Comparison of nocturnal ecosystem respiration measured with eddy covariance and chamber system at two agricultural sites in Sanjiang plain

Measurements of nocturnal ecosystem respiration were conducted in a rice paddy of Sanjiang from July to September in 2004 and in a soybean field from June to September in 2005,using eddy covariance(EC) and dark chamber-gas chromatography (DC-GC).The differences of simultaneous data measured by two methods were not significant when night turbulent mixing was well.The correlations between two methods can be improved under more large friction velocity.EC measurements were poorly correlated to night ecosystem respiration estimated by chamber models for the variability inherent to EC measurements(R~2=0.06,0.23 for rice and soybean,respectively).However,this correlation can be improved by aggregating measurements over longer time period(R~2=0.16,0.75 for rice and soybean field,respectively).In general,whatever comparison with simultaneous chamber measurements or modeled results,eddy covariance measurements were consistently lower 4%～30% than chambers,and this bias varied with crop growing stage,which were maximum in middle and minimum in early and late growing stage.