The NITROGEN CYCLE

Doc Brown's Chemistry KS4 science GCSE/IGCSE/O level Revision Notes

PART E A description of the complexity NITROGEN CYCLE

All my GCSE/IGCSE/US grade 8-10 Chemistry Revision notes

The nitrogen cycle is described in terms of the function of nitrifying bacteria, lightning and production of nitrogen oxides, function of denitrifying bacteria, action of nitrogen-fixing bacteria, part played by leguminous plants (legumes) in nitrogen fixation, bacteria decomposers, action of soil bacteria, plants absorbing nitrates from the soil. These revision notes on the nitrogen cycle, should prove useful for the new AQA GCSE chemistry, Edexcel GCSE chemistry & OCR GCSE chemistry (Gateway & 21st Century) GCSE (9–1), (9-5) & (5-1) science courses.

PART E 6. The Nitrogen Cycle for the gaseous element N_2(g)

• Nitrogen is an extremely important element for all plant or animal life! It is found in important molecules such as amino acids, which are combined to form proteins. Protein is used everywhere in living organisms from muscle structure in animals to enzymes in plants/animals.
• Nitrogen from the atmosphere:
  • Action of nitrifying bacteria, e.g. they function in the root nodules of certain plants like peas/beans (the legumes), can directly convert atmospheric nitrogen into nitrogen compounds in plants e.g. nitrogen => ammonia => nitrates which plants can absorb.
    • However, most plants can't do this conversion from nitrogen => ammonia, though they can all absorb nitrates, so the 'conversion' or 'fixing' ability might be introduced into other plant species by genetic engineering.
  • The nitrogen from air is converted into ammonia in the chemical industry, and from this artificial fertilisers are manufactured to add to nutrient deficient soils. However, some of the fertiliser is washed out of the soil and can cause pollution.
  • The energy of lightning causes nitrogen and oxygen to combine and form nitrogen oxides which dissolve in rain that falls on the soil adding to its nitrogen content.
    1. N_2(g) + O_2(g) ==> 2NO_(g), then 
    2. then 2NO_(g) + O_2(g) ==> 2NO_2(g) 
    3. NO_2(g) + water ==> nitrates_(aq) in rain/soil
    4. Incidentally, reactions 1. and 2. can also happen in a car engine, and NO₂ is acidic and adds to the polluting acidity of rain as well as providing nutrients for plants!
• Nitrogen recycling apart from the atmosphere:
  • Nitrogen compounds, e.g. protein formed in plants or animals, are consumed by animals higher up the food chain and then bacterial and fungal decomposers break down animal waste and dead plants/animals to release nitrogen nutrient compounds into the soil (e.g. in manure/compost) which can then be re-taken up by plants. 
• Nitrogen returned to the atmosphere:
  • However, the action denitrifying bacteria will break down proteins completely and release nitrogen gas into the atmosphere.
• Diagram of the nitrogen cycle

• More 'biological detail' of the NITROGEN CYCLE with reference to the above diagram so you can show an understanding of how nitrogen is recycled.
  • a) Nitrogen gas in the air (78%, ~⁴/₅th) cannot be used directly by most plants and all animals.
    • No animals and only a few specialised plants can directly use the very unreactive nitrogen from air, but all plants nitrogen in some form to synthesise amino acids and proteins for growth and maintenance and for DNA in cell reproduction.
    • However, nitrogen can be changed into nitrogen compounds like nitrates which the plants can use.
    • Animals rely on plants or other animals in the food chain for their source of nitrogen compounds e.g. protein in grass, crops or other animals.
  • b) Action of Nitrogen-fixing bacteria living in root nodules of plants or in the soil, their function is to fix nitrogen gas from the atmosphere into a chemical form the plant can metabolise.
    • Leguminous plants like peas, lentils, clover and beans can absorb nitrogen from the air via their root nodules (swellings on the root surface) which contain enzymes capable of converting ('fixing') atmospheric nitrogen into soluble nitrate - a nutrient essential for amino acids, proteins and therefore plant growth.
      • Legumes and their root nodule bacteria are an example of mutualism (see section 3.19 b) because the plant root supplies the bacteria with carbohydrate food and minerals and the bacteria supplies the plant in the form of the nitrate ion.
      • The process of converting nitrogen in air into nitrogen compounds is sometimes called 'nitrogen fixation'.
  • c) The action of lightning can convert nitrogen gas into nitrates.
    • The very high electrical energy discharges from lightning activates nitrogen and oxygen molecules to react and form nitrogen oxides. These dissolve in rain to form nitrates which end up in the soil when rainwater trickles into the soil.
  • d) Action of decomposers: Their function is to break down dead animals and plants
    • Decomposers, e.g. various organisms like bacteria, fungi or worms can break down dead animals or plants. They break down proteins to amino acids.
  • e) Action of soil bacteria: Their function is to convert proteins and urea into ammonia or ammonium ions.
    • Decomposer bacteria in the soil can change proteins from dead plants/animals and urea in animal urine/droppings into ammonia/ammonium ion compounds.
    • d) plus e) is sometimes called putrefaction by putrefying bacteria.
  • f) Action of nitrifying bacteria: Their function is to convert this ammonia to nitrates - the process of nitrification
    • Nitrifying bacteria oxidise ammonia/ammonium ions from the decayed material to form nitrates, the nitrate ion can be absorbed by plants through their root systems.
  • g) Plants absorb nitrates from the soil.
    • Plants absorb nitrates (soluble in water) in the moisture that the roots absorb from the surrounding soil.
    • Plants can use the nitrate ion in forming amino acids from which the plant can make its proteins.
  • h) Nitrates are needed by plants to make proteins for growth.
    • Nitrates are an essential nutrient for plants to synthesis amino acids and hence proteins.
  • i) Nitrogen compounds pass along a food chain or web of food chains.
    • All food chains involve the passing of carbon compounds e.g. sugars, carbohydrates, fats and proteins up to the next trophic level i.e. the consecutive eating along a food chain (and waste produced on the way).
      • e.g. grass ==> cow ==> human
      • Plants make their own protein from nitrates, but animals must obtain it from plants or other animals. In fact the protein is broken down in digestion to amino acids and each animal makes its own proteins from these amino acid residues.
  • j) Action of denitrifying bacteria: Their function is to convert nitrates to nitrogen gas.
    • Particular bacterial organisms can remove the oxygen from nitrate compounds to form the element nitrogen gas.
    • These denitrifying bacteria live in anaerobic conditions like waterlogged soils and use the nitrate ion to respire.
    • This is the opposite function of the nitrogen-fixing bacteria (b).

Associated links

Index: A Reversible Reactions  *  B Reversible reactions and Equilibrium 

C The Haber Synthesis of ammonia  *  D(a) The Uses of ammonia-nitric acid-fertilisers 

D(b) Fertilisers-environmental problems  *  E The nitrogen cycle (this page)

Foundation tier (easier) multiple choice QUIZ on ammonia, nitric acid and fertilisers etc.

Higher tier (harder) multiple choice QUIZ on ammonia, nitric acid and fertilisers etc.

Advanced A Level Notes - Equilibrium (use indexes)

Advanced A Level Chemistry Notes p-block nitrogen & ammonia

See also Carbon cycle, nitrogen cycle, water cycle, decomposition - decay investigation, biogas  gcse biology revision notes

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