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.

Nitrous oxide emissions from inland waters: Are IPCC estimates too high?

“…This corresponds to about 3% of all N 2 O emissions and about one third of IPCC's previous estimate of 1.7 Tg N year −1 in the 4th Assessment Report for the same systems. Several studies have highlighted that emissions from rivers might be underestimated (Beaulieu et al, 2011) or significantly overestimated (Hu, Chen, & Dahlgren, 2016;Macdonald, Nadelko, Chang, Glover, & Warneke, 2016) in the IPCC assessments (Table 1). A recent review of estuarine emissions (Murray, Erler, & Eyre, 2015) also suggested that these aquatic systems could emit about three times more N 2 O (0.31 Tg N year −1 ) than the latest IPCC estimate.…”

Tail‐drain sediments are a potential hotspot for nitrous oxide emissions in furrow‐irrigated Vertisols used to grow cotton: A laboratory incubation study

“…Quantifying indirect N 2 O emissions from leaching and runoff requires measurement of NO 3 − –N and N 2 O concentrations in groundwater, surface drainage, rivers, and estuaries, but not direct measurement from transported sediments. Macdonald, Nadelko, Chang, Glover, and Warneke () demonstrated the relatively small scale of indirect N 2 O emissions from furrow‐irrigated cotton paddocks through analysis of water from irrigation channels and storage areas but conceded that further indirect N 2 O emissions (from sediments) would occur as the irrigation furrows, tail drains, and channels dried down (not measured). Fertilized upslope soils that remain uncropped are not specifically included as a source of indirect emissions, but perhaps should be.…”

The current status of nitrogen fertiliser use efficiency and future research directions for the Australian cotton industry

“…4 and 5 indicated that excess N is being applied to the fields in response to a lack of lint production. Unfortunately, the majority of this N is lost via denitrification to the atmosphere, with a portion also lost in surface run-off, transformed and lost as the powerful greenhouse gas nitrous oxide (N 2 O), or lost to deep drainage, potentially affecting groundwater systems (Macdonald et al 2016b). These losses further emphasise the importance of improving NFUE across the industry.…”

“…Recirculation of tail-water is the practice of Australian cotton industry, with moderate concentrations of residual N in supply channels leached from the hills in the fields (Macdonald et al 2016b). As such, N losses can occur from the irrigation network external to the field regardless of whether intentional fertigation is occurring.…”