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Improving nitrogen use efficiency

By Carmen Abad - Fertiberia and Bryan Sánchez - Fertiberia Chair

Reconciling agricultural productivity with water and air quality objectives

  • Paper presented by Wageningen University to the International Fertilizer Society at a conference in Cambridge, UK, on 12 December 2019.

In Europe, approximately 60% of the nitrogen applied is absorbed by crops, the rest is lost to the environment. Since the 1990s, Nitrogen Use Efficiency (NUE) has increased from 51% to 60%, but not enough to meet environmental targets.

Nitrogen (N) losses resulting from the use of nitrogenous agricultural inputs affect air and water quality. High N concentrations in runoff water lead to eutrophication of surface water, ammonia (NH3) emissions have a major impact on terrestrial ecosystems, and high nitrate (NO3) concentrations in groundwater affect drinking water quality. These agricultural inputs include mineral nitrogen fertilisers, organic fertilisers from livestock manure and biosolids, as well as biological N fixation.

Globally, there are large differences in the impacts generated by these nitrogen inputs between regions. In many African countries, as well as in large areas of Latin America and Southeast Asia, N inputs are insufficient to maintain soil fertility, which involves risks of soil degradation. On the other hand, many developed and fast-growing economies have large N surpluses. Therefore, in many parts of the world, increased N inputs are needed to avoid soil degradation and increase crop yields, while in others, N application can be reduced while maintaining or even improving yields and reducing environmental impacts. Ultimately, detailed studies are needed that take into account the particular conditions of each case.

In the EU-27 (excluding Croatia and including the UK), models based on N balances are used to assess where the so-called "critical emissions" of N in its different forms (N, NH3 and NO3) are exceeded in relation to the impacts mentioned above. For this purpose, the N inputs required to achieve the target yields and productions and the actual inputs are calculated. When the actual or required inputs exceed the critical inputs, the required reduction and increase in NUE is calculated to achieve the actual or target yields, while simultaneously achieving the air and water quality objectives.

For this study the INTEGRATOR model was used, whereby the following are calculated:

  • Actual N inputs, which is the sum of inputs through mineral fertilisers, manure, biosolids, atmospheric deposition and biological N fixation, while N absorption consists of the N removed from the soil through harvesting.
  • The N inputs required to reach the target crop yield are defined as the maximum yield that can be obtained from a crop with 80% water availability. To calculate these inputs, actual inputs and the ratio of target to actual yield are multiplied, assuming that all additional N required is mineral fertiliser and all other inputs remain constant. N absorption and losses in the required inputs are obtained by scaling the actual absorption and losses with the ratio of target to actual yields. These calculations assume that the NUE of the required inputs is the same as the NUE of the actual inputs.
  • N losses to air and water. Emissions of gaseous N compounds and leaching and runoff to surface water are due to N inputs from faeces and urine in manure storage, free-range grazing, manure and mineral fertiliser application, atmospheric deposition, N fixation and crop residue use. These losses are calculated by multiplying the inputs by emission, leaching or runoff factors.
  • Critical N inputs in relation to environmental impacts, which are those where critical losses to water and air are not exceeded in order to meet environmental objectives. These are obtained by identifying critical values for the defined N indicators in air and water; assessing the critical N losses in air and water, which correspond to the critical values of the identified N indicators; and assessing the critical N inputs and the N fertilisation and excretion rates related to critical N losses. Critical N inputs refer to sources that can be managed by the farmer, while the defined N indicators are critical NH3 emissions determined by critical N deposition in ecosystems, NO3 concentration in leachate to groundwater and N concentration in runoff to surface water.

The results show that EU-27 N inputs are on average 27% lower than the inputs needed to reach the target crop yield. However, for NH3 emissions and N runoff to surface water, the actual inputs are 31% and 43% higher than the critical N inputs, respectively. On the other hand, with regard to NO3 leaching to groundwater, these inputs are 1% below the limit. It is therefore necessary to increase the NUE.

An overall reduction of N inputs by 30% seems to be a reasonable estimate to meet air and water quality objectives while achieving the target yields. However, in the EU-27 there are also large differences between regions. Inputs are far exceeded in areas with high N applications, such as Ireland, the Netherlands, Belgium and Luxembourg, Brittany in France and the Po valley in Italy because of intensive livestock production.

As mentioned above, the actual NUE in Europe is 60%. This value has to increase to 72% to protect surface water quality at actual crop yields and to 74% to achieve target crop yields. It is, therefore, feasible to reduce the environmental impact of agriculture by increasing this factor, while at the same time boosting crop production in Europe.

However, in some areas, due to their special soil and weather characteristics, it would be necessary to increase NUE by 90% to achieve the environmental objectives by reaching the target yield. In these areas, the solution would be to reduce N application, at the expense of reducing crop yields.

Spain

Regarding Spain, this study indicates that in some areas the required N inputs are different from the actual inputs, which causes a clear yield gap (difference between actual and potential yield).

On the other hand, in certain areas NH3 emissions are exceeded because the characteristics of the area lead to low limits, even though it is not a country with a high livestock density like Ireland or part of the UK.

Furthermore, it is necessary to significantly increase the NUE in those areas of Spain where critical runoff to surface water is exceeded to protect their quality.

Ultimately, the management of agricultural inputs with nitrogen in Spain needs to be improved in order to minimise environmental impacts and improve agricultural crop yields. For this, an efficient use of nitrogen-based mineral fertilisers is key, always applying the right fertiliser, in the right quantity and at the right time and place, adapting to the needs of each crop and each specific situation with the help of experts who advise on the way in which fertilisation should be carried out.

References: IFS Proceedings 842, Required changes in nitrogen inputs and nitrogen use efficiencies to reconcile agricultural productivity with water and air quality objectives in the EU-27 by Prof. Wim de Vries and Lena Schulte-Uebbing, Wageningen University.

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