Dynamics of CO2 exchange and its environmental controls in an urban green-land ecosystem in Fuzhou City

The area of urban green-land is expanding dramatically as a strategy to counter rapid urbanization. Urban green-land ecosystems with plantations as their main vegetation type have great potential to sequester atmospheric carbon. Continuous measurements of CO2 flux were made using eddy covariance technique, from January 2014 to January 2016, in Fuzhou City to quantify the seasonal dynamics of net ecosystem CO2 exchange (NEE) and its responses to environmental factors. Gross ecosystem productivity (GEP), ecosystem respiration (Re), and net ecosystem productivity (NEP=–NEE) showed strong seasonal pattern, with CO2 uptake dominating during the growing season from April to November, and a respiratory release of CO2 dominating during the non-growing season. Ecosystem respiration (Re) had a positive relation with GEP and gross primary productivity, and the fitting line was less than 1, with the straight intercept not zero, which shows that the balance of urban green-land ecosystem was greatly influenced by the precipitation and distribution of precipitation during the growing season. An exponential equation of the net carbon exchange to simulate the soil temperature (Ta) showed that NEE increased exponentially with the Ta, and the interpretation ratio was more than 80％. GEP increased exponentially with the Ta, and there was no optimum temperature, i.e., the highest temperature had no inhibitory effect on the GEP. NEP decreased with increasing Ta, when Ta 5.0 ℃, but increased when Ta 5.0 ℃. NEE and photosynthetically active radiation (PAR) meet the optimal hyperbolic relationship when PAR 350 μmol·(m2·d)–1, where the ecological system breathing was greater than the photosynthetic production, and when PAR 500 μmol·(m2·d)–1, where the ecosystem carbon uptake was saturated. The ecosystem quantum yield (α) and maximum photosynthesis (Amax) showed apparent seasonal patterns, both peaking in July. Correlation analysis showed that the NEE residuals for different months were negatively correlated with rainfall and positively correlated with Ta and vapour pressure deficit. The present results could contribute to the carbon budget of urban ecosystems and help create carbon-oriented management strategies for sustainable urban development under global climate change.