EMISSIONS OF NITROGEN GASES FROM SUGARCANE SOILS

IT IS increasingly evident that emissions of nitrogen gases from agriculture to the atmosphere represent a loss of N to agriculture and can have important environmental effects. The main gases of concern are the direct greenhouse gas nitrous oxide (N2O) and the indirect greenhouse gases nitric oxide (NO) and nitrogen dioxide (NO2), the last two being known collectively as NOx, and ammonia (NH3). Nitrous oxide is a powerful greenhouse gas with a global warming potential 298 times that of CO2. In the atmosphere, the indirect greenhouse gases take part in chemical reactions and eventually, they and their products are deposited on earth where they can undergo nitrification and denitrification with the consequent formation of N2O. Previous work with sugarcane showed that emissions of NH3-N to the atmosphere from surface applications of urea could be as much as 40% of the N applied. New practices that bury the fertiliser can reduce NH3 losses although whole of season reductions have not been quantified previously; nor have those of NOx. This paper reports emissions of N2O, NOx and NH3 from a rain-fed, fertilised, trashblanketed sugarcane soil at Mackay, Queensland. Emissions were measured using near-continuous automatic chamber and micrometeorological techniques for the whole of the 2006–2007 season and for the first 2 months of the 2007–2008 season. The nitrogen fertiliser was mostly urea applied at a rate of 150 kg N/ha into slits 10 to 15 cm deep. Nitrous oxide emissions accounted for around 5 kg N/ha, or 3% of the applied N in the 2006–2007 season, with NOx and NH3 together accounting for around 1.5 kg N/ha, or about 1% of applied N. Allowing for an estimated loss of 20 kg N2-N/ha through denitrification, the net loss would amount to 26.5 kg N/ha or 17 % of the N applied. The main drivers of N2O and NOx emissions appeared to be the availability of a mineral N source and the water content of the near-surface soil. However, NH3 emissions appeared to have a source different from the soil, possibly the trash and the sugarcane foliage. Our emission measurements indicate that only insignificant amounts of N2O are likely to be formed through the deposition pathway in the main sugarcane growing regions and that burying urea fertiliser is very effective in preventing N losses through NH3 volatilisation. Only 0.3% of the fertiliser applied was lost as NH3 over the 2006–2007 growing season.