NITROUS OXIDE EMISSIONS FROM SUGARCANE SOILS: EFFECTS OF UREA FORMS AND APPLICATION RATE

THE COMBINATION of high nitrogen (N) fertiliser application rates and cane residue retention with the wet and warm climate in the Australian sugarcane regions provides favourable conditions for the production of nitrous oxide (N2O). Using manual gas sampling chambers, we measured N2O emissions from different N fertiliser and trash management practices on two sugarcane soils throughout the entire cropping seasons. One of the trials was near Murwillumbah NSW where the cane was burnt before harvest each year; the other site was located near Mackay where green cane trash blanketing was practiced. The annual cumulative N2O emissions from the farmers’ fertilisation practices of 160 and 150 kg N/ha for the Murwillumbah and Mackay sites respectively were 44 kg N2O/ha and 5.7 kg N2O/ha, with the emission factors (% of fertiliser N emitted as N2O-N) being 10.0% and 1.32%. Polymer coated urea reduced annual N2O emission by 30% on the Murwillumbah site but increased it by 50% on the Mackay site. Although addition of nitrification inhibitor (3,4- dimethylpyrozole phosphate, DMPP) to urea reduced N2O emissions from the fertilised band, the plot-scale emissions were not significantly different between the tretaments with or without DMPP at the Murwillumbah site (P=0.05). No significant effect was observed for the nitrification inhibitor at the Mackay site. Removal of trash from the soil surface decreased N2O emissions by 30% and 24% respectively from the Murwillumbah and Mackay soils, showing the importance of organic carbon in regulating N2O fluxes.