Fertilizing potential of nitrogen in human excreta across diverse countries and food systems

Human excreta represent a significant but underutilized source of nitrogen (N) for agriculture. Few studies have quantified the full fertilizing potential of human excreta across national contexts, or under a range of diverse food systems. This study provides the first systematic assessment of the potential contribution of N in human excreta to current cropland fertilization in a wide range of countries, covering 80% of the global population. Using food supply and demographic data, we estimate N in human excreta for each country and compare it to current cropland total N inputs, including synthetic fertilizers, manure, biological fixation, and atmospheric deposition. Assuming a 100% recovery rate of human excreta to estimate an upper-bound potential, under the current situation, we find that human excreta (37 MtN at the global scale) could provide 15%–30% of current cropland N inputs in most countries, 30%–50% in sub-Saharan Africa, and over 50% in major agricultural importers (e.g. Japan, Korea, Algeria). In contrast, large agricultural exporters with high external inputs, such as Argentina or the United States, show lower potential (5%–10%). For the whole world, the rate is 20%. For most countries, N excreted by humans is also of the same order of magnitude as (and often superior to) livestock manure currently applied to croplands. In addition to an analysis of the current situation, we assess how improved food system efficiency-via more vegetarian diets, reduced losses and increased manure recycling thanks to better coupling of husbandry and crop cultivation-could affect the potential contribution. Under deeply transformative agro-ecological scenarios, the global contribution of human excreta to N fertilization could exceed 30%. This work focuses on maximum potentials; future research should focus on realistic recovery rates (<100%) and adjust our results accordingly.