The biogeochemical imprint of human metabolism in Paris Megacity: A regionalized analysis of a water-agro-food system

Megacities are facing a twofold challenge regarding resources: (i) ensure their availability for a growing urban population and (ii) limit the impact of resource losses to the environment. This paper focuses on two essential resources – nitrogen and phosphorus – and challenges their sustainable management in the water-agro-food system of Paris Megacity. An in-depth analysis of the nitrogen and phosphorus imprint of Paris Megacity was conducted, originally centered on human metabolism through consumption and excretion of these two elements. Upstream, the whole agricultural production that feeds Paris Megacity was scrutinized and nitrogen and phosphorus flows in the agro-system were fully documented. Downstream, the analysis of solid waste and wastewater management in Paris Megacity showed the fate of nitrogen and phosphorus imported into the city. Paris Megacity appears to rely on a very complex and international agro-food system, requiring high levels of chemical fertilizers and strongly impacting the environment through nutrient environmental losses. On the other hand, solid waste and wastewater management appears to be mostly disconnected from the agro-food system: even if the release of nitrogen and phosphorus into the environment has largely decreased in recent years, their recycling rate remains very low. This overview of the water-agro-food system of Paris Megacity suggests that an optimal management of nitrogen and phosphorus in the three subsystems (agriculture, waste management and sanitation) should be integrated within a comprehensive approach linking agriculture and urban residues. This analysis thus constitutes a groundwork on which paradigm shift scenarios of the global water-agro-food system could be constructed.