1. Introduction

An assessment of most promising measures to decrease nitrate leaching on a EU scale was made using the MITERRA-EUROPE model. Two measures were selected based on the results from the »Monitoring & indicators, »Farming practices: review and assessment and »Governance & policy support parts of the FAIRWAY research programme together with questionnaires filled in by case study leaders. The two measures selected for analysis using MITERRA-EUROPE were

• balanced N fertilization, which aims at reducing N fertilizer inputs by adjusting application rates to the N requirement of the crops, thereby decreasing the N surplus.
• growing a cover crop after the main crop, so that the residual mineral N in the soil after harvest of the main crops can be reduced through uptake by the cover crop.

2. Methodology

2.1 MITERRA-EUROPE

The model MITERRA-EUROPE was developed in a service contract for DG Environment (Velthof et al., 2007; 2009). MITERRA-EUROPE is a deterministic emission and nutrient flow model, which calculates greenhouse gas (CO₂, CH₄ and N₂O) emissions, nitrogen emissions (N₂O, NH₃, NOx and NO₃), N and P flows, soil organic carbon stock changes, and soil erosion on an annual basis, using emission factors and leaching fractions. The model was developed to assess the effects and interactions of policies and measures in agriculture on N losses on a NUTS-2 (Nomenclature of Territorial Units for Statistics) level in the EU-28 (Velthof et al., 2009; De Vries et al., 2011). The model was originally based on the models CAPRI (Common Agricultural Policy Regionalised Impact) and GAINS (Greenhouse Gas and Air Pollution Interactions and Synergies), and was later supplemented with a N leaching module, a soil carbon module based on RothC (Merante et al., 2014) and a module for greenhouse gas mitigation measures. In addition, a module for water-induced soil erosion was included based on the Revised Universal Soil Loss Equation (RUSLE) approach (Panagos et al., 2015a; 2015b).

Input data consist of activity data (e.g., livestock numbers, crop areas, and crop yields from CAPRI, Eurostat and FAOSTAT), soil data (LUCAS), climate data (WorldClim), GHG emission factors (IPCC, UNFCCC), and NH₃ emission factors, excretion factors and manure management system data (GAINS, UNFCCC). The N flow calculation module is schematically presented in Figure 2. The N leaching fractions used in MITERRA-EUROPE are presented in the map in Figure 3. The model includes measures to simulate carbon sequestration and mitigation of GHG, NH₃ emissions, and NO₃ leaching.

Figure 2
Figure 3

The MITERRA-Europe model is described in more detail in Velthof et al. (2007; 2009) and Lesschen et al. (2011), and the most recent input data is described in Duan et al. (2021).

2.2 Measures and scenarios

The effect of balanced N fertilization and the growth of cover crops were assessed with MITERRA-EUROPE.

Balanced N fertilization

A detailed description of the balanced N fertilization measure in MITERRA-EUROPE is given in Velthof et al. (2007; 2009). Briefly, the total supply of plant-available N is equal to the total N demand of the crop. The crop N demand is calculated as the total N content of the crop (harvested part + crop residue) times an efficiency factor. Crops are not able to take up all N in the soils, because of limited density of roots in the soil. It is assumed that on average 25% more available N must be present in the soil than the amount of N in the harvested crop and crop residue. This factor differs among crops (different rooting systems) and regions (different soils and growing conditions), but as a first approach one efficiency factor is used. If the amount of plant-available N is higher than the crop demand, less N must be applied in order to achieve balanced N fertilizer application. Only the mineral N fertilizer input is decreased. However, most farmers always will apply some fertilizer and they will not only apply manure (e.g. because they do not have the equipment, manure is not easily available, they are afraid of seeds of weed in manure, cannot apply manure on wet soils with heavy machinery etc.).

Cover crops

Based on the literature survey and meta-analyses described in »Management practices that reduce nitrate transport, an average reduction of N leaching by 45% was assumed. In addition we assume that N losses through surface runoff are reduced by 25%, following Velthof et al. (2009). When the cover crop is incorporated into the soil, the N taken up by the cover crop becomes available for the following main crop. This means that the amount of N supplied by fertilizer applications during the following growing season can be reduced. Hence, we assumed a reduction in N fertilizer application based on the amount N in the cover crops. Data on CN ratios in cover crops are scarce, and show quite high variation, with low CN ratios for leguminous cover crop species and higher ratios for non-legumes. We assumed an average CN ratio of 35, which results in an average uptake of 42 kg N/ha by the cover crop. This amount is added to the crop residues in the model calculations, and part of this N is mineralized over time and becomes available for the following crop. The model calculates the amount of mineral N fertilizer that can be saved as a result. Scenarios were defined in which cover crops were either used in isolation (resulting in reduction of leaching of 45%), or in combination with balanced N fertilization, taking the N release from incorporated crop residue into account.

