Characteristics And Effect Factors Of CO₂ Fluxes Variation Of Nanji Wetland Ecosystem In Poyang Lake

At present,under the background of global change,the carbon cycle of wetland ecosystems has become a hotspot subject of scientification investigations.Wetlands affect the fixation and release of greenhouse gases such as CO₂ and CH₄.Therefore,research of carbon fluxes in wetland ecosystems has of great significance.As the largest freshwater lake in China,Poyang Lake plays an important role in regional carbon cycle.Monitoring and research of CO₂ flux patterns in Poyang Lake wetlands and its response to water level variations are among essentials for the region.Based on the eddy covariance technique,this research continuously monitored the CO₂ fluxes of Nanji wetland and its atmospheric interface from April 2015 to September 2017.This research analyzed the meteorological factors measured in the Nanji Wetland Comprehensive Experimental Station,revealed dynamic characteristics of net ecosystem CO₂ exchange of the Nanji wetland under the different time scales,and discussed the affecting factors of the change.The study showed that:The average diurnal variation of every month net ecosystem CO₂ exchange in the Nanji wetland has obviously differences.During the non-flooding period（April-May,August-October,2015,April 2016,September 2016-May 2017,September 2017）,the average diurnal variation of NEE showed roughly a"U shaped"curve,with different peaks and a wide range of diurnal variation.The wetland ecosystem absorbed CO₂during daytime and released CO₂ at night.The net ecosystem CO₂ exchange in this wetland ranged from-17.45⁶.11μmol˙m^-2˙s^-1.During the flooding period（July2015,May-August 2016,June-August 2017）,the average diurnal variation of NEE didn`t show a distinct characteristics,and the fluctuation range was minimum.The net ecosystem CO₂ exchange ranged from-2.89⁹.81μmol˙m^-2˙s^-1.The seasonal variation of net ecosystem CO₂ exchange of Nanji wetland was not significant.However there were significant differences between years.The daily minimum and maximum cumulative values of NEE in the wetland ecosystem were-332.29 g˙m^-2d^-1and 986.38 g˙m^-2d^-11 respectively.There was a significant difference in the monthly cumulative NEE of Nanji wetland,and there was a significant difference in the CO₂ flux in each month.The monthly cumulative NEE varied from-1172.74g˙m^-2˙month^-11 to 18221.80 g˙m^-2˙month^-1.During the flooding period,the monthly cumulative NEE of the wetland all was positive.From the results of existing data,during the observation period,the total amount of NEE in 2015 was-1918.08 g˙m^-2.At this time,the wetland ecosystem absorbed CO₂ and showed as a carbon sink.In 2016,the total amount of NEE was 4646.28g˙m^-2.At this time,the wetland ecosystem released CO₂ to the atmosphere,which represented as a carbon source.In 2017,the total amount of NEE was 4000.58 g˙m^-2.The wetland ecosystem represented as a carbon source.Due to the annual change of the water level in Poyang Lake,the influencing mechanism of its affecting factors is unique from other wetland ecosystems.Research have shown that CO₂ exchange in the net ecosystem of the wetland ecosystem is related to photosynthetic active radiation,air temperature,soil temperature and water level.The correlation coefficient between the net ecosystem CO₂ exchange and photosynthetic active radiation was the largest,indicated that in the day,the variation of net ecosystem CO₂ exchange in the wetland ecosystem was mainly affected by photosynthetic active radiation.