Reaching the ecological targets of the Water Framework Directive will require efforts by agriculture in order to reduce nitrogen and phosphorus loads on surface waters. The question is how much the loads by leaching and run-off from agricultural land need to be reduced, and what is the effectiveness of possible measures.
Article source: Water Matters
The European Water Framework Directive (WFP) is aimed at the sustainable protection of ecosystems and water supplies. Despite a slight improvement, the nutrient concentrations in 2027 will probably still be too high in approximately half of all regional waters3. A reduction of nitrogen and phosphorus loads by leaching and run-off is required in order to achieve the ecological targets. For a realistic assessment of the suitability of measures, insight is required into the sources of nitrogen and phosphorus in the surface water, and into the effectiveness and cost of the various measures.
Reduction targets for regional waterbodies
For regional waters the target for agriculture is defined as a reduction of the nitrogen and phosphorus loads on surface waters from agricultural land required in order to achieve a “good” ecological state in surface waters. This target is derived from the exceedance of the standards for the nitrogen and phosphorus concentration in regional bodies of water. Using data from water and nutrient balances, these exceedances have been translated to the required reduction in the nitrogen and phosphorus load at the outflow and discharge points. Account is thereby taken of retention in the surface water. Retention is a collective term for various dissipation, capture and subsequent delivery processes of nitrogen and phosphorus. Examples are denitrification (nitrogen) and the binding of phosphate to soil particles.
Agriculture’s share in the reduction target: method
The source ‘agriculture’ contributes part of the nutrient load to surface waters. This contribution is calculated by calculating a nutrient balance sheet for each waterbody. Incoming items are: Leaching and run-off from agricultural land and and nature areas, supply from adjacent upstream catchments, supply from point sources and other diffuse sources4. The load passing the outflow point is the outgoing item. Nitrogen and phosphorus disappear by retention, hence the load at the outflow point is lower than the sum of the sources.
The contribution from agricultural land has been calculated using the STONE model which describes crop uptake, nutrient cycles in the soil and leaching processes. Using a sensitivity analysis, contributions from agricultural land are then unravelled into five sub-items: atmospheric deposition on agricultural land; fertiliser; soil processes; drainage of ditch water which is infiltrated during the summer and upward seepage (see figure 1, top left - see further the cited background report). The soil is an important source in peatland areas due to the mineralisation of the peat; in recent reclaimed marine clay areas nutrients can be released from nutrient-rich sediment layers. The source ‘fertiliser’ is defined as the leaching and run-off caused by fertiliser applications, recently and in the past. Only part of the sources listed above can be influenced.
The required reduction in the N and P load has been calculated based on the contribution from the various sources to the nutrient load and the required reduction at the outflow point. After offsetting the influence of retention, the requirement for all diffuse sources and point sources has been calculated.
Agriculture’s share in the reduction target: results
A substantial reduction in emissions will be required in order to meet the targets for nitrogen and phosphorus concentrations in regional surface waters. The sum of the required reductions at all outflow points amounts to 24 million kilograms of N and 2.3 million kilograms of P per year (figure 1, bottom right). Counting back to the load reductions at the sources and taking account of retention in the surface water, a reduction of 28 million kilograms of N and 3.1 million kilograms of P per year is required (figure 1, bottom left).
Which sources constitute ‘agriculture sources’ is a subject to debate. In practice the different points of view results in a range for the reduction to be achieved by agriculture for both nitrogen and phosphorus. For the Netherlands as a whole, the load on surface waters from agricultural land must be reduced by 5.7 - 8.1 million kilograms of nitrogen and 0.38 - 1.19 million kilograms of phosphorus annually2. This is equivalent to 10-20% of the total leaching of nitrogen and 10-40% for phosphorus. Figure 2, in which the upper limit of the ranges has been adopted5, shows that the differences between the areas are substantial.
Effects of measures
The STONE model has been used to explore the effects of a number of measures on nitrate concentrations in the groundwater (leaching) and the nitrogen and phosphorus load on surface waters2:
- Changing the cultivation plan for arable and horticulture crops in the southern sand region: replacing potatoes by a crop with a smaller nitrogen surplus, or: partly replacing a ‘late harvest’ crop by an ‘early harvest’ crop combined with growing a catch crop. This can reduce the nitrate concentration in the groundwater by 9-13 mg per litre and the nutrient load on surface waters by 5-10%. The effect on the leaching and run-off of phosphorus is limited.
