NITROUS OXIDE EMISSIONS FROM A SUGARCANE SOIL UNDER DIFFERENT FALLOW AND NITROGEN FERTILISER MANAGEMENT REGIMES

NITROUS OXIDE (N2O) EMISSIONS from a sugarcane soil at Mackay were monitored during a fallow period and the subsequent sugarcane cropping year using both automatic and manual gas sampling chambers to determine the effects of legume fallow, N fertiliser application and a nitrification inhibitor (DMPP). Substantial N2O emissions were observed from the bare fallow soil (5.3–6.0 kg N2O-N/ha in seven months), although no nitrogen fertiliser was applied. Growing soybean during the fallow period did not significantly affect N2O emissions but substantially increased the emissions in the months after incorporation of the soybean biomass (2.5 t/ha without harvest of grain). The cumulative N2O emissions from the unfertilised soil amounted to 6.3 kg N2O-N/ha/year and 16.4 kg N2O-N/ha/year during the twelve-month sugarcane cropping following bare fallow and soybean fallow, respectively. Application of N fertiliser (urea) at 150 kg N/ha to the sugarcane grown on the bare-fallowed soil increased N2O emissions by 6–9 kg N2O-N/ha/year. Addition of the nitrification inhibitor in urea reduced N2O emissions by 4.2 kg N2O-N/ha/year from the fertilised soil. Application of fertiliser at 75 kg N/ha to the sugarcane following soybean fallow increased N2O emissions by 4.5–5.4 kg N2O-N/ha/year as compared to the no fertiliser treatment. The high N2O emissions, particularly following soybean residue incorporation and N fertiliser application, demonstrated the need for developing best management strategies to reduce N losses and N2O emissions while increasing N use efficiency by sugarcane crops. Further studies are required to ascertain the efficacy of nitrification inhibitors in other soil and climate conditions and to investigate optimum soybean residue management practices.