Feeding the city: Food consumption and flow of nitrogen, Paris, 1801–1914

The flows of foodstuffs (and the nitrogen they contain) through the city of Paris in the 19th and early 20th century were evaluated. Between 1801 and 1914, the fivefold increase in the population of Paris, as well as the threefold increase in the number of horses used in urban transport, gave rise to increased needs for food and feed. The corresponding inputs of nitrogen increased from 6000 tN/year in 1817 to 25,000 tN/year from the rural hinterland to the city. The corresponding per capita inflows were relatively stable throughout the period and may be divided into four more or less equal parts (flour, meat, other human foodstuffs, forage), each representing about 6 gN per inhabitant per day. In total, the demand for foodstuffs was of the order of 24 gN per inhabitant per day, one quarter of which was for transport. The fate of this dietary nitrogen after consumption changed a lot with the techniques used for exploiting urban excreta of all kinds, particularly of nitrogen, which was in great demand until the development of synthetic fertilizers. Dietary nitrogen flow diagrams are established for the years 1817, 1869 and 1913, and reveal an increasing improvement of the agricultural reuse (from 20 to 40% of the inflowing N).