Contribution of the cotton irrigation network to farm nitrous oxide emissions

Abstract

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 0.0025kg 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±0.339kgha–1. By contrast, N2O emissions estimated by the IPCC methodology were 0.843±0.022kgha–1 irrigation surface area. Over the same period of measurement, direct land-surface emissions were 1.44kgN2O-Nha–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 ~2.4–4% of the total N2O emissions and <0.02% of the applied N fertiliser.