Nitrogen management is the key for low-emission wheat production in Australia: a life cycle perspective

Farm management affects the global greenhouse gas (GHG) budget by changing not only soil organic carbon (SOC) stocks and nitrous oxide (NO) emissions but also other pre-farm, on-farm and off-site emissions. The life cycle assessment (LCA) approach has been widely adopted to assess the "carbon footprint" of agricultural products, but rarely used as a tool to identify effective mitigation strategies. In this study, the global warming impacts of no-till (NT) vs. conventional till (CT), stubble retention (SR) vs. stubble burning (SB), and N fertilization (NF) vs. no N fertilization (N0) in an Australian wheat cropping system were assessed using in situ measurements of NO fluxes over three years, SOC changes over forty years and including other supply chain GHG sources and sinks. The results demonstrated the importance of full GHG accounting compared to considering SOC changes or NO emissions alone for assessing the global warming impacts of different management practices, and highlighted the significance of accurately accounting for SOC changes and NO emissions in LCAs. The GHG footprints of wheat production were on averaged 475kg carbon dioxide equivalent (CO-e) ha (or 186kg CO-e t grain) higher under NF than N0. Where fertilizer N was applied (70kgNha), the life cycle emissions were 200kg CO-e ha (or 87t grain) lower under NT than CT and 364kg CO-e ha (or 155t grain) lower under SR than SB. Classification of the emission sources/sinks and re-calculation of published data indicated that under the common practices of SR combined with NT, N-related GHG emissions contributed 60-95% of the life cycle emissions in the predominantly rain-fed wheat production systems in Australia. Therefore, future mitigation efforts should aim to improve N use efficiency, explore non-synthetic N sources, and most importantly avoid excessive N fertilizer use whilst practising NT and SR