Localising the nitrogen imprint of the Paris food supply

Biogeosciences Discussions This discussion paper is/has been under review for the journal Biogeosciences (BG). Abstract The Seine watershed has long been the food-supplying hinterland of Paris, providing most of the animal and vegetal protein consumed in the city. Nowadays, because of the land specialisation of agriculture made possible by the shift from manure-based to synthetic nitrogen fertilisation, the Seine watershed, although it exports 80 % of its 5 huge cereal production, still provides most of the cereal consumed by the Paris agglomeration. The meat and milk supply originate, however, mainly from regions in the North and West of France, specialised in animal farming and importing about 30 % of their feed from South America. As it works today, this system is responsible for a severe nitrate contamination of surface groundwater resources. Herein two scenarios of 10 re-localising Paris's food supply are explored, based on organic farming and local provision of animal feed. We show that for the Seine watershed it is technically possible to design an agricultural system able to provide all the plant-and animal-based food required by the population, to deliver sub-root water meeting the drinking water standards and still to export a significant proportion of its production to areas less suitable 15 for cereal cultivation. Decreasing the share of animal products in the human diet has a strong impact on the nitrogen imprint of urban food supply.