Emissions of methane and nitrous oxide from Australian sugarcane soils

Climatic conditions and cultural practices in the sub-tropical and tropical high-rainfall regions in which sugarcane is grown in Australia are conducive to rapid carbon and nitrogen cycling. Previous research has identified substantial exchanges of methane (CH₄) and nitrous oxide (N₂O) between sugarcane soils and the atmosphere. However, that research has been mostly short-term. This paper describes recent work aimed at quantifying exchanges of CH₄ and N₂O from fertilised sugarcane soils over whole growing seasons. Micrometeorological and chamber techniques provided continuous measurements of gas emissions in whole-of-season studies in a burnt-cane crop on an acid sulfate soil (ASS) that was fertilised with 160kg nitrogen (N)ha^-1 as urea in the south of the sugarcane belt (Site 1), and in a crop on a more representative trash-blanketed soil fertilised with 150kg urea-Nha^-1 in the north (Site 2). Site 1 was a strong source of CH₄ with a seasonal emission (over 342 days) of 19.9kg CH₄ha^-1. That rate corresponds to 0.5-5% of those expected from rice and wetlands. The many drains in the region appear to be the main source. The net annual emission of CH₄ at Site 2 over 292 days was essentially zero, which contradicts predictions that trash-blankets on the soil are net CH₄ sinks. Emissions of N₂O from the ASS at Site 1 were extraordinarily large and prolonged, totalling 72.1kgN₂Oha^-1 (45.9kgNha^-1) and persisting at substantial rates for 5 months. The high porosity and frequent wetting with consequent high water filled pore space and the high carbon content of the soil appear to be important drivers of N₂O production. At Site 2, emissions were much smaller, totalling 7.4kgN₂Oha^-1 (4.7kgNha^-1), most of which was emitted in less than 3 months. The emission factors for N₂O (the proportion of fertiliser nitrogen emitted as N₂O-N) were 21% at Site 1 and 2.8% at Site 2. Both factors exceed the default national inventory value of 1.25%. Calculations suggest that annual N₂O production from Australian sugarcane soils is around 3.8ktN₂O, which is about one-half a previous estimate based on short-term measurements, and although ASS constitute only about 4% of Australia's sugarcane soils, they could contribute about 25% of soil emissions of N₂O from sugarcane. The uptake of 50-94tCO₂ha^-1 from the atmosphere by the crops at both sites was offset by emissions of CH₄ and N₂O to the atmosphere amounting to 22tCO₂-eha^-1 at Site 1 and 2tCO₂-eha^-1 at Site 2.