Figure 3.

Schematic of the N fluxes linked to agriculture in the Seine (upper panels) and Ebro (lower panels) watersheds used as examples. (a) Description of the current situation based on data from [6–8]. (b) Construction of a hypothetical organic and demitarian scenario for both basins, based on the following constraints: (i) human consumption of animal protein is set to 35–40% of the total current intake of proteins; (ii) no synthetic fertilizers are allowed; (iii) livestock is adjusted to the local requirements of animal proteins as far as the local vegetal protein production is able to satisfy the requirements of both livestock and human populations (no import of feed is allowed) and (iv) nitrate concentration of arable land subroot water is kept below the drinking water standard (11 mg N l⁻¹). The crop production of arable land and the N soil surplus are calculated from the total N soil inputs by manure, symbiotic N₂ fixation and atmospheric deposition, using the same relationship as observed in the current situation of each watershed (figure 5). See the electronic supplementary material, §S4 for further examples.