Consequential LCA between NPK fertilizers from microbial, animal, plant, and mineral origin highlights resource constraints and environmental impacts

Since the production and use of mineral fertilizers lead to severe environmental impacts, initiatives such as the European Green Deal promote organic farming and nutrient circularity. However, many organic fertilizers (those containing organic carbon and nutrients of exclusively biological origin) face constraints due to resource availability, and it is paramount to understand their environmental impacts to prevent unforeseen effects. This study employed consequential life cycle assessment (LCA) to compare the environmental impact of mineral fertilizer to three types of organic fertilizers: animal-, plant-, and innovative microbe-based, considering market constraints and marginal suppliers. Organic fertilizers were complemented with mineral products to yield NPK compound fertilizers containing 1 t of nitrogen, 0.44 t of phosphorus, and 0.83 t of potassium (multi-substance functional unit). The findings reveal that many commonly used organic fertilizers currently face constraints, being unable to meet increased demand. Consequently, for the transition to a circular and organic agrifood system, the focus should shift toward untapped resources like microbial fertilizers sourced from food and beverage industry wastewater, and sewage sludge. Constrained markets vary with time and geographical location, emphasizing the importance of context-specific decision-making. Surprisingly, the impact of mineral fertilizers is relatively low across categories, but the gap between them and energy-intensive organic fertilizers narrows in an industrial symbiosis scenario where waste heat is available. The LCA results counterintuitively favoured mineral fertilizers, but an additional assessment of soil organic carbon estimated the contribution of organic fertilizers to the "4 per 1000" initiative's goal for soil carbon storage. While organic fertilizer application alone may rarely achieve this goal, combining it with other management practices like crop rotation offers a promising strategy. One scenario including fertilizer transport showed the potential for reduced environmental impacts by applying locally available by-products as organic fertilizer instead of imported mineral fertilizers. Overall, the results call for caution when transitioning from mineral to organic fertilizers as their availability and environmental impact are not self-evident.