Nutrient substitution for secondary fertilizer: Is current practice comprehensive enough? A review to reveal the LCA methodological challenges

Purpose: As LCA is widely applied for benchmarking and decision-making, the need to clarify the ambiguity within fundamental methodological issues is imperative. Nutrient substitution, a subcategory of substitution, where credits are given for secondary fertilizer, is one of the common means to solve multi-functionality in LCA studies. This review aims to unravel the unique challenges associated with nutrient substitution, given the increasing relevance attributed to this topic. Methods: A systematic review of LCA studies available in Scopus and Web of Science (WoS) has been conducted. Studies about the recovery of nutrients from waste streams to produce fertilizer were scrutinized. As this review focuses on nutrient substitution methodology, only studies applying substitution for secondary fertilizer were included. PRISMA checklist has been used for reporting and completeness check of the review. Results are demonstrated from system modeling and explicit substitution procedure perspectives, supplemented by an investigation on sensitivity analysis. Results and discussion: As a general caveat, poor documentation and low transparency have been observed. Substitution has been used to model attributional (ALCA) and consequential LCA (CLCA) systems. The choice of functional unit combined with nutrient substitution in ALCA could attribute impacts to other functions than those studied. The determination of system boundary, especially the incorporation of the Use on Land (UoL) stage and avoided UoL emissions, is not always in accordance with the selected system modeling. Furthermore, there is no consensus on calculating the nutrient substitution rate. Single and aggregated factors comprising internal product quality, external-environmental, and external-societal variables have been identified. A prevalent observation among most studies is the absence of a sensitivity analysis pertaining to the nutrient substitution rate. Conclusion: The consistency of nutrient substitution cannot be achieved without an unambiguous definition and connotation of substitution and system modeling. The exclusion of the UoL phase not only limits the scope of a study but also fails to reflect quality differences between primary and secondary products. The key lies in elevating awareness regarding the intricacies of nutrient substitution, which consequently necessitates a rigorous definition and integration of influential factors when calculating substitutability.