Nitrogen fertiliser: the fallen hero of agriculture

Spraying fertiliser on oilseed rape near Barton Grange, North Lincolnshire,  UK. Image by David Wright - geograph.org.uk (CC BY-SA 2.0)

Spraying fertiliser on oilseed rape near Barton Grange, North Lincolnshire, UK. Image by David Wright – geograph.org.uk (CC BY-SA 2.0)

In many ways, the development of nitrogen fertilisers was a big success story for agriculture. All plants need nitrogen to grow, but most can only absorb it from the soil in the form of ammonia or nitrates. Unfortunately, there are limited amounts of ammonia and nitrates in soil, which limits plant growth and poses a challenge for agriculture. Animal dung and plant waste are naturally rich in ammonia and nitrates and have been used for centuries to promote plant growth, but it was the development of an industrial process to manufacture ammonia in the early 20th century that revolutionised agriculture. Synthetic nitrogen fertilisers are now widely used in all but the World’s poorest countries, and are estimated to have increased crop yields by 35-50% since the 1940s (1).

However, nitrogen fertilisers have some major drawbacks. When applied to fields, only about 50% of the nitrogen in fertiliser is actually absorbed by plants (2). The excess can be washed out of the soil into rivers and lakes where it promotes the growth of algae, leading to algal blooms that can kill other life forms by blocking out light and lowering the oxygen content of the water. Also, excess nitrogen fertiliser in soil can be converted into nitrous oxide by some bacteria. Nitrous oxide is a very potent and long-lived greenhouse gas, and worryingly, the levels of it in the atmosphere are rising. Agriculture is a major contributor to the increase and accounts for over 80% of total nitrous oxide emissions (3).

The production of nitrogen fertiliser is also environmentally damaging. Industrial production of ammonia requires lots of energy, accounting for around 50% of the total fossil fuels burned in agriculture (3). Burning fossil fuels produces the infamous greenhouse gas carbon dioxide, so all in all, the production and use of nitrogen fertilisers is pretty disastrous for the environment.

Even if we put aside the environmental concerns, it is not always practical to use nitrogen fertilisers to improve crop yields. The cost of producing and transporting nitrogen fertilisers puts them out of reach of many farmers in developing countries, especially in many African nations. Many of these countries are experiencing rapid population growth, so increases in crop yields are sorely needed. Fertilisers are set to become even more expensive as fossil fuels are depleted, so farmers in these countries need alternative, more affordable solutions to supply nitrogen to their crops.

In the efforts to develop less polluting, and cheaper ways to supply nitrogen to crops some advances have already been made. Slow- and controlled-release fertilisers release ammonia and nitrate into the soil more gradually over a period of weeks or months. Using these, together with better practices for fertiliser application to fields, can help reduce the amount of excess nitrogen in the soil that could be washed away or converted to nitrous oxide.

Nitrogen fixing bacteria live within these nodules (pink bumps) on the root of Medicago italica. Image by Ninjatacoshell (CC BY-SA 3.0)

Nitrogen fixing bacteria live within these nodules (pink bumps) on the roots of the legume Medicago italica. Image by Ninjatacoshell (CC BY-SA 3.0)

However, there is a much more elegant solution to the nitrogen problem in nature. Many members of the legume family of plants—including peas, beans and clover—can team up with bacteria from the soil that can produce ammonia from nitrogen gas. The bacteria live within the roots of the plant and provide the plant with ammonia in exchange for sugars. The plant controls the relationship so the amount of ammonia available roughly matches its needs throughout its lifespan. The relationships can be so successful that excess ammonia can be released into the soil and many legumes have been used in traditional farming practises to increase soil fertility for other crops.

Since legumes have such a neat solution to accessing nitrogen it is not surprising that scientists are researching ways to use it in agriculture. Large-scale cultivation of ammonia-producing bacteria could be a less energy-intensive and cheaper way to produce ammonia than the current industrial process. Since ammonia is produced most efficiently within the host legume plant, mimicking this environment in an industrial setting is likely to be key. Another possibility would be the genetic-modification of non-legume crops—such as wheat or rice—with genes from legumes to enable them to be able to form relationships with the bacteria. This would be no simple task, but if it is achieved, it would be an environmentally friendly and sustainable way to provide nitrogen to crops. Also, it would be relatively easy to distribute the seeds of genetically-modified crop varieties to farmers in poorer countries so they could benefit too.

It is unlikely that the nitrogen problem will be solved with a “one size fits all” technology. Instead, the combination of new technologies with improved farming practises is likely to be much more successful at providing nitrogen to our crops while reducing the impact of agriculture on the environment.

References:

  1. Smil, V. (1991). Population growth and nitrogen: an exploration of a critical existential link. Population and Development Review. 17:569-601.)
  2. Matthews, E. and A. Hammond (1999). Critical consumption trends and implications degrading Earths ecosystems. World Resources Institute.
  3. UK Observatory monitoring framework – indicator data sheet. https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/285188/agindicator-dd2-27feb14.pdf

FAO (2011) http://www.fao.org/docrep/014/i2456e/i2456e00.pdf

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7 thoughts on “Nitrogen fertiliser: the fallen hero of agriculture

  1. Pingback: Morsels For The Mind – 16/01/2015 › Six Incredible Things Before Breakfast

  2. Are you aware of any work in respect of Nitrogen and the impact on soil flora and fauna. It is my recollection that artifical N cause bacteria to increase and fungi to decrease, but can’t find anything about this.

  3. Hi Neil,
    I’m not an expert on soil microbes but I have been doing a bit of research. The effects of fertiliser on soil microbes has not been as well studied as the effects on plants. Excess nitrogen in soil promotes the activities of denitrifying bacteria, which convert the ammonia back into nitrogen gas so I would imagine that these species might be more common in high nitrogen soils but I don’t know if this has been proven.

    It seems that nitrogen fertiliser can change the composition of species of bacteria and archaea in the soil (reference: only read abstract as don’t have access to full text http://www.ncbi.nlm.nih.gov/pubmed/18764871). Phosphorous, which present in most artificial fertilisers alongside nitrogen, seems lower the amount of symbioses between arbuscular mycorrhizal fungi and plants in soil (Ref: again only read abstract http://www.ncbi.nlm.nih.gov/pubmed/25163116). I would imagine that this could affect the types of fungi present in the soil.

    I hope this helps. I know a few people who study soil microbe populations so I will try to find out more.

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