Climate impact of alternative organic fertilizers using life cycle assessment

Abstract Anaerobic digestion is a common method for managing liquid manure and other biomasses, generating biogas as a renewable energy source. The resulting digestate can be processed into organic fertilizers to enhance nutrient recycling, but its environmental impact warrants investigation. In this study, a life cycle assessment was conducted to examine the impact of fertilizers derived from cattle slurry and grass–clover co-digestion on global warming (measured in CO 2 equivalents) compared to untreated cattle slurry (CA). The different treatments analyzed include CA, digestate, liquid fractions (LFs) from digestate separation, and an enriched liquid nitrogen–sulfur product derived from post-processing of biogas and drying of the solid fraction. The functional units of this study were 100 kg of total nitrogen in the final organic fertilizer (FU1) with the cradle-to-processing gate boundary, and the harvesting of 1 ton of spring barley dry matter (FU2) with the cradle-to-field application boundary. The carbon footprint ranged from 24% to 49% of the baseline scenario for FU1, and from −6% to 177% of the baseline scenario for FU2. The main contributors to the carbon footprint of fertilizers included greenhouse gas emissions from storage and field application. However, biogas production from anaerobic digestion, together with the concurrent mitigation of CH 4 emissions during storage, contributed most to a reduction in the overall global warming potential associated with anaerobic digestate and its LF. This study showed large climate prospects in replacing untreated slurry as organic fertilizer with alternatives resulting from its anaerobic digestion and post-treatment.