Human–excreted nutrient sinkholes and recirculation hotspots worldwide

Despite increasing calls and efforts to reframe human excreta management as part of food and farming systems, these signals have so far not been picked up in most political and academic agendas of befitting communities. This partly results from the fact that human nutrient excretion appears at aggregate levels to be of little significance with respect to agricultural supply and requirements, although recent work on samples of cities or individual cities indicate that the contrary may hold at more local levels. Based on recently published datasets, this study maps 3385 human–excreted nutrient sinkholes worldwide at a 3 × 3 km resolution within which nutrient excretion often largely surpasses nutrient supply to agriculture. Nutrient sinkholes of varying sizes globally accrue a total of 10 Tg yr –1 of nitrogen surplus and 0.8 Tg yr –1 of phosphorus surplus, corresponding respectively to 24% and 18% of human N and P excretion. Subsequent analysis of current nutrient inputs to cropland in areas surrounding these sinkholes preliminarily leads to identifying 467 hotspots of nutrient recirculation potential.