CROP NET CARBON-DIOXIDE EXCHANGE-RATE AND RADIATION USE EFFICIENCY IN SOYBEAN

Plant growth is directly related to the fraction of the C fixed by photosynthesis that is converted into phytomass. We report here the results of a study aiming at the estimation of the net CO2 exchange rates (CNCER) of a soybean crop over the growing season. Hourly values of CNCER were estimated based on eddy correlation measurements of CO2 fluxes above the canopy, on chamber measurements of CO2 fluxes at the soil surface, and on the assumption that root respiration can be computed as a fraction of plant daily photosynthesis. Mean daytime CNCER varied between 0.70 and 0.83 mg m(-2) s(-1) for a leaf area index (LAI) greater than 1, with a maximum hourly value of 1.48 mg m(-2) s(-1). Absolute mean nighttime CNCER values were usually <20% of daytime values. Hourly CNCER varied nonlinearly with intercepted photosynthetically active radiation (IPAR). The accuracy of the CNCER estimates was evaluated by comparing the radiation use efficiency of the soybean crop calculated using CNCER estimates with radiation use efficiency calculated using phytomass accumulation. Agreement between the two approaches was within 10% for LAI > 2, but calculated CNCER overestimated radiation use efficiency based on phytomass accumulation for smaller values of LAT.