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 NO3l -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.

net greenhouse gas impact of nitrogen has been quantified globally (Butterbach-Bahl et al., 2011;Erisman et al., 2011), but information on its spatial variation is lacking. Objectives and outline of this thesisThis thesis aims to provide information on policy-relevant nitrogen indicators that capture trade-offs between nitrogen's adverse impacts and benefits, to support policies on sustainable nitrogen management in Europe and globally. This objective is divided into three subobjectives:Objective 1: To estimate spatially explicit critical agricultural nitrogen losses, surpluses and inputs related to targets for air and water quality, and aggregate spatially explicit thresholds to derive safe nitrogen boundaries at the regional, continental, and global level. (Chapters 2 and 4)Objective 2: To assess to what degree redistributing nitrogen inputs and increasing nitrogen use efficiency can contribute to meeting food demand while remaining within safe boundaries for nitrogen losses to air and water. (Chapters 3 and 4)