Assessment of spatially explicit actual, required and critical nitrogen inputs in EU-27 agriculture

In this study we investigated critical nitrogen (N) inputs and N losses and their exceedances (actual inputs or losses minus critical inputs or losses) for agricultural soils in the EU-27 region in view of adverse effects. Critical N inputs were calculated based on a critical (i) ammonia emission rate related to critical N loads in view of biodiversity protection, (ii) N concentration in runoff to surface water of 2.5 mg N l-1, related to the protection of aquatic ecosystems and (iii) nitrate (NO3-) concentration in leachate to groundwater of 50 mg NO3- l-1, based on the safe limit for drinking water. In addition, we calculated the required N inputs to achieve target crop yields, which were set at 80% of the water-limited yield potential. Calculations were performed with the INTEGRATOR model for ca 40,000 unique soil-slope-climate combinations and then aggregated at NUTS3-, country- or EU-27 level. Results show that critical N inputs at EU-27 level as compared to actual (year 2010) N inputs are 31% lower to protect biodiversity and 43% lower to protect surface water quality. Critical N inputs are most strongly exceeded in regions with high livestock densities, such as Ireland, the Netherlands, Flanders in Belgium, Brittany in France and the Po valley in Italy. Inversely, required N inputs to attain target crop yields at EU-27 level are ca. 40% higher. Especially in Eastern Europe, there is a large potential to increase yields by increasing N fertilization, but this generally requires strongly improved N use efficiency (NUE), since critical N inputs are mostly lower. Using a maximum plausible NUE, the surface water criterion cannot be achieved on 17% and 25% of all agricultural land at actual crop yield and target crop yields, respectively. Similarly, a maximum plausible reduction in NH3 emission fractions also still causes exceedances of critical N loads in view of biodiversity protection. Reducing agricultural production is needed to protect biodiversity and/or water quality in those regions.