EFFECTS OF UREA FORMULATION ON SUGARCANE YIELD, NITROGEN UPTAKE AND NITROUS OXIDE EMISSION IN TROPICAL QUEENSLAND

THERE IS a genuine need to explore new management practices on sugarcane farms to improve fertiliser nitrogen (N) efficiency and reduce nitrous oxide (N2O, a potent greenhouse gas) emissions while maintaining crop yield. A field experiment was conducted at Ingham in northern Queensland from October 2012 to October 2013 to assess the efficacy of two ‘enhanced efficiency’ nitrogen fertilisers: polymer-coated urea (PCU) and nitrification inhibitor-coated urea (NICU). N2O emissions were measured using both manual and automatic gas sampling chambers. The N release from PCU into soil was considerably slower, which resulted in lower mineral N concentrations in the first 1–2 months after application and maintained higher mineral N levels during the mid to late cropping season than the conventional urea treatments. Lower NO3– contents in soil were recorded in the NICU treatment than the conventional urea treatment in the initial three months. The annual cumulative N2O emissions amounted to 11.4–18.2 kg N/ha with no significant differences between urea forms and between fertiliser application rates (0, 100 and 140 kg N/ha). The similar N2O emissions suggested that N2O production in this soil was mainly driven by other factors such as rainfall rather than soil mineral N concentrations. The urea formulation did not affect sugarcane yield at the same N application rate. Reduction in the fertiliser application rate from the recommended 140 kg N/ha to 100 kg N/ha decreased sugarcane yield for the conventional urea and PCU treatments but not for the NICU treatment. Crop N uptake also decreased with the decreasing N application rate for the conventional urea, but not for PCU and NICU. These results demonstrated that the coated fertilisers may potentially reduce N application rates without causing N deficiency to the crop. However, further studies are required to investigate the optimal management practices such as application time and rate in relation to soil and climatic conditions.