed the light response of net ecosystem CO₂ exchange by changing maximum light energy conversion rate and maximum photosynthetic rate.（5）Tidal cycles in these marshes affect their groundwater level and the physicochemical properties of soil,such as pH,salinity,nutrient content and temperature,and subsequently may modify wetland-atmosphere CO₂ exchange.Our study showed that the mean daily NEE in tidal flooding days was significantly higher than that in non-tidal flooding days（P<0.01）.Tides flooding enhanced the daytime net CO₂ uptake mainly by promoting daytime photosynthesis.In addition,flooding increased the nighttime basal respiration from the tidal salt marsh and its temperature sensitivity.Both light energy maximum conversion rate and maximum photosynthetic rate in short-term flooding days were higher than that in long-term flooding days.Thus,it is necessary to take into account the influence of tides on the magnitude of NEE in order to accurately estimate the carbon sink-source status of tidal salt marshes.（6）During the growing seasons of 2011,2012,2013,2014 and 2015,the tidal salt marsh acted as CO₂ sinks of-16.30,-23.05,-30.44,-21.32 and-17.65 g C m^-2.The linear regression analysis showed that in the growing season scale,the relationships between NEE,GPP and R_ecoco of the tidal salt marsh and soil temperature,air temperature,precipitation and EVI could be well fitted with linear equations.However,more years of continuous data set is needed in order to prove the significance of the fitting.