Contribution of the cotton irrigation network to farm nitrous oxide emissions

Nitrous oxide (N2O) is a potent greenhouse gas, and agriculture is the dominant source of N2O-N emissions. The Australian cotton industry requires high inputs of N to maintain high lint quality and yields; however, over-fertilisation with N is symptomatic of the industry. Up to 3.5% of N fertiliser applied is lost directly from cotton fields as N2O gas. Excess N may also be lost via erosion, deep-drainage, leaching and runoff, and may subsequently form indirect N2O emissions. The estimate by the Intergovernmental Panel on Climate Change (IPCC) suggests that 0.0025 kg N2O-N is produced indirectly from groundwater and surface drainage for each kg N lost via runoff and leaching, although this estimate carries a large degree of uncertainty. This study is the first to address the lack of indirect N2O emission data from irrigated cotton-farming systems. Indirect emissions were determined from total N concentrations in irrigation runoff by using the IPCC emission factor and from measurements of dissolved N2O during the first four irrigations (October-December 2013). Total indirect N2O emissions from the surface of the irrigation network over 3 months when estimated by the dissolved-N2O method were 0.503 +/- 0.339 kg ha(-1). By contrast, N2O emissions estimated by the IPCC methodology were 0.843 +/- 0.022 kg ha(-1) irrigation surface area. Over the same period of measurement, direct land-surface emissions were 1.44 kg N2O-N ha(-1) field. Despite relatively high emissions per surface area, the irrigation network is only a minor component of the total farm area, and indirect emissions from the irrigation system contribute similar to 2.4-4% of the total N2O emissions and <0.02% of the applied N fertiliser.