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.