Agricultural use of organic residues in life cycle assessment: Current practices and proposal for the computation of field emissions and of the nitrogen mineral fertilizer equivalent

Agricultural utilization of organic residues is often included in Life cycle assessment (LCA) studies on livestock and crop production as well as waste and wastewater treatment. A review on the current state-of-the-art practices in agricultural use of organic residues in LCA studies is presented. This reveals that agricultural use of organic residues in LCA studies can be represented in several ways and at different levels of detail. About 100 published references were thoroughly analyzed showing that agricultural use of organic residues usually replaces the use of a mineral fertilizer (substitution of avoided mineral fertilizer). The mineral fertilizer equivalents (MFE) applied are rarely documented, although LCA results can be significantly affected by the way avoided impacts are modeled. Accounting of field emissions from organic residue application varies with the topic of the LCA study. To facilitate nitrogen MFE and field emission calculations, an Excel-tool is proposed for determining the nitrogen MFE of organic residues, direct nitrogen field emissions from organic residue applications, as well as avoided emissions (avoided mineral fertilizers). Computation of the nitrogen MFE of organic residues is based on their nitrogen content and composition, and on nitrogen emissions from field applications of the organic residues. Nitrogen field emissions were estimated using simple models and average climate and soil conditions. A global sensitivity analysis revealed that the choice of the application method, which determines the extent of incorporation into the soil, is the main cause of uncertainty in calculated nitrogen MFE values.