The nitrogen metabolism of growing sub-Saharan cities and their prospect for shifting from regional sinks to sustainable city region food systems

Urban development trajectories in sub-Saharan Africa are unsustainable. Fast-growing cities constitute nutrient sinks relying on nutrient-poor hinterlands. We propose a pragmatic waste flow assessment providing authorities with a cross-sectoral view of a city's nutrient sink status. Following a nested approach, we focus on the origin and fate of a city's nutrient-containing waste flows, constituting a partial urban metabolism. Application of this method for nitrogen to Maradi, Niger, and Ouagadougou, Burkina Faso, shows that the city of Maradi is a nitrogen sink in a still rather sustainable city region food system. Maradi's Territorial Sustenance index, expressing net N provision from within the territorial system as a share of urban throughput, is around 79%. But Maradi may well be set to evolve towards a situation similar to that of Ouagadougou: a large nitrogen sink with no significant city-hinterland recycling. Ouagadougou exhibits a Territorial Sustenance index of about 5%. Urination constitutes the dominant nitrogen loss pathway and urine-collecting initiatives could provide valuable fertilizer adapted to local agricultural requirements and constraints, increasing the urban system's sustainability by enhancing regional food provision as well as by reducing sanitation-induced urban water pollution.