Harvested food carries a fraction of the nitrogen applied through fertilization; the remainder is typically lost into the environment, impairing planetary sustainability. Using a global agriculture model that integrates key drivers of food production and nitrogen cycling, we simulated upper bounds to global feeding capacity-and associated nitrogen pollution-as a function of nitrogen limitation under organic and industrial fertilization regimes. We found that the current agricultural area could feed similar to 8-20 billion people under unconstrained industrial fertilization and ca. 3-14 billion under organic fertilization. These ranges are inversely correlated with animal proteins in human diets, and are a function of feed-food competition, grassland-to-cropland allocation and-in the case of organic fertilization-nitrogen use efficiency. Improved nitrogen use efficiency is required to bring nitrogen pollution within planetary sustainability limits and is also essential in narrowing down food productivity gaps between organic and industrial fertilization regimes.