[email doc b: comment? query?] * [privacy-policies-disclaimer] * ]SEARCH doc b's website]

(3) The Nitrogen Cycle and its important role in the Earth's biosphere

• 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.
  • Therefore nitrogen containing molecules are passed along food chains.
  • Nitrogen in air cannot be absorbed by plants until it is converted into ions such as nitrite (NO₂^-) or nitrate (NO₃^-).

   

• Nitrogen from the atmosphere - nitrogen fixation processes:
  • Air contains ~78% nitrogen gas.
  • N₂ is a very stable molecule and the word fixation means to convert the nitrogen in air into some chemical form that is soluble in water and plants can use - this can be synthetically in the Haber process or by nitrogen-fixing bacteria.
  • Action of nitrifying bacteria, (nitrogen-fixing 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.
    • This is a naturally occurring nitrogen fixation process.
    • Some nitrogen-fixing bacteria live in soil whilst others live in the swelled nodules in the roots of legumes.
    • When legume plants die and decompose the nitrogen is returned to the soil.
    • Nitrogen ions can leak out of the modules during plant growth.
    • The nitrogen-fixing bacteria have a mutualistic relationship with the plants - the bacteria get sugary food from the plant and the plant gets nitrogen ions from the bacteria (to make amino acids and proteins).
    • However, most plants cannot 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, combining with hydrogen, is converted into ammonia (NH₃) in the chemical industry, and from this artificial fertilisers are manufactured to add to nutrient deficient soils.
    • This done in the Haber process - a synthetic fixation process.
    • However, some of the fertiliser is washed out of the soil and can cause pollution - eutrophication.
  • The energy of lightning is so high it causes nitrogen and oxygen to combine and form nitrogen oxides which dissolve in rain that falls on the soil adding to its nitrogen content. This is also described as a natural nitrogen fixation process - atmospheric elemental nitrogen converted to a compound that enters the soil for plants to use.
    1. N_2(g) + O_2(g) ==> 2NO_(g)
    2. 2NO_(g) + O_2(g) ==> 2NO_2(g) 
    3. NO_2(g) + water ==> NO₂^-_(aq) or NO₃^-_(aq) nitrates/nitrites 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! See air pollution and acid rain
    5. Note that the Haber synthesis of ammonia is a synthetic method of nitrogen fixation.
• 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.
  • 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.
  • The bacteria and fungi break down proteins in rotting plants and animals and urea in animal waste and convert the nitrogen into ammonia, which is oxidised to nitrite or nitrate ions - so returning the nitrogen to the soil in a form that can be absorbed by plants.

   

• Nitrogen returned to the atmosphere:
  • However, the action denitrifying bacteria will break down proteins completely and release nitrogen gas (N₂) into the atmosphere.

• More 'biological detail' of the NITROGEN CYCLE with reference to the above diagram so you can show an understanding of how nitrogen is recycled.
  • 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.
  • 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.
  • There are four types of bacteria involved in the nitrogen cycle
    • (1) Action of Nitrogen-fixing bacteria (nitrifying bacteria) living in root nodules of plants or in the soil, their function is to fix nitrogen gas directly from the atmosphere into a chemical form the plant can metabolise.
    • Members of the leguminous family contain symbiotic bacteria called rhizobia within the root nodules, producing nitrogen compounds that help the plant to grow and compete with other plants. When the plant dies, the fixed nitrogen is released, making it available to other plants and this helps to fertilize the soil.
    • So, 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 bacterial enzymes capable of converting ('fixing') atmospheric nitrogen into soluble ammonia/ammonium ions, these are oxidised to the nitrate ion - a nutrient essential for synthesising amino acids, proteins for plant growth.
    • • Legumes and their root nodule bacteria are an example of mutualism, because the plant root supplies the bacteria with carbohydrate food (sugars) and minerals and the bacteria supplies the plant with nitrogen in the form of the nitrite  or nitrate ion - different species of nitrifying bacteria produce different ions.
      • This mutual relationship benefits both the plant and its associated bacteria.
      • The process of converting nitrogen in air into nitrogen compounds is sometimes called 'nitrogen fixation'.
    • When these leguminous plants decompose the nitrogen compounds in the tissues and root nodules is returned to the soil - a good composting action.
    • Ions containing nitrogen can diffuse out of the nodules during plant growth.
    • Leguminous plants have a mutualistic relationship with bacteria.
    • The bacteria get food like sugars from the plant and the plants get nitrogen ions from the bacteria to use in making proteins.