The level of implementation of cover crops was derived from Eurostat data at NUTS II level from the agri-environmental indicator ‘Soil Cover’, which is based on information from the Farm System Survey (FSS) of 2016. This indicator provides information on the soil cover and distinguishes the following classes: normal winter crop, cover crop, multi-annual plants, plant residues and bare soil. The current cover crop share has been derived from these data (Figure 4). In total, the area under cover crops in 2016 was about 7.6 million ha in the EU-28 (Figure 4). The technical potential of cover crop implementation was set by the area of land that was classified as ‘bare soil’ or ‘plant residues’ in 2016. For the scenarios, we assumed that cover crops could be applied on 40 to 80% of this technical potential (Figure 5), which would increase the total area under cover crops to 16.8 to 33.7 million ha. For some of the NUTS II regions no complete information was available on soil cover in the Eurostat data, this area for which no data was recorded was disregarded during the calculations.

Figure 4
Figure 5

Scenarios

The reference scenario was the year 2017 with cover crops implementation in NUTS II regions according FSS 2016. The following six scenarios with measures were used to calculate the changes in NO₃  leaching, N runoff, and other N emissions with MITERRA, based on the reference year 2016:

1. without cover crop implementation.
2. with a reference cover crop implementation rate FSS 2016 and balanced N fertilization.
3. with a cover crop implementation rate of 40% of the technical potential.
4. with a cover crop implementation rate of 40% of the technical potential and balanced N fertilization.
5. with a cover crop implementation rate of 80% of the technical potential.
6. with a cover crop implementation rate of 80% of the technical potential and balanced N fertilization.

3. Results and discussions

Figure 6 shows a map of the EU of the calculated nitrate concentration in reference year 2016 and for the scenario with balanced N fertilization. Figure 7 shows the EU map with results of N leaching and runoff in kg N per ha per year in the reference year 2016 and the change in leaching compared to the reference for three of the six scenarios. In Table 2, the results of simulations of the measures are summarised on EU-28 level for the main environmental N indicators, including emissions to nitrous oxide (N₂O) and ammonia (NH₃).

Figure 6
Figure 7

Calculated nitrate concentration exceeds the threshold of 50 mg per l in regions with intensive agricultural systems, including the Netherlands and Flanders (Belgium) and regions in Spain and Greece (Figure 6).

Balanced N fertilization strongly reduced nitrate leaching (Figure 7) and improved water quality (Figure 6). On EU scale, N leaching decreased with 22%, N surface runoff by 8%, and N leaching to surface water by 19%. Inputs of N fertilizer were reduced by 13% across the EU when balanced N fertilization was applied. On average, the soil N surplus decreased by 16%. Balanced N fertilization also reduced the emissions of N₂O (5%) and NH₃ (3%) on EU level. Clearly, balanced N fertilization, in which the N application is adapted to the N demand of the main crop is a promising measure to reduce NO₃  leaching to groundwater as well as gaseous N emissions to the atmosphere. This measure requires specific knowledge of the N demand of the main crop (both in total, depending on the expected yield, and over time during the growing season), the N supply by the soil and by applied organic fertilizers, 4R strategies (N application ar Right time, Right place, Right rate, and Right type) that may enhance the efficiency of added N, and unavoidable N losses through denitrification (gaseous losses of N₂ and N₂O) or nitrate leaching during wet periods in the growing season. Farmers can use »Decision Support Tools, soil and plant analyses, and precision farming techniques to implement successful N balance fertilization practices.

The growth of cover crops after the main crop is selected by FAIRWAY's research programme as a most promising measure in both »Farming practices: review and assessment and »Science & policy support. The literature review in »Farming practices: review and assessment showed an average reduction of 45% of NO₃ leaching by cover crops. In addition, the release of N from cover crops after incorporation into the soil can reduce the need for N fertilizer during the following growing season, thereby mitigating the risk of N leaching, and N₂O and NH₃ emissions. Cover crops are already commonly grown in many regions in EU, especially Denmark, the Netherlands, Flanders, and parts of Germany and France (Figures 4 and 5). In these regions, the growth of cover crops is part of the Nitrates Directive action plan to reduce nitrate leaching. In the scenario where the use of cover crops was omitted, the average NO₃ leached to groundwater and N leached to surface water was 2-4 % higher on EU level than in the reference scenario with cover crop 2016 implementation (Table 2).

