城市绿地生态系统碳交换动态及其与环境控制因子的关系

Dynamics of CO_2 exchange and its environmental controls in an urban green-land ecosystem in Fuzhou City

随着城市的不断发展,城市绿地面积逐年增加,绿地生态系统潜在的碳汇和碳源功能显得非常重要。2014年1月至2016年1月,基于涡度技术对福州市绿地生态系统进行了碳通量连续两年的定位观测,并探讨了城市绿地生态系统碳交换年际动态特征及其环境调控因子。结果表明,城市绿地生态系统年总生态系统生产力(GEP)、生态系统净生产力(NEP)和生态系统呼吸(R_e)呈现出一致的变化规律,其中生态系统碳交换特征表现为生长季(4月-11月)以吸收CO_2为主,非生长季以释放CO_2为主。城市绿地R_e均与GPP、GEP呈正比关系,并且拟合线性斜率<1,受生长季的降水量影响较大。采用指数方程对碳交换和土壤温度(T_a)进行模拟,生态系统日平均净碳交换量(NEE_(day))随T_a的增加呈指数增长模型,T_a可解释80%以上的变异;GEP随着T_a的增加逐渐增加,当T_a达到最高时,并未对GEP产生抑制作用;当气温低于5℃时,城市绿地生态系统以呼吸作用为主导,当气温高于5℃,城市绿地生态系统净吸收大气CO_2为主导。光合有效辐射(PAR)与NEE呈直角双曲线关系,当光补偿点PAR小于350μmol·(m^2·d)^(-1)时,NEE为正值,此时生态系统呼吸大于光合生产;当PAR大于500μmol·(m^2·d)^(-1)时,生态系统呼吸与光合生产持平,其碳吸收量基本达到饱和。生态系统最大光合速率、白天平均生态系统呼吸强度、表观量子效率(α)与温度和PAR的季节变化趋势相一致,均在7月最大和5月最小。相关性分析可知,不同月份NEE残差均与降水量呈负相关关系,与饱和水汽压差(VPD)呈正相关关系。上述研究结果为估算、模拟和预测城市绿地生态系统碳交换提供了基础数据支持和理论基础。

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 at- mospheric 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 domi-nating 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 pro ductivity, and the fitting line was less than 1, with the straight intercept not zero, which shows that the bal- ance 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 tempera- ture had no inhibitory effect on the GEP. NEP decreased with increasing T., 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 t, mol （m2 d）-1 , where the ecosystem carbon uptake was saturated. The ecosystem quantum yield （a） and maximum photosynthesis （Am,x） 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 T, 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.