A substance flow analysis to assess the potential benefits of livestock based circularity for nutrient use efficiency and carbon return to soils in the agro-food-waste system of a tropical island

CONTEXT: Livestock is often considered as a potential hub of circularity but evidence of circularity benefits for the sustainability of agro-food-waste systems (AFWS) is lacking in the literature. METHODS: This study assessed the consequences of livestock based circularity on nutrient and carbon management in the AFWS of an isolated tropical island. A substance flow analysis of nitrogen (N), phosphorus (P), and carbon (C) cycles was conducted at the level of the AFWS of French Reunion Island. Eight livestock based circularity levers and their combinations, aimed at increasing recycling between sub-sectors, reducing losses at the sub-sector level, and adjusting inputs to needs at the sub-sector level, were compared to the baseline situation using four indicators: (i) N, P and C cycling indexes as circularity indicators, (ii) N and P use efficiency, (iv) N and P self-sufficiency and (iv) humified C inputs to cultivated soils as a proxy of potential additional C storage. RESULTS: Simulating livestock based scenarios improved the cycling index by from +0% to +17% depending on the substance and scenario considered. These increases in AFWS circularity were positively correlated with an increase in AFWS N and P use efficiency, but not necessarily with an increase in the humified C inputs to cultivated soils. The combination of all livestock based scenarios increased the AFWS N, P, and C cycling index by +12%, +17%, and +4% respectively, AFWS N and P use efficiency increased by respectively, +13% and +26%, and humified C inputs to cultivated soils by +6%. The study shows that the benefits of circularity levers differ with the substance and emphasizes the importance of considering multiple substances simultaneously to improve the accuracy of assessments of different management strategies. SIGNIFIANCE: This study provides a reference method to compare circularity strategies within AFWS and to assess their consequences for food system sustainability. In particular, the proposed method can help determine (i) the most effective strategies and (ii) the potential co-benefits or trade-offs between different strategies, depending on the substances considered.