Establishing long-term nitrogen response of global cereals to assess sustainable fertilizer rates

Insight into the response of cereal yields to nitrogen (N) fertilizer is fundamental to improve nutrient management and policies that can sustain economic crop benefits and food sufficiency with minimum N pollution. Here, we propose a new method to assess long-term (LT) regional sustainable N inputs. The core is a novel scaled response function between normalized yield and total net N input. The function was derived from 25 LT field trials for wheat, maize and barley in Europe, Asia and North America and fitted by a 2nd order polynomial (R2=0.82). Use of response functions derived from common short-term field trials, with soil N not in steady state, gives risks of soil N depletion and N pollution. The scaled LT curve implies that total N input required to attain the maximum yield is independent of this maximum yield as postulated by Mitscherlich in 1924. The unique curve was incorporated in a simple economic model with valuation of externalities of N surplus as a function of regional per-capita GDP. The resulting LT sustainable N inputs range between 150 and 200 kgN/ha and this interval narrows with increasing yield potential and decreasing GDP. The adoption of LT response curves and external costs in cereals may have important implications for policies and application ceilings for N use in regional and global agriculture and ultimately the global distribution of cereal production.