Increasing the area of cover crops to 40% of the technical potential reduced N leaching to ground and surface water by 2 – 4% on EU level (Table 2). Application of balanced N fertilization in combination with cover crops (at 40% implementation) strongly reduced N leaching; on EU level by 19% for nitrate leaching. Implementation of cover crops to 80% of the technical potential further reduced N leaching, up to 36% for nitrate leaching in combination with balanced N fertilization (Table 2). Model results showed that using a combination of balanced N fertilization and cover crops could lead to large reductions in N leaching and runoff, specifically in Flanders, the Netherlands, and the northern part of Italy (Figure 7). A reduction of more than 20% in N leaching and runoff by implementation of a combination of cover crops and balanced N fertilization could be achieved in many areas in the EU, including Flanders, the Netherlands, parts of Germany, the northern parts of Spain and Portugal, the northern part of Italy, regions in Poland, Czech republic, Croatia, Bulgaria, and Greece (Figure 7).

Additionally, the effect of combining N balanced fertilization and the growth of a cover crop is larger than the sum of the single effects of both measures (e.g., balanced N fertilization reduces total N leaching and runoff with 17%, and the growth of cover crop at 40% implementation reduces total N leaching and runoff with 3%, whereas the combination of both measures results in a reduction of 22%). The synergy of these measures is due to the fact that the combination of both measures accounts for the N supply from incorporated cover crop, by which N fertilizer input can be reduced (in this example 15% with the combination of measures, compared to 13% reduction with only balanced N fertilization).

As a trade-off, implementation of cover crops, may increase N₂O emissions and N losses through denitrification (Table 2). This is due to incorporation of organic C and N, which increases denitrification (the process during which N₂O is produced). This effect has also been reported in review studies (e.g. Basche et al., 2014). However, when the growth of a cover crop is combined with balanced N fertilization, emission of N₂O is reduced (Table 2). This shows that the risk on pollution swapping can be reduced if a combination of measures is taken.  

Table 2. Relative emissions of nitrous oxide (N₂O), and ammonia (NH₃), N leaching to groundwater, N surface runoff, N leaching to surface waters, total N leaching + surface runoff, N surplus on the soil N balance, and N fertilizer use in EU 28 at different measures compared to the reference year 2016 (reference 2016 = 100%).

4. Conclusions

The main conclusions of the assessments of most promising nitrate measures using MITERRA-EUROPE are:

• Balanced N fertilization, in which the N application is tuned to the N demand of the crop, is a promising measure to reduce nitrate leaching to groundwater and gaseous N emission to the atmosphere. Balanced N fertilization reduced nitrate leaching to groundwater on EU scale in 2016 by 22%, N surface runoff by 8%, and N leaching to surface water by 81%. Balanced N fertilization also reduces the emissions of N₂O (5%) and NH₃ (3%).
• Cover crops are already grown in many regions in EU, and especially in Denmark, the Netherlands, Flanders and parts of Germany and France. Omitting cover crops in 2016 resulted in a 3% increase in the nitrate leaching to groundwater and N leaching to surface water across the EU level.
• Increasing the area of cover crops to 40% of the technical potential reduced N leaching to ground and surface water by 3%. Implementation of cover crops to 80% of the technical potential further reduced N leaching (7%).
• Application of balanced N fertilization in combination with cover crops (at 40% implementation) strongly reduced N leaching; on EU level by 19% for nitrate leaching up to 36% for nitrate leaching in combination with balanced N fertilization.
• Reduction of more than 20% in N leaching and runoff by implementation of a combination of cover crops and balanced N fertilization can be achieved in many areas in EU, including Flanders/Belgium, the Netherlands, parts of Germany, the northern parts of Spain and Portugal, the northern part of Italy, regions in Poland, Czech republic, Croatia, Bulgaria, and Greece.
• The reduction of the combination of N balanced fertilization and the growth of a cover crop on N leaching is larger than the sum of the single effects of both measures.
• Cover crops increase N₂O emission. However, when the growth of a cover crop is combined with balanced N fertilization, emission of N₂O is reduced. The risk on pollution swapping can be reduced if a combination of measures is taken.

Note: For full references to papers quoted in this article see

» References