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Nitrogen cycle

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Biology

The nitrogen cycle describes the process of converting nitrogen to different chemical forms. These nitrogen forms circulate between the terrestrial environment, marine environment and the atmosphere.

The main processes in the nitrogen cycle include nitrogen fixation, ammonification, nitrification and denitrification (Figure 1). Different organisms can access different forms of nitrogen.

Nitrogen Cycle [+] the nitrogen cycle [+] StudySmarterImage Figure 1 The nitrogen cycle. Sources: Wikimedia Commons CC BY-SA 4.0

The steps in the nitrogen cycle

The nitrogen cycle is broken down into key steps:

  • Nitrogen fixation
  • Nitrification
  • Assimilation
  • Ammonification
  • Denitrification
  • Anaerobic ammonia oxidation

Nitrogen fixation

Nitrogen fixation refers to the conversion of nitrogen gas (N2) to nitrogen-containing compounds. This process occurs naturally via lightning, artificially by the Haber-Bosch process, or specialised nitrogen-fixing microorganisms.

Nitrogen fixation by lightning

In the atmosphere, two nitrogen atoms in the nitrogen molecule are held together by a strong triple covalent bond, i.e., the atoms share three pairs of electrons. Lightning carries enough energy to break the covalent bonds of the atmospheric nitrogen. When the nitrogen molecule splits, it bonds to atmospheric oxygen forming nitrogen oxides (NOx).

'x' in NOx refers to the number of atoms.

Nitrogen fixation by the Haber-Bosch process

Human activities have become a large source of nitrogen fixation. Burning fossil fuels produces fixed nitrogen, while industrial nitrogen fixation is a byproduct of the Haber-Bosch process to produce ammonia. During the process, nitrogen gas combines with hydrogen gas in a high temperature and a high-pressure environment. The ammonia produced is widely used in synthetic fertilisers.

Nitrogen fixation by microorganisms

Free-living nitrogen-fixing bacteria can transform atmospheric nitrogen into inorganic nitrogen forms that plants can access. Plants will use inorganic nitrogen to make amino acids.

Mutualistic nitrogen-fixing bacteria, such as Rhizobium, live in plant nodes (e.g., peas and beans) and acquire carbohydrates from the plant. In turn, they provide amino acids to the plant. The relationship between these bacteria types and the plants is symbiotic or mutualistic.

Symbiotic or mutualistic relationships occur when two organisms live in close physical proximity, and both organisms provide advantages to one another. Examples include nitrogen-fixing bacteria and their associated plant.

Nitrification

Nitrification is a two-step oxidation reaction process converting ammonium ions into nitrate ions. The process involves:

  1. Oxidation of ammonium ions (NH4+) to nitrite ions (NO2-) by Nitrosomonas and Nitrococcus bacteria.
  2. Oxidation of nitrate ions (NO2-) to nitrate ions (NO3-) by Nitrobacter bacteria.

Nitrifying bacteria gain energy through these oxidation reactions. Both of the reactions require oxygen to occur; therefore, to increase productivity within agriculture, the soil needs to be kept well aerated.

Assimilation

Assimilation occurs when plants and animals use nitrate ions and ammonia to make amino acids and proteins. Nitrate ions and ammonia are formed by nitrogen fixation and nitrification, as we have just read.

Amino acids are the molecules that makeup proteins.

Ammonification

Ammonification involves the conversion of organic nitrogen (nitrