| With the advance of urbanization, the coverage area of urban green space is increasing, owing to our pursuits for better life quality. Urban green space can bring about various ecological benefits, such as releasing oxygen, absorbing cabon, reducing temperature, increasing humidity, absorbing harmful gases, falling dust and reducing noise by photosynthesis and transpiration. All these effects can greatly improve environmental quality. Consequently, green space plays an important role in urban ecosystem, which can maintain the ecological balance, improve urban environment and beautify city landscape. In this paper, continuous eddy-covariance measurements of CO2fluxes were conducted in2009and2010at Dongguan botanical garden, in order to analyze the daily and annual carbon exchange status and find out the environmental effects on carbon sequestration.The major results were:(1) net ecosystem CO2exchange (NEE) was-91.9gC-m-2·a-1in2009and-116.5gC·m-2·a-1in2010, indicating that urban green space was a net carbon sink. The NEE varied seasonally with environmental factors. Urban green space was a carbon source from December to March and a carbon sink from April to November each year.(2) The annual average apparent light use efficiency (a) was0.00134±0.00035mgCO2·μmol-1Photons and maximum photosynthetic productivity (Pmax) was1.006±0.283mgCO2·m-2·s-1.(3) The annual ecosystem respiration (Reco) was1370.4gC·m-2·a-1in2009and1384.8gC·m-2·a-1in2010. Reco varied with changes in the soil temperature during the year. The simulated Reco during the night was less than the daytime result produced by the Michaelis-Menten model at the same temperature.(4) Because the urban green space was a mixture of forest and grassland, the NEE of the urban green space was smaller than that of a forest ecosystem, but larger than a grassland ecosystem. Compared with other urban green spaces, the NEE in Dongguan was lower than northern urban forests due to the high Reco caused by high temperatures.(5) Correlation analyses between the NEE and meteorological factors like the PAR, air temperature (Ta) and saturation vapor pressure differential (VPD) were performed. The partial correlation coefficient of the PAR was larger than the others, indicating that the PAR was the most important factor to the NEE. The partial correlation coefficient of Ta was negative during the winter and positive during the summer. The partial correlation coefficient of VPD was positive year-round and was higher under high VPD conditions.(6) Absolute value of NEE increased with PAR, while it was less than the light saturation point (1500μmolPhotons·m-2·s-1), however, NEE slightly decreased when PAR was higher than the light saturation point.(7) Ecosystem respiration (Reco increased with rising Ts. Reco was more sensitive to Ts when temperature was low (Q10of Lloyd-Taylor equation was1.8at10℃) comparing with high temperature (Q10of Lloyd-Taylor equation was1.43at30℃).(8) VPD could act on CO2exchange by affecting stomata conductance, when VPD rose, CO2release and fixation increased.(9) Precipitation had significant effect on Reco by amplifying soil moisture. After precipitation, Reco was15.8%higher than before at25℃.(10) Green space was wider in northeast and southwest direction around the tower, therefore, NEE was larger while those directions were main contribution areas, indicating that urban carbon fixation ability would be improved by afforesting. |
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