Effects of urea formulations, application rates and crop residue retention on N2O emissions from sugarcane fields in Australia

Abstract

High fertiliser nitrogen (N) application rates, crop residue (trash) retention and the wet and warm climatic conditions in sugarcane (Saccharum officinarum) cropping may favour nitrous oxide (N2O) production in soil. We measured N2O emissions from a silty clay loam with high organic carbon content (98 g kg−1) in northern New South Wales (NSW) and a sandy loam with moderate organic carbon content (16 g kg−1) in central Queensland (QLD), Australia, to quantify whole season emissions under different N management regimes. High N2O emissions occurred mostly in the first 4–6 months following N fertiliser application (October–March) around the summer season. At farmers’ fertiliser application rates (160 or 150 kg N ha−1 as urea), annual N2O emissions reached 28.2 and 3.6 kg N2O–N ha−1, and the emission factors of fertiliser N were 10.0% and 1.32% in NSW and QLD, respectively. Under 80 kg urea–N ha−1, the annual emissions decreased to 23.2 and 2.6 kg N2O–N ha−1 at the NSW and QLD sites, respectively. Emissions of N2O following polymer-coated urea application decreased by ∼31% at the NSW site but increased by 50% at the well-drained QLD site compared to conventional urea. Application of urea coated with the nitrification inhibitor 3,4-dimethylpyrozole phosphate (DMPP) decreased N2O emissions from the fertilised area by ∼36% but this reduction did not translate into statistically significant effects on the plot-scale emissions at the NSW site. DMPP did not significantly reduce annual N2O emissions at the QLD site. Removal of cane trash from the soil surface decreased N2O emissions by 24–30%, demonstrating the promoting effects of trash retention on N2O emissions. We conclude that minimising fertiliser N application rates provides a consistently effective option for reducing N2O emissions from sugarcane farms but the efficacy of polymer- or DMPP-coated urea varies with soil and climatic conditions.