Water, carbon and nitrogen fluxes from a tropical crop sugarcane

The high soil moisture regimes, high soil temperatures and high levels of available carbon that characterise Australian sugarcane culture can be expected to promote high evaporation rates and intensify the normal processes of nitrogen and carbon cycling in sugarcane soils that lead to greenhouse gas production. This has been demonstrated by previous work, but evaporation rates and rates of emissions of greenhouse gases for sugarcane that have been reported so far are either estimates or are based on short–term measurements that cover only a few days and are not representative of the whole of the growing season. This paper reports micrometeorological studies of the exchanges of solar radiation, heat and water vapour and the greenhouse gases CO2, CH4 and N2O, made over the complete growing seasons of two sugarcane crops. One is a 2nd ratoon crop growing on an acid-sulfate soil (ASS) in a sub-tropical region in the south of the Australian sugarcane belt, where the practice is to burn the foliage of the crop immediately before harvest. The other is a crop of 5th ratoon trash-blanketed cane in the tropical north of the belt, where the soil and cultural practices are considered to be more representative of the industry. The trash-blanket comprises the foliage stripped from the crop during harvesting and left as a mulch on the surface of the ground. Both crops were fertilised with 160kg ureaN/ha. The growing season for the southern crop was 342 days and that for the northern crop 292 days. The measurements included eddy covariance measurements of evaporation and CO2 exchange in the air layer above the crop and above-crop flux-gradient determinations of fluxes of CH4 and N2O. Emissions of CO2, CH4 and N2O from the ground surface were made with both manual, static chambers and dynamic, closed, automatic chambers. Evaporation rates from both crops were similar, averaging around 2mm/d for their growing seasons. Both crops sequestered similarly large amounts of CO2 from the atmosphere, 52t/ha for the southern crop and 56t/ha for the northern crop, but there were different contributions from “soil” respiration, 28t/ha in the south and 10t/ha in the north. Emissions of N2O from the southern crop were remarkable at 46 kgN or 22 t CO2-e per hectare. Those from the northern crop were closer to expectation at 5 kgN or 2 t CO2-e per hectare. CH4 emissions from the southern crop were unusually large also, amounting to 52 kg or 1 t CO2-e per hectare. They were virtually zero in the north. The methodologies used and the soil, agronomic and climatic factors controlling the energy and gas emissions will be discussed.