Abstract: The net ecosystem exchange (NEE) of CO₂ above urban green space is controlled by various environmental factors. To quantitatively analyze the environmental response of the NEE can provide valuable basis for the evaluation of carbon fixation effect of green space ecosystem and the optimization of urban green space distribution. Based on the CO₂ flux data of Dongguan Botanical Garden in 2009 and 2010 measured by eddy-covariance method, this paper analyzed the relationships between the NEE and the environmental factors (photosynthetic active radiation (PAR), soil temperature (T_s), saturation vapor pressure differential (VPD), precipitation, and land cover distribution). The results showed that the absolute value of NEE increased with increasing PAR when the PAR was less than the light saturation point (1500 μmol photons·m^-2·s^-1), but presented a decreasing trend when the PAR was higher than the light saturation point. The ecosystem respiration (R_eco) increased with rising T_s, and was more sensitive to the T_s when the temperature was low (the Q₁₀ of Lloyd-Taylor equation was 1.8 at 10 ℃), compared with that at high temperature (Q₁₀ was 1.43 at 30 ℃). The VPD could act on CO₂ exchange by affecting plant stomata conductance. When the VPD rose, both the CO₂ release and the CO₂ fixation increased. Precipitation had significant effect on the R_eco through increasing soil moisture content. After precipitation, the R_eco at 25 ℃ was 15.8% higher than that before precipitation. There was a wider green space in northeast and southwest directions around the observation station. When these directions were the main contribution areas, the absolute value of NEE was larger, indicating that increasing urban green space could improve urban carbon fixation effect.

Key words: mangrove wetland, CO₂ flux, intertidal, Aegiceras corniculatum, Kandelia obovate.