Quantifying net ecosystem carbon dioxide exchange of a short-plant cropland with intermittent chamber measurements

[1] An approach for quantifying the net ecosystem exchange (NEE) of carbon dioxide, which is subject to a rectangular hyperbolic relationship between NEE and photosynthetic active radiation, was adapted to a typical wheat-rice rotation ecosystem under a subtropical monsoon climate in the Yangtze River delta. Adaptation schemes were established; these relied upon intermittent measurements and thereby parameterization of photosynthesis, canopy respiration, root respiration, root-to-shoot ratio, and plant growth, using manual chambers and conventional methods. To apply the adapted approach for NEE estimation at daily, seasonal, and annual scales, data from hourly air temperature, hourly photosynthetic active radiation, shoot biomass at maturity, measured soil heterotrophic respiration during the nonflooded growing season, and intermittently observed ecosystem respiration are all required. Indirect verification showed that this approach was capable of yielding seasonal NEE estimates comparable with those of field measurements using meteorological techniques such as eddy covariance (EC). The daily NEE fluxes were calculated for two wheat-rice rotations. Then the total NEE during the rice-growing seasons, the wheat-growing seasons, the nonrice periods of the year, and the entire rotation cycles were respectively estimated as -7.08 to -7.54, -1.49 to -1.58, -0.83 to -0.92, and -7.91 to -8.46 t C ha(-1) in the 2001-2002 rotation, and -7.35 to -7.82, -2.63 to -2.80, -2.17 to -2.33, and -9.51 to -10.15 t C ha(-1) in the 2002-2003 rotation. Slight carbon gains occurred during the rice seasons (-0.14 to -0.62 t C ha(-1)), but obvious carbon losses occurred during the nonrice periods (2.27 to 3.13 t C ha(-1)) and over the entire rotation cycle (1.66 to 2.96 t C ha(-1)). These carbon losses were due to low rates of crop residue incorporation and lack of organic manure application. This study implies that the adapted approach applies for field trials requiring multiple field plots of a short-plant ecosystem. This approach may provide a methodological alternative to fill the measurement gap for quantifying NEE on fragmented terrains at high temporal and spatial resolutions.