Recirculation of human-derived nutrients from cities to agriculture across six continents

Recovering human-derived nutrients can advance circular economies by linking increasingly urban global populations with local cropland, offsetting unsustainable fertilizer use and improving access in low-income countries. For 56 of the world's largest cities, we analyse co-location of urban nutrients with surrounding agricultural needs (that is, the degree to which recoverable nutrients spatially align with crop demands), defining paths forward to close urban nutrient cycles. Estimated nutrient transport distances, which may constrain what recovery strategies are locally feasible, span two orders of magnitude and are often shorter among European, African and Asian cities due to high local cropland density. We further examine how growing nutrient-intensive crops and recovering highly concentrated nutrient products could impact distance and energy requirements. Broadly, locations with high cropland density, nutrient-intensive crops and compact urban area may find agricultural nutrient reuse particularly impactful and achievable, creating opportunities to boost productivity by coupling urban water and regional agriculture systems.