Sugarcane fields: sources or sinks for greenhouse gas emissions?

The quantities of greenhouse gases emitted into the atmosphere from sugarcane fields, and their contribution to the total emissions from Australian agriculture, have never been estimated with any degree of certainty. This review was conducted to collate the available information on greenhouse gas emissions from the Australian sugarcane crop. Estimates were made for the emissions of the 3 major greenhouse gases―carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O)―from known or suspected sources. Sinks for the sequestration of the gases also have been identified. CO2 was found to be emitted during burning of the crop and from trash-blanketed and bare sugarcane fields. Total emissions from these sources in the 1994 season were estimated at 7·6 Mt CO2-C/year. However, the sugarcane crop was identified as a major sink for C, with uptake by the crop in 1994 estimated at 13· 4 Mt CO2-C/year. N2O emanating from sugarcane soils via denitrification following application of fertiliser accounted for 45-78% of total gaseous N emissions. Estimates of N2O emissions from all land under sugarcane in 1994 totalled 4·4 kt N2O-N/year from denitrification with a further 6·3 kt N2O-N emitted from areas that are still burnt. This review suggests changes in management procedures that should limit the opportunities for denitrification in the soil and thus reduce N2O emissions. Methane evolution occurs during the smouldering phase, following burning of the crop, with production estimated at 6·7 kt CH4-C/year in 1994. CH4 oxidation in soil was identified as an important process for removal of atmospheric CH4, as were trash-blanketed soils. Although these figures are our best estimate of gaseous production from sugarcane fields, there still remains a degree of uncertainty due to sampling variability and because of the extrapolation to the entire sugarcane area. However, the coupling of new laser techniques with known micrometeorological methods will allow for a more precise sampling of greenhouse gas emissions over a larger area. Estimates would thus be more representative, resulting in a greater degree of confidence being placed in them by the sugar industry.