Long Term Carbon Dioxide Flux Measurements In Sanjiang Plain, Northeastern China

Eddy covariance (EC) was used to measure the fluxes of carbon dioxide, water andenergy in different ecosystems during growing season from 2004 to 2006 in sanjiang Plain,China. Measurements were carried out at wetland, rice (reclaimed from wetland) andsoybean sites. We investigated the daily and seasonal variation of the net ecosystemexchange (NEE) and their response to environmental factors. The effects of climate changeon the inter-annual variation in growing season NEE of wetland was been examined. Theeffects of land use change from wetland to cropland on energy and mass fluxes was alsobeen discussed. The main results are as follows:Spectra analysis suggested that the instrument effects including the frequency responseof the sonic anemometer and of IRGA and sensor separation did not obviously damp thehigh-frequency fluctuations. Good agreements of air temperature and air vapor densitymeasured by EC with of that did by routine meteorological sensors, along with CO₂concentration measured by EC with that by chromatography, guarantee the satisfaction ofEC system to flux measurements. There is not significant difference of flux corrected by2D rotation and Planar fit method with the raw flux, due to the relative homogeneous andflat surface layer. As the methods of quality control, the application of stationary test andintegral turbulence test indicated that most low quality data were usually been found in theearly morning and later afternoon hours during which extremely non-stationary conditionswas observed. The estimated flux footprint showed that nearly100ï¼…Dmax (the largestcontribution to the measured flux per unit area) was located in the measuring plot wefocusing on. The night CO2 storage term (Fs) estimated by single height measurementshowed that Fs account for 6ï¼…of nighttime NEE on average and could not be ignoredwhen friction velocity was less than 0.12ms^-1. Comparison of nocturnal NEE measuredwith EC and dark chamber(DC) which was a independent measurement of CO₂ fluxshowed eddy covariance measurements were consistently lower 4％～30ï¼…than DC andthis underestimation was varied with the change of growing stage.The diurnal and seasonal variation of NEE in wetland ecosystem was obviously, withshift maximum mean monthly emission of 0.11, 0.15 and 0.13 mg CO₂ m^-2s^-1in nighttime to maximum uptake of-0.26,-0.43, -0.36 mg CO₂ m^-2s^-1in daytime from June toAugust in 2004 and 2005. The peak daily NEE of -8.0gCm^-2d^-1was observed at July 13 in2004. Daytime NEE were related to the photosynthetically active radiation (PAR), a highvapour press deficit(≥15hPa) corresponding with high temperature was found to reducethe assimilation rate by on average 50ï¼…. Nighttime NEE(nighttime ecosystem respiration)were related to the temperature, which could be described by a simple exponentialfunction(R² varied from 0.52 to 0.6 in different growing seasons) and produced a relativehigh Q₁₀value of from 1.9 to 3.3. A significant relationship between night ecosystemrespiration and water table did not exist. Wohlfahrt model that took account of LAI couldexplain more variation of nighttime ecosystem respiration (R² varied from 0.6 to 0.75 indifferent growing seasons) than simple exponential function and could be used for gapfilling of low quality nighttime data. Combined with the daily NEE of 0.96 gCm^-2d^-1measured by chamber technique during the non growing season, the annual sums of NEEwas-146±41.3, -44±21.9å’Œ5±14g C m^-2a^-1, varied from carbon sink to neutral.The cumulative NEE of rice ecosystem was-351 gC m^-2during growing season of 2005,which reclaimed from wetland, 218 gC m^-2more than that of wetland, however, Thecumulative NEE of soybean ecosystem was only -61gC m^-2, 71 gC m^-2less than that ofwetland, it could be ascribed to 1) high soil respiration of soybean ecosystem and 2) high Ncontent of soybean plant.