Assessment of the needed increase in nitrogen use efficiency in European agricultural soils in view of water quality

Wim de Vries1,2 and Hans Kros2

1Wageningen University and Research, Environmental Systems Analysis Group, PO Box 47, 6700 AA Wageningen, the Netherlands
2Wageningen University and Research, Environmental Research (Alterra), PO Box 47, 6700 AA Wageningen, the Netherlands

Background: The intensification of European agriculture, including large inputs of nitrogen (N) to soil by fertilizers and manure, has led to an increase in crop growth but also to adverse effects on the environment. In several regions in Europe, high N inputs have led to: (i) eutrophication of surface waters due to increased N runoff, (ii) increased nitrate (NO3) levels in drinking water reservoirs due to elevated NO3 leaching and (iii) (i) loss of terrestrial biodiversity due to increased emission and deposition of ammonia (NH3). Inversely, in other regions there is still room for increasing N inputs without exceeding critical thresholds for N losses. When current N inputs exceed critical N inputs, environmental objectives can only be reached at lower N input, which likely would cause a loss in crop production, unless the nutrient use efficiency (NUE) is increased. When the NUE is increased, the current N input can be lowered, since the same crop yield can be reached with less N fertilizer, due to an enhanced N uptake fraction, while the critical N input increases since a lower fraction of N is lost to the environment. The NUE increase that is required to attain the current crop production while protecting surface water was assessed in a European wide study. The approach is relevant in view of discussion on the use of planetary N boundaries and the need for downscaling those boundaries at the regional level and country level.

Methods: We calculated critical N inputs and their exceedances (current N inputs minus critical N inputs) for agricultural soils in the EU-27 region in view of N runoff to surface water and related effects on aquatic ecosystems. In addition, critical N inputs were calculated in view of critical NO3 leaching to ground water and critical NH3 emissions to air, respectively. The derivation of critical N inputs in view of adverse environmental impacts, consisted of three consecutive steps, i.e.: (i) identification of critical values for defined N indicators, (ii) back-calculation of critical N losses to surface water or air that correspond to critical values for N indicators and (iii) back-calculation of critical N inputs from critical N losses. The INTEGRATOR model was used to calculate current and critical N inputs at EU27 level. The calculated spatially explicit critical N inputs in view of losses to air and water were compared with current N inputs (the year 2010). For areas where critical N inputs were below current N inputs, we calculated the needed increase in NUE to attain environmental objectives at current crop yields.

Results: The calculations at EU-27 level showed that the critical N inputs were approximately 20% lower than current (year 2010) N inputs, using either critical N concentration in surface water or critical N deposition as criterion. The calculated NUE values that are needed to attain the current crop yield while not exceeding critical environmental thresholds nearly always ranged between 50 and 90%. In several regions, the current N input is much higher than the critical N input and vice versa. There is thus a clear need for a spatial reallocation of the N inputs to N deficient regions and an increase in NUE in highly productive regions to avoid environmental impacts, such as eutrophication of surface waters, while maintaining crop production.