Measured CO2 Concentration and Flux at 16 m Height during Corn Growing Period on the North China Plain
在华北玉米生育期观测的16 m高度CO2浓度及通量特征

In order to better understand the global climate change,it is necessary to assess the budget of CO2 exchange as accurately as possible between the biosphere and atmosphere over representative ecosystems.Eddy covariance technique,which has been adopted as a standard method in the global carbon flux observation network(FLUXNET),allows us to directly measure the exchange rate of CO2 across the interface between vegetation canopy and atmosphere.However,the results from different studies are not comparable because they use different interpreting and correcting methods to estimate CO2 flux.In this paper,the authors present the variation of CO2 concentration and flux measured at 16 m height by eddy covariance system over a typical corn canopy surface on the North China Plain.Three methods are used to calculate CO2 flux,which are direct fluctuation covariance,WPL method(Webb et al.,1980),and Liu method(Liu,2005).It is found that the direct estimation by fluctuation covariance contains errors and should be corrected.WPL method,which has been used to correct these errors in recent years,overcorrects the value of negative CO2 flux. Liu method compromises the results between direct estimation and WPL correction,and has the advantage of no any theoretical assumption as that of WPL correction.Therefore the authors consider that the CO2 flux from Liu method is reasonable.Based on Liu method,the authors then determine the net budget of CO2 at measurement height at the scales of day and the whole experiment period,respectively.The main results of this study show as follows.(1) The variation of CO2 concentration can reflect the influences of human's field activities(e.g.tillage and firing the residual) and corn plants growing.Tillage and firing the residual induce a sharp increase of CO2 concentration,while corn growth results in the vale value of CO2 concentration in U form.(2) CO2 concentration holds diurnal cycle during the whole period of measurement,but CO2 flux has not visible diurnal cycle until the corn canopy develops to some degree.The peak(vale) value of CO2 concentration usually appears at sunrise(sunset),related to the altering direction of CO2 flux.The vale value of CO2 flux in the daytime is more distinct than the peak value in nighttime.(3) The maximum carbon taking by corn canopy occurs at the abloom stage,followed by spin-ripe stage and jointing stage.(4) Corn surface serves as carbon sink to atmosphere from young canopy stage to mature stage,but serves as a carbon source at other periods.The net exchange of CO2 flux at the measurement height over the whole period is-176 g/m2(C).(5)The analysis of ecosystem carbon balance indicates a weak sink to the atmosphere(about 660 kg/hm2(C)).Because of omitting the carbon storage in the atmosphere under the measurement height,the true carbon sink may be stronger than this value.It is noted that the different methods mentioned above might bring as large as 160% bias to the net CO2 exchange estimations in th