Greenhouse gas emission reductions in subtropical cereal-based cropping sequences using legumes, DMPP-coated urea and split timings of urea application

Abstract

To contribute to national greenhouse gas emissions (GHG) reduction targets, grain growers need strategies that minimise emissions associated with grain production. We used life cycle assessments (LCAs) with field-measured production inputs, grain yields and proteins, legume nitrogen (N₂) fixation, and soil nitrous oxide (N₂O) and methane (CH4) emissions, to explore mitigation strategies in 3-year crop sequences in subtropical Australia. The sequences were: canola plus 80 kg/ha fertiliser nitrogen (80N)-wheat 85N-barley 65N (CaNWtNBaN), chickpea 0N-wheat 85N-barley 5N (CpWtNBa), chickpea 0N-wheat 5N-chickpea 5N (CpWtCp), and chickpea 0N-sorghum 45N (CpSgN). We also assessed the impacts of split fertiliser N application and urea coated with DMPP, a nitrification inhibitor, on the LCA for the CaNWtNBaN sequence. Total pre-farm plus on-farm GHG emissions varied between 915 CO₂-e/ha (CpSgN) and 1890 CO₂-e/ha (CaNWtNBaN). Cumulative N₂O emitted over the 3-year study varied between 0.479kg N₂O-N/ha (CpWtCp) and 1.400 kg N₂O-N/ha (CaNWtNBaN), which constituted 24-44% of total GHG emissions. Fertiliser production accounted for 20% (CpSgN) to 30% (CaNWtNBaN) of total emissions. An extra 4.7 kg CO₂-e/ha was emitted for each additional kg N/ha of applied N fertiliser. Three-year CH4 emissions ranged from 1.04 to 0.98 kg CH4-C/ha. Split N and DMPP strategies could reduce total GHG emissions of CaNWtNBaN by 17 and 28% respectively. Results of the study indicate considerable scope for reducing the carbon footprint of subtropical, dryland grains cropping in Australia.