Environmental Impacts of Sugarcane Production.

Sugarcane is a preferred crop for biofuel production. It is a C4 plant with low stomatal resistance. Coupled with high air temperatures, high levels of solar radiation and high and frequent rainfall in the tropical and subtropical regions where it is usually grown, these attributes lead to high photosynthetic rates. However, they also lead to high evapotranspiration (ET) rates and high soil biological activity, affecting soil respiration and nitrogen transformations. We report whole of growing season rates of carbon dioxide exchange, ET and emission of the greenhouse gases nitrous oxide and methane from two Australian sugarcane crops. Automatic chambers were employed for measuring soil emissions and micrometeorological techniques for exchanges between crop and atmosphere. As expected, carbon dioxide assimilation and ET were large. Over the growing seasons of 10 to 11 months, net assimilation varied from 60 to 132 t carbon dioxide/ha and ET from 970 to 1280 mm. Nitrous oxide emissions were also large, varying from 5 to 46 kg Nitrous oxide-N/ha and methane emissions varied from virtually zero to 20 kg/ha. Expressed in greenhouse terms, the net sequestration of carbon dioxide from the atmosphere by the two crops varied from 40 to 94 t carbon dioxide/ha, but this was offset by emissions of nitrous oxide and methane varying from 2 to 22 t carbon dioxide equivalents /ha. We conclude that sugarcane production for biofuel entails some costs in terms of water use and greenhouse gas emissions, which can be quite large. A companion paper by Chen et al. will model the main outcomes of the study.