GREENHOUSE GAS EMISSIONS FROM SUGARCANE SOILS AND NITROGEN FERTILISER MANAGEMENT: II By

Emissions of the greenhouse gases carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4) from a sugarcane crop have been measured for the whole of the 2005–2006 growing season of 342 days. The investigation employed chambers and micrometeorological techniques. The ratoon crop, grown on an acid sulfate soil (ASS) at Murwillumbah NSW, received 160 kg urea-N/ha. Rainfall for the period was 1879 mm so that the soil was frequently wet. Soil respiration over the season was 29.8 t/ha of CO2 and the CO2 sequestered from the atmosphere was 51.4 t/ha. Soil respiration thus amounted to 37% of the net assimilation by the crop. Soil moisture conditions were near optimal for N2O production through both nitrification and denitrification and emissions were large and prolonged, totalling 45.9 kgN/ha over the 342 days and persisting at substantial rates for 5 months. Emissions from unfertilised plots totalled 11.3 kgN/ha. The corresponding emission factor for N2O was thus 22% of the fertliliser N applied. The N2O several times higher than those measured for other sugarcane soils. Also in contrast to other sugarcane soils, ASS appear to be sources of CH4, not sinks. The average emission rate over the season was 0.154 kg/ha/d, which is 2 to 10% of rates for rice and wetlands. For the growing season, the net sequestering of CO2 by the crop from the atmosphere of 51.4 t/ha was offset by the emission to the atmosphere of 23 t/ha CO2-e through N2O and CH4.