Why mass allocation with representative allocation factor is preferential in LCA when using residual livestock products as organic fertilizers.

The production impact of agricultural residues are generally not accounted for in LCAs when they are given a second life as feedstocks for bio-fuels/plastics/fertilizers. Such is the case for the organic fertilizers manure and blood meal (stemming from livestock) used in organic cultivation. This raises questions on how realistic comparisons are between conventional cultivation systems and organic systems: the flows of resources and associated impacts are not represented equivalently in both systems. The aim of this study is to conduct sensitivity analyses for different possible allocation procedures and to select the most preferable one. The cultivation of organic apples in Flanders is used as case study. Considering no production impacts for organic fertilizers was firstly assessed as it is the generally used approach in LCA. In system expansion, the production impacts of two products: a mineral fertilizer and an organic plant-based fertilizer, are allocated to apple cultivation as a substitute for the organic fertilizers. For mass and economic allocation, the production impact is considered as a fraction of the impacts of the livestock system based on mass flows and economic value, respectively. Several possible allocation factors were assessed and price variations were considered. The different allocation procedures lead to diverging results, underlining the importance of selecting an appropriate procedure. Accounting no production impact is not advised since organic growers do not carry any environmental burden for a product they need for fertilization. System expansion causes too much uncertainties, needing to make speculative scenarios for factors lying outside the studied system. Economic allocation causes the impact of a product to change with its price while production stays the same. Therefore, mass allocation is preferable - though more harmonization research is needed - since no parameters from outside the system are needed, leading to a stable and close approximation of reality. For this case study, the amount of N available in fertilizers as a fraction of live weight and the mass of N in manure, is chosen as the least worse option. Our general recommendation is that the allocation factor needs to be chosen such that it is a representation of the function of the organic fertilizers and is comparable between different fertilizers.