     

    (2) Action of decomposers - soil bacteria:

    • The function of decomposers, bacteria, fungi and worms in the soil, is to break down the remains of dead animals and plants converting proteins and urea (from animal waste) into soluble ammonia or ammonium ions - so can be absorbed by the roots of plants.
    • This decomposition (decay) returns nitrogen to the soil.
    • This process is sometimes called putrefaction by putrefying bacteria.
    • The ammonium ions are oxidised to nitrate ions (see below), which plants can absorb through their roots to complete the recycling process.
    • (3) Action of nitrifying bacteria:
    • Nitrifying bacteria oxidise ammonia/ammonium ions from the decayed material to form nitrites, which are further oxidised to nitrate ions that be absorbed by plants through their root systems.
    • (4) Action of denitrifying bacteria:
    • Denitrifying bacteria convert nitrates back into nitrogen gas which is returned to the atmosphere.
    • Therefore they are of no benefit to any living organism.
    • These particular bacterial organisms can remove the oxygen from nitrate compounds to form the element nitrogen gas.
    • The action of denitrifying bacteria is of no use to any living organism!
    • The function of denitrifying bacteria is the opposite of the nitrogen-fixing bacteria (1).
    • Denitrifying bacteria are most often found in waterlogged soils, conditions unsuitable for most plant or animal life.
    • These denitrifying bacteria live in anaerobic conditions like waterlogged soils and use the nitrate ion to respire.
    • These denitrifying bacteria use the oxygen rich nitrate ion (NO₃^-) as an oxidant instead of oxygen gas.

     

  • Plants absorb nitrates from the soil.
    • Plants get their nitrogen from 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.
    • Nitrogen compounds are then passed along the food chains e.g. animals eat plants (herbivores) and animals eat animals (carnivores).
  • Nitrates are needed by plants to make proteins for growth.
    • Nitrates are an essential nutrient for plants to synthesis amino acids and hence proteins.
  • 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.

     

• Ways in which farmers can increase the nitrate content of soil
  • As plants grow they will use up the available nitrates in soil, which can become deficient in essential nutrients.
  • When the crops are harvested, the nitrate content of the soil is now reduced - its ended up as protein in the grain.
  • Unless the nitrate is replaced, the nitrate content in the soil will decrease with each crop grown.
  • To avoid the soil becoming infertile leading to poor plant growth and deficiency disease, the nitrate must be replaced.
  • There are three ways of doing this.
    1. Spreading an organic fertilisers like animal manure or some composted plant material which is decomposed by bacteria/fungi to release nitrogen compounds into the soil. This is a good method because it is essentially recycling organic substances from animal waste or plant material, both returning nitrogen compounds to the soil on decomposition.
    2. Spreading synthetic fertilisers (e.g. NPK products) made from ammonium and nitrate ion compounds. Can you name them? However, the use of artificial fertilisers can create pollution problems like eutrophication if overused. See the notes on ...

       Biodiversity, land management, waste management, maintaining ecosystems - conservation

    3. Crop rotation avoids growing the same crop in the same field over and over again. Several different crops are grown each year in a seasonal cycle. The cycle should include a leguminous plant (nitrogen-fixing crop) like beans or peas that will naturally return nitrates to the soil for another crop the following year - the plant itself can also be ploughed into the field.

 

See also in  the GCSE chemistry notes

The Haber Synthesis of ammonia - nitrogen fixation

Manufacture and uses of ammonia-nitric acid-fertilisers, use of NPK fertilisers, environmental issues

---

Key points - Summary of ideas about the nitrogen cycle

Based on the syllabus-specifications for students taking the AQA, Edexcel and OCR GCSE level biology examinations (~US grades 9-10).

The Nitrogen Cycle and Its Importance in Nature

Introduction

Nitrogen is an essential element for all living organisms, as it is a fundamental component of proteins, nucleic acids (DNA and RNA), and other biomolecules.

Although nitrogen gas (N₂) makes up approximately 78% of the Earth's atmosphere, most organisms cannot use it in this form.

The nitrogen cycle is the process by which nitrogen is converted into various chemical forms and recycled through ecosystems, making it available for biological use.

---

Key Processes in the Nitrogen Cycle

The nitrogen cycle consists of several key stages that convert nitrogen from one form to another:

1. Nitrogen Fixation
   Atmospheric nitrogen (N₂) is converted into ammonia (NH₃) or ammonium (NH₄⁺) by nitrogen-fixing bacteria. These bacteria can be:

   • Free-living bacteria (e.g., Azotobacter) in the soil.

   • Symbiotic bacteria (e.g., Rhizobium) found in the root nodules of leguminous plants (such as peas, beans, and clover).

   • Industrial processes (e.g., the Haber process) that convert nitrogen gas into ammonia for agricultural fertilizers.

2. Nitrification
   Ammonia (NH₃) or ammonium (NH₄⁺) is converted into nitrites (NO₂⁻) and then into nitrates (NO₃⁻) by nitrifying bacteria:

   • Nitrosomonas converts ammonia into nitrites.

   • Nitrobacter converts nitrites into nitrates.

   • Nitrates are absorbed by plants through their roots and used to build proteins and DNA.

3. Assimilation
   Plants take up nitrates from the soil and incorporate them into organic molecules (e.g., amino acids, proteins, and nucleic acids). When animals consume plants, they obtain nitrogen-containing compounds, which become part of their biological systems.

4. Ammonification
   When plants and animals die, decomposers (e.g., fungi and bacteria) break down their organic matter, releasing nitrogen in the form of ammonia (NH₃) or ammonium (NH₄⁺) back into the soil.

5. Denitrification
   Denitrifying bacteria (e.g., Pseudomonas) convert nitrates (NO₃⁻) back into nitrogen gas (N₂), returning it to the atmosphere and completing the cycle. This process typically occurs in anaerobic conditions (low oxygen environments, such as waterlogged soils).

---

Importance of the Nitrogen Cycle

The nitrogen cycle is crucial for maintaining ecosystem stability and supporting life:

• Provides Essential Nutrients: Enables plants to obtain nitrogen for growth, which supports herbivores and the entire food chain.

• Regulates Soil Fertility: The conversion of nitrogen forms maintains soil quality, impacting agriculture and food production.

• Balances Atmospheric Nitrogen: Prevents excess accumulation of nitrogen compounds in ecosystems, which could lead to pollution.

• Supports Biodiversity: Different organisms play specific roles in the cycle, promoting ecological interactions.

---

Human Impact on the Nitrogen Cycle

Human activities can disrupt the nitrogen cycle, causing environmental problems such as:

• Fertilizer Overuse: Excessive use of artificial fertilizers can lead to nitrate pollution in water sources, causing eutrophication (excessive growth of algae that depletes oxygen).

• Deforestation: Reduces nitrogen fixation by removing plants that host nitrogen-fixing bacteria.

• Burning Fossil Fuels: Releases nitrogen oxides (NOₓ), contributing to air pollution and acid rain.

Efforts such as crop rotation with nitrogen-fixing plants, reducing fertilizer use, and improving waste management help mitigate these impacts.

Summary of learning objectives and key words or phrases

Be able to construct a simplified diagram of the nitrogen cycle and be able to interpret and explain a more complex diagram of the nitrogen cycle described above.

Be able to show an understanding of how nitrogen is recycled (NITROGEN CYCLE diagram above):