Modern analysis of an ancient integrated farming arrangement: life cycle assessment of a mulberry dyke and pond system

Purpose: Integrated agriculture and aquaculture (IAA), as typified by the mulberry dike-pond system (DPS) of the Pearl River delta of southern China, is often cited as an example of sustainable intensified production due to its characteristic closed loop recycling of nutrients. In this study, we tackle two issues that have been hardly addressed in previous analyses of aquaculture production, greenhouse gas emissions (GHGe) from the pond and the role of labour. Methods: Previous assessments led us to revisit the sustainability of the DPS system as a model for a well-studied IAA system using a life cycle assessment (LCA) methodology. Our study quantifies on-farm CH4 and N2O emissions and indirect emissions embedded in inputs, using the global warming potential (GWP) metric. To model the indirect impact of the high labour requirements of the system, a simple methodology based on metabolizable energy requirements is proposed. Results and discussion: Our GHGe assessment suggests that using fish ponds to treat organic waste results in higher net emissions than alternative waste processing options (e.g. composting), even when the co-production of fish is accounted for. The majority of total system GWP100 (97 %) can be attributed to methane from the fertilised ponds. Food required to meet labour requirements plays an important role, from 11 to 22 % of total environmental impact. Conclusions: Methane from semi-intensive ponds fertilised with organic waste appears to be a significant source of GWP, calling into question the environmental sustainability of IAA systems such as the mulberry DPS. Improving sustainability in such systems will require better understanding of GHGe from waste-fed aquaculture ponds, notably with respect to on-farm N2O and CH4.