- Improving the soil structure. This can be done with measures to restore the soil (breaking the ploughpan), preventative measures (incl. adapted agricultural machinery, increasing the organic matter content by providing compost) and by opting for deep-rooting agricultural crops. This can reduce the nitrate concentration in the groundwater by 7-10 mg per litre and the nutrient load on surface waters by 7-26%. The effect on the leaching and run-off of phosphorus is unclear because this is often determined - more than for nitrogen - by surface run-off and shallow transport routes. These transport routes, which in practice are partly determined by extreme weather conditions, soil characteristics and field relief, are hard to describe in calculation models.
- Improving nutrient utilisation, incl. through better placing and timing of fertiliser and maximum use of catch crops. On average this can result in a reduction in the nitrate concentrations in groundwater of 8-18 mg per litre for arable crops on sandy soil. The nutrient load on surface waters is reduced by 12-23%. The effect is smaller for clay soil, and the effect on phosphorous is unclear for all soil types.
- Modifying the water management of agricultural fields by:
- Installing controllable tile drains in wet soils which have not previously been drained. This leads to an increase of the nitrogen load on surface waters by an average of 33% and a reduction of the phosphorus load of 25%.
- Replacing existing tile drains (fixed level) with new drain tubes with a controllable level leads to a reduction of nitrogen leaching by 27-35%. Phosphorus leaching increases by 9-16%, but it can lead to reduction for some clay soils.
- The use of submerged drain tubes in wet peatlands leads to 24% and 11% reductions of the load on surface waters by nitrogen and phosphorus respectively.
- Installing ironsand-coated drainage pipes in sandy flower bulb fields in the western sandy region leads to a 60 to 90% reduction of the phosphorus leaching.
In order to meet the standards for nitrogen concentrations in regional surface waters by 2027, the load from agricultural land needs to be reduced by more than 40% and sometimes more than 70% in parts of the Southern and Eastern sandy region. The measures considered can result in a reduction of the nitrate leaching to the groundwater ranging from 5-10% to 15-25% in the sandy regions. It appears that it will be possible to meet the reduction targets for a large number of waterbodies by using a combination of the measures, but for the clay and peat areas the measures will only partly contribute to achieving the reduction target.
In order to meet the standards for phosphorus concentrations, the load on surface waters from agricultural land must be reduced by 40 to 70% in the Western Netherlands, the Southern sandy region and Twente, an in some areas by more than 70%. This reduction target cannot be met with the measures examined or a combination thereof. Because a large proportion of the leaching and run-off is determined by phosphate stocks already present in the soil, it can only be minimally influenced. Alongside the measures considered here, measures are needed which tackle transport routes and/or measures with a purifying effect on the surface water.
- Groenendijk et al (2014) Bronnen van diffuse nutriëntenbelasting van het oppervlaktewater (Sources of diffuse nutrient load in the surface water); Evaluatie Meststoffenwet 2012 (Fertiliser Act 2012 evaluation): ex post partial report. Wageningen, Alterra. Report 2328.
- Groenendijk et al (2016) Landbouw en de KRW-opgave voor nutriënten in regionale wateren (Agriculture and the WFD requirement for nutrients in regional waters). Het aandeel van landbouw in de KRW-opgave, de kosten van enkele maatregelen en de effecten ervan op de uit- en afspoeling uit landbouwgronden. (The share of agriculture in the WFD requirement, the costs of several measures and their effects on the leaching and run-off from agricultural land) Wageningen, Alterra. Teport 2749.
- PBL (2015) Waterkwaliteit nu en in de toekomst (Water quality now and in the future). Final report on ex-ante evaluation of the Dutch plans for the Water Framework Directive. The Hague: PBL. PBL publication 1727
- www.emissieregistratie.nl version 2013
- PBL (2017) Evaluatie Meststoffenwet 2016 (Fertiliser Act 2016 evaluation): Synthesis report. The Hague: PBL.
- Van Gerven et al (2009) Retentieschatting van N en P in het oppervlaktewater op verschillende schaalniveaus (Retention estimate of N and P in the surface water at various scale levels). Wageningen, Alterra. Report 1848.