NITROGEN DYNAMICS AND NITROUS OXIDE EMISSIONS IN IMPROVED SUGARCANE FARMING SYSTEMS

THE AUSTRALIAN SUGAR industry is under increasing environmental pressure to improve nitrogen (N) use efficiency. This will require a greater understanding of processes affecting N availability and crop N demand in sugarcane farming systems. A field experiment was established at Mackay to improve understanding of: nitrogen availability in the soil when legume break crops are grown in the farming system; the effect of tillage on N availability; and nitrous oxide emissions from these farming systems to determine appropriate management strategies for reducing greenhouse gas emissions. The experiment was arranged as a split plot with tillage (zonal or no tillage) prior to planting sugarcane as the main plot and fallow management (bare (BF) or soybean fallow (SF)) as the sub-plot. The soybean crop achieved above ground dry biomass of 6.1 t/ha 100 days after planting. Sugarcane received 138 kg N/ha in BF plots and 18 kg N/ha following soybean. High levels of soil mineral N were present at the commencement of the experiment. In BF, mineral N moved down the soil profile prior to sugarcane planting. In SF, some mineral N was captured for soybean crop growth. N2O emissions were significantly greater in BF than SF (1.9 vs. 1.0 kg N2O-N/ha) during the fallow period, most likely due to greater mineral N availability. During the sugarcane cropping period, significantly higher cumulative N2O emissions were observed in BF (2.3 kg N2O-N/ha) than SF (1.5 kg N2O-N/ha), most likely due to differences in mineral N availability following fertilisation. Emissions were low compared to other sugarcane studies. Tillage had no effect on mineral N availability or N2O emissions. This was most likely due to the low level of soil disturbance in the zonal tillage treatment. Cane and sugar yield were similar across farming systems treatments. These results reinforce recommendations for management of legume residues on the soil surface and reduced N fertiliser rates on sugarcane following good legume crops.