NET ECOSYSTEM CO2 EXCHANGE OF A SUGAR CROP ON THE PLANT GROWING CYCLE

The sugarcane crop (Saccharum officinarum L.) contributes about two thirds of world sugar production. It is a C4 metabolism plant with high capacity for CO2 assimilation. The CO2 assimilation by the canopy of a crop can be determined by measuring the net exchange of CO2 between the ecosystem (vegetation and soil) and the atmosphere (NEE). In this study, we did NEE measurements of a sugarcane crop and compared them with the values reported for other plant species. We also analyzed the relationship of NEE with the rate of evapotranspiration (ET) and the photosynthetically active radiation (PAR) absorbed by the canopy. The crop instantaneous NEE rate was -28.23 mu mol CO2 m(-2) s(-1) (the negative sign indicates that the net flux of CO2 is from the atmosphere into the crop canopy) and was approximately equal to or greater than that reported for several species of grasses and forests. The daily average CO2 retention is the difference between the NEE integrated during the day and night, which was 44.94 kg C ha(-1) d(-1). This was also equal to or greater than the values reported for other species and crops. There was a parabolic relationship between NEE and the ET rate where the maximum NEE rate was 25.92 mu mol m(-2) s(-1) at an ET of 5.477 mu mol m(-2) s(-1). The relationship between NEE and absorbed PAR was similar: the maximum NEE rate was 23.08,mu mol m(-2) s(-1), with an absorbed PAR of 1127.5 mu mol m(-2) s(-1).