SUGARCANE FIELDS - SOURCES OR SINKS FOR GREENHOUSE-GAS EMISSIONS

The quantities of greenhouse gases emitted into the atmosphere from su garcane fields, and their contribution to the total emissions from Aus tralian agriculture, have never been estimated with any degree of cert ainty. This review was conducted to collate the available information on greenhouse gas emissions from the Australian sugarcane crop. Estima tes were made for the emissions of the 3 major greenhouse gases-carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O)-from known or s uspected sources. Sinks for the sequestration of the gases also have b een identified. CO2 was found to be emitted during burning of the crop and from trash-blanketed and bare sugarcane fields. Total emissions f rom these sources in the 1994 season were estimated at 7.6 Mt CO2-C/ye ar. 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 e manating from sugarcane soils via denitrification following applicatio n of fertiliser accounted for 45-78% of total gaseous N emissions. Est imates 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 em itted from areas that are still burnt. This review suggests changes in management procedures that should limit the opportunities for denitri fication in the soil and thus reduce N2O emissions. Methane evolution occurs during the smouldering phase, following burning of the crop, wi th production estimated at 6.7 kt CH4-C/year in 1994. CH4 oxidation in soil was identified as an important process for removal of atmospheri c CH4, as were trash-blanketed soils. Although these figures are our b est estimate of gaseous production from sugarcane fields, there still remains a degree of uncertainty due to sampling variability and becaus e of the extrapolation to the entire sugarcane area. However, the coup ling of new laser techniques with known micrometeorological methods wi ll 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 in dustry.