Abstract: Wetland ecosystem stores a large amount of carbon in the soil and plays an important role in the terrestrial carbon cycle. Meanwhile,wetland ecosystem,especially wetland in the arid area,is sensitive to climate change and very fragile. Its carbon cycle is easily influenced by environmental factors and human activity. It is significant to study carbon cycle of wetland ecosystem in the arid area. With CO₂ flux observation over a Phragmites-dominated wetland in Zhangye oasis,Gansu province,northwest China(from June,2012 to August,2014), the seasonal change of CO₂ flux and environmental impact factors of the wetland were analyzed first. And combining with fraction of absorbed photosynthetically active radiation(FPAR) from MODIS remote sensing data, light use efficiency(LUE) of the wetland was also estimated. The results showed that there was a distinct seasonal variation of CO₂ flux. The mean gross primary productivity(GPP) over two years was 1.06 kg C·m^-2·a^-1,and the mean net ecosystem exchange(NEE) over two years was -0.40 kg C·m^-2·a^-1. The wetland was apparently a carbon sink. The night-time ecosystem respiration exponentially increased with air temperature and soil temperature, while there was a negative correlation between daytime CO₂ flux values and incident solar radiation,and the relation changed seasonally. The maximum value of LUE was 1.51 g C·MJ^-1 and the average LUE during the growing seasons was 1.07 g C·MJ^-1. LUE was generally related with air temperature,and the value of LUE increased with temperature increasing,which means the higher conversion efficiency of solar energy to dry matter when the air temperature is close to the optimum temperature for vegetation growth.

Key words: wetland, eddy covariance, CO₂ flux, temperature, radiation, light use efficiency, remote sensing