Science & Technology in Action

7th Edition

Protecting Water Quality – The Nitrates Directive

Department of Agriculture, Food and the Marine

The Nitrates Directive requires EU Member States to identify surface freshwaters and groundwaters, which exceed or could exceed a concentration of 50 mg of nitrate per litre, and also to identify surface waters, which are eutrophic or could become eutrophic, that is, enriched so much that they become polluted.
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Contains the full lesson along with a supporting toolkit, including teachers’ notes.

Lesson excerpt

Nitrogen is a major component of nucleic acids, proteins and vitamins and is essential for life as we know it. However, much of the world's nitrogen is in the form of unreactive atmospheric nitrogen (N2) and remains largely inaccessible to living organisms. Nitrogen fixation involves the conversion of N2 to chemical forms that plants, and consequently animals via the food chain, can use. Nitrogen-fixing bacteria metabolise nitrogen gas to ammonia, which can be subsequently converted to nitrate. These bacteria are found in free-living form in saltwater, freshwater and soil. Some species live symbiotically in the roots of leguminous plants such as clover; the plant receives essential nitrate while the bacteria have access to essential compounds such as carbohydrates. Atmospheric nitrogen is re-generated whendenitrifying bacteria metabolise nitrogen compounds in the soil and in decaying organic matter.

Nitrogen Fertilisers
In the early 20th century the development of the Haber-Bosch process allowed the artificial fixation of nitrogen gas toammonia, which was then oxidised to produce nitrate and eventually artificial nitrate fertilisers. Artificial fertilisers had larger and less variable amounts of available nitrogen than animal excreta or seaweed which farmers had traditionally used as organic fertilisers. The production of artificial fertilisers meant that people no longer had to rely on nature's cycles to slowly replenish a field after grazing or crop production. Land formerly deemed unproductive was now helping sustain a growing world population. Improved mechanisation made fertilisers easier to apply. While artificial fertilisers remained inexpensive they were often applied in larger quantities than growing crops required and often at inappropriate times. Over time, higher fertiliser costs and a growing awareness of the problems associated with excessive fertiliser use have prompted action.

Environmental Impacts of Nitrogen
If organic or chemical nitrogen fertiliser is not taken up by plants it can be lost to the environment. It may be denitrified into nitrogen oxide compounds which can damage crops and contribute to the global greenhouse effect. Nitrogen dioxide is also involved in the formation of ozone in the troposphere. Run-off of nitrogen fertiliser due to heavy rain and leaching of nitrogen downwards through the soil can cause pollution of surface water and groundwater. 

Quiz questions

  1. The Nitrates Directive includes controls on phosphate application to agricultural land. false
  2. Increased cultivation of crops for biofuels has been accompanied by increased nitrate levels in the Mississippi and the Gulf of Mexico true
  3. It is not necessary to regularly review the designation of a site as a nitrate vulnerable zone. false
  4. It is more desirable to deliver a field's projected fertiliser requirement in one large application rather than over several smaller applications. false
  5. Faster nitrate-driven growth in plants is always associated with higher quality food. false
  6. Nutrient run-off is more likely from sloping river banks. true
  7. Fertiliser is best applied in very dry weather true
  8. Red clover is not as good for nitrogen fixation as white clover but it is a favoured flower of many species of bumblebee. true