Estimation of nighttime ecosystem respiration over a paddy field in China

Abstract. Accurate estimation of terrestrial ecosystem respiration is crucial for developing regional- to global-scale carbon budget databases. This study evaluated nighttime ecosystem respiration under low turbulence conditions at a paddy field in China during the 2004 growing season. Data from turbulent flux with storage change and alternatively from CO₂ concentration profiles measured from the surface to 32 m height were investigated and compared. Conditions were separated into windy and calm using a friction velocity (u_∗) threshold. On calm nights, the vertical gradient of CO₂ concentration was higher near the canopy level and decreased with height. No differences were detected in terms of quantity and seasonality between the eddy covariance-observed nighttime ecosystem respiration (R_e) and the alternatively calculated R_e under calm conditions. Nighttime underestimation of paddy ecosystem respiration was low, even under calm conditions. Under stable atmospheric conditions, nighttime "loss" of CO₂ flux may result mainly from CO₂ being stored in air below the sensor height, and CO₂ drainage loss could be small because advection is small. Because the addition of measurement-height storage change is preferable for reducing nighttime underestimation, u_∗ filtering and low turbulence data elimination are not required for the paddy ecosystem. Alternatively, under low turbulence conditions, nighttime flux can be calculated from concentration profiles, but actual measurement of the nocturnal boundary layer height is very important. For gap-filling of nighttime CO₂ flux data for a paddy ecosystem, development of multiple regression functions based on the crop biomass/leaf area index in association with field water status is preferable to a single regression function using air/soil temperature.