| Coastal wetlands, the interfaces between terrestrial and ocean ecosystems, can be as an efficient sink due to their higher primary productivities and lower carbon decomposition rates. However, the global climate change can effect their carbon sinks. Using the eddy covariance technique, we measured CO2 flux between the ecosystem and atmosphere, environmental and biological factors over a reed wetland in the Yellow River Delta. O ur objectives were(1) to characterize diurnal and seasonal variations of the net ecosystem CO2 exchange(NEE) of the ecosystem;(2) to identify the main abiotic and biotic drivers of NEE at different timescales;(3) to illustrate the effect of sunny and cloudy days on NEE(4) quantify the carbon sink ability of the coastal wetland.Results show:(1) Diurnal patterns of NEE among different months were very similar in shape but varied substantially in amplitude. From May to October, average NEE for each month was negative(a CO2 sink) during the daytime and positive(a CO2 source) during the nighttime in wetland. While during growing season, the maximum of the averaged daily CO2 uptake and release rate in wetland ecosystem were-10.0 μmol CO2m-2s-1 and 3.6 μmol CO2m-2s-1.(2) The values of NEE, photosynthetic active radiation(PAR), air temperation(T), soil temperature(Tsoil) and soil water content(SWC) were higher during the peak growing season(from July to September) and lower during the initial(from January to June) and late(from October to December) growing season. Average monthly PAR reached its maximum in June(500.8 μmolm-2s-1), and then decreased gradually. Annual mean temperature was 12.1 oC with minimum mean daily temperatures of-1.3 oC in December and maximum of 27.7 oC in August, respectively. Soil temperature at the 10 cm depth in the wetland was 12.6 oC, while it was 22.0 oC during the growing season. The total precipitation during the growing season was 434 mm with nearly 68% of the annual precipitation. The abovegroud biomass(AGB) and leaf area index(LAI) increased through the growing season until a maximum value was reached in August(635.5 gm-2) and July(0.61), respectively.(3) During each month of the growing season, the response of daytime NEE to PAR can be expressed by a rectangular hyperbolic function. The seasonal variations of parameters(apparent quantum yield(α), maximum photosynthesis rate(Amax) and daytime ecosystem respiration(Reco, dayt ime)) could be represented as single peak curves and be described by quadratic models. The parameters of α and Reco, dayt ime reached maximum in August(0.08 μmol CO2μmol-1PAR, 10.8 μmol CO2m-2s-1), while Amax reached its maximum in July(25.4 μmol CO2m-2s-1).SWC and Tsoil were the main factors controlling the nighttime CO2 flux. An exponential relationship existed between nighttime NEE(Reco,nightt ime) and soil temperature at 10 cm depth under different SWC levels. The results showed that the temperature sensitivity of ecosystem respiration(Q10) evidently declined with increases in soil moisture. Q10 reached its maximum(2.8) when SWC was less than 40%. The temperature sensitivity of ecosystem respiration(Q10) were 3.6 in 2011, while it was 2.5 during the growing season.(4) Diurnal pattern of NEE showed a distinct U- like course on both sunny and cloudy days, but they varied substantially in amplitude. During the daytime, NEE on sunny days was significantly higher(P < 0.01) than that on adjacent cloudy days, when data were averaged over the 12 paired sampling dates. The response of daytime NEE to PAR can be expressed by a rectangular hyperbolic function on both sunny and cloudy days. There was a significant reduction(P < 0.01) in Amax on cloudy daytime compared to the adjacent sunny daytime. Similarly, there was a statistically significant decrease(P < 0.01) in Reco,dayt ime on cloudy daytime as compared to that of adjacent sunny daytime. Though there were significant exponential relationships between Reco,dayt ime and air temperature on both sunny and cloudy days. In addition, Q10 on cloudy days(1.9) was significantly lower as compared to that of sunny days(5.5). Stepwise multiple regression analyses suggested that changes in PAR and T together explained 63% of the changes in NEE between sunny and cloudy days. By taking advantage of the natural shift of sunny and cloudy days without disturbance to the plant-soil system, the results indicated that cloud cover significantly inhib it the absorption capacity of CO2 in the wetland. Thus, it is necessary to take into account the influence of shift of sunny and cloudy days on NEE when predicting the ecosystem responses to future climate change in the wetland.(5) The wetland was a carbon sink of 247.2 g C m-2 in 2013. Approximately 757.0 g C m-2was assimilated by GPP, and 1003.7 g C m-2 was released by Reco. During the growing season, Reco, GPP and NEE were nearly 91%, 89% and 95% of the annual values, respectively. Key words: Yellow River Delta, wetland, ecosystem CO exchange, light response,... |
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