Assessing methods to attribute soil greenhouse gas emissions to a crop in life cycle assessment of cropping systems

There is an increased demand for a reduction in greenhouse gas (GHG) emissions and LCA has been widely applied to agricultural systems to assess GHG. However, no consensus has been found on how to attribute soil GHG emissions in agricultural LCA. In this study, the objectives were: (i) to compare methods of attribution (year period, planting to planting, harvest to harvest), and (ii) to assess advantages and disadvantages of each method when used to attribute CO2 and N2O emissions to a crop in the LCA of cropping system with no winter crops. Soil CO2 and N2O emissions were estimated over 28 years using the biogeochemical DNDC model for 4 different scenarios based on a field experiment in Manitoba, Canada. Model results were used in the Crop.LCA tool to estimate global warming potential (GWP) on ha basis. Results showed no significant differences among methods when considering the full cropping system, however large differences were found on a year basis. Inter-annual variability was found to be higher than the difference across methods. Larger differences among methods were found for the cropping system where residues remained on the field. Thus a multimethod approach is suggested together with a long term LCA assessment to assess this system. However, the choice of methods to employ is a compromise between accuracy and applicability with regards to the LCA objectives.