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Static and transparent automatic chamber (AC) technique is a necessary choicefor measuring net ecosystem exchange (NEE) of carbon dioxide (CO2) incircumstances where eddy covariance (EC) technique is not applicable.However, a comparison of the two techniques for measurements on croplands hasseldom been undertaken. We carried out NEE observations in a cotton field(for one year) and a winter wheat field (for one cropping season) using bothAC and EC techniques, to (a) compare the NEE fluxes measured using eachtechnique, and (b) test the NEE measurement performance of an automaticchamber system (AMEG), which was designed for simultaneous flux measurementsof multiple gases. The half-hourly NEE fluxes measured with the twotechniques were in approximate agreement, with the AC fluxes being 0.78(cotton) and 1.06 (wheat) times the size of the EC fluxes. When integrated todaily timescale, the fluxes of the two techniques were in better agreement,showing an average ratio of 0.94 and 1.00 for the cotton and wheat,respectively. During the periods with comparable field conditions and normalperformance of both instruments, the cumulative NEE fluxes revealed smalldifferences between the two techniques (−9.0% ~ 7%, with amean of 0.1%). The measurements resulted in an annual cumulative NEE of−40 g C m−2 yr−1 (EC) and −42 g C m−2 yr−1(AC) in the cotton field, and a seasonal cumulative NEE of−251 g C m−2 (EC) and −205 g C m−2 (AC) in the wheatfield. Our results indicate that, for cropland populated by short plants, theAMEG system and the data processing procedures applied in this study are ableto provide NEE estimates comparable to those from EC measurements

Comparison between eddy covariance and automatic chamber techniques for measuring net ecosystem exchange of carbon dioxide in cotton and wheat fields