Restoring nutrient circularity: A review of nutrient stock and flow analyses of local agro-food-waste systems

To reduce environmental issues resulting from excess nutrients, conserve valuable resources and safeguard future food security, natural nutrient cycles in agro-food-waste systems need to be restored. To this end, nutrient stock and flow analyses of the agro-food-waste system can be undertaken. There is currently no standardized method for the systematic analysis of nutrient stocks and flows to support nutrient circularity of those systems at a local scale. This review of 57 studies summarizes the current knowledge on nutrient stock and flow analysis of agro-food-waste systems in local areas and proposes a six-step framework. About a third of the reviewed studies analyzed the complete agro-food-waste system, including crop production, animal production, food and feed processing industry, consumption and waste management. Furthermore, the main focus tends to be on phosphorus (P), and to a lesser extent on nitrogen (N), potassium (K) and (organic) carbon (C). Only a few studies combined the analyses of different nutrients, even though nutrient use efficiency relies on obtaining the optimal stoichiometric balance. The proposed framework for nutrient stock and flow analyses encompasses the inclusion of the complete agro-food-waste system and simultaneous analysis of N, P, K and C to facilitate assessment of the full potential to restore local nutrient circularity. Moreover, the local study area needs to be sufficiently large to include all the subsystems and sufficiently small to facilitate transportation of nutrients. Following this six-step framework, analyses will be able to identify hotspots, based on which effective measures to restore local nutrient circularity can be developed.