Does Growing Grain Legumes Or Soil Liming Lower Greenhouse Gas Emissions From Wheat Production in a Semi-Arid Climate?.

We investigated if including a grain legume (lupin) in a cropping rotation, or applying lime to an acidic soil, decreased greenhouse gas (GHG) emissions from rain-fed wheat produced in a semi-arid environment. The life cycle assessment (LCA) incorporated in situ GHG flux measurements rather than international default values. Nitrous oxide (N₂O) and methane (CH₄) fluxes were measured for two years on a sub-daily basis using an automated system. The randomized-block design included two cropping rotations (lupin-wheat, wheat-wheat) by two liming treatments (0, 3.5 t ha^-1) by three replicates. The lupin-wheat rotation only received nitrogen (N) fertilizer during the wheat phase (20 kg N ha^-1), while the wheat-wheat received 125 kg N ha^-1 across two growing seasons. Including a grain legume in the rotation did not enhance soil N₂O emissions; total N₂O losses were approximately 0.1 kg N₂O-N ha^-1 after two years for both rotations when averaged across liming treatment. Liming decreased cumulative N₂O emissions from the wheat-wheat rotation by 30% by lowering emissions following summer-autumn rainfall, but had no effect on emissions from the lupin-wheat rotation. Methane uptake was less from the wheat-wheat rotation (601 g CH₄-C ha^-1 after two years) than from either lupin-wheat rotations (967 g CH₄-C ha^-1), however liming the wheat-wheat rotation increased CH₄ uptake (1078 g CH₄-C ha^-1) to a value similar to the lupin-wheat rotation. Including a grain legume in the cropping rotation decreased total GHG emissions from wheat production by 56% on a per hectare basis, and 35% on a per tonne of grain basis, primarily by lowering N fertilizer inputs. Applying lime increased GHG emissions from wheat production by varying amounts depending upon the assumed rate of lime dissolution. Accounting for soil liming in agricultural LCA would be improved by better information about its effect on carbon dioxide (CO₂) emissions and non-CO₂ GHGs.