NITROUS OXIDE EMISSIONS FROM SUGARCANE SOILS AS INFLUENCED BY WATERLOGGING AND SPLIT N FERTILISER APPLICATION

NITROGEN FERTILISER application to sugarcane soils as surface broadcast or subsurface application can lead to loss of N as gas although the amount and forms of gaseous N loss, including nitrous oxide (N2O), remain unclear. We measured N2O emissions from a soil (Hydrosol – Humic gley) over 12 months during 2003–2004 from a third ratoon sugarcane crop (Q155) at Rocky Point, south east Queensland. The N treatments consisted of 0, 100 and 200 kg N/ha, as single sub-surface applications of liquid urea, or split-fertiliser application with sub-surface liquid urea and granular ammonium nitrate applied on the row. Experimental flooding was applied to a subset of 0, 100 and 200 kg N/ha (single application) plots soon after the first application of fertiliser and trash blanketing to observe short-term effect of waterlogging on soil N2O emissions. Soil N2O emissions increased significantly with N fertiliser application rates during wet summer months. The cumulative annual N2O emissions were 2.6, 3.6 and 6.6 kg/ha for 0, 100 and 200 kg N/ha, respectively. Timing and application of N fertiliser as split compared to single fertiliser application had no effect on N2O emissions for 100 kg N/ha treatments; however, lower N2O emissions in split compared to single application of 200kg N/ha fertiliser was observed. The onset of waterlogging to soils increased soil moisture in cane rows for 4–5 weeks compared to non-waterlogged treatments. No effect of waterlogging on N2O emissions was observed in the 100 kg N/ha treatment; however, waterlogging of the 200kg N/ha treatment resulted in an increased pulse of N2O emissions which lasted nearly three weeks. We estimated that the emission of N2O as a percentage of applied fertiliser N varied between 1.2 and 6.7 % thus, indicating that N2O emissions from Australian sugarcane soils may be higher and more variable than the emission factor of 1.25% that is currently applied in the National Greenhouse Gas Inventory (NGGI).