Quantifying the impact of an abrupt reduction in mineral nitrogen fertilization on crop yield in the European Union.

Contemporary crop production in Europe relies on nitrogen (N) fertilization. Fertilizer prices soared in 2021-2022, and remained at historical high levels in 2023. These high prices invoked an immediate concern on the possible consequences for Europe's food production. In this study, we use a biogeochemical model framework to estimate the impact of reducing mineral N fertilization on crop yields in the European Union (EU). First, crop yields simulated with the biogeochemical DayCent model are evaluated against subnational yield data averaged for 2015-2018 reported by Eurostat and National Statistical Institutes in the EU for soft wheat, barley, grain maize and rapeseed. Then, we simulate three different scenarios where mineral N fertilization across the EU is abruptly reduced by respectively 5, 15 and 25 %, and compare yields to the projected baseline for contemporary conditions (2019-2022). The model evaluation gives r values ranging from 0.28 (rapeseed) to 0.61 (soft wheat) and root mean square errors (RMSE) ranging from 0.6 (rapeseed) to 1.95 t ha (maize). The model shows a reduction in yield per crop at the EU level up to 2.1, 6.4 and 11.2 % with the 5, 15 and 25 % reduction scenario, respectively. Different crops show different percentage reduction in yield following a reduction in mineral N fertilization, showing a legacy effect over the years and depending on the availability of organic fertilizer. The strongest relative yield reduction occurs for soft wheat for all three scenarios. Even with 25 % drop in mineral N fertilization, maize yield in the Netherlands, Belgium and Denmark is not significantly reduced, because of the high N surplus and large share of organic fertilization in these countries. This process-based modelling study provides spatially explicit, high resolution information on the response of crop yields to N fertilizer input reductions, helping policy-makers in decision-making on food security and environmentally-friendly food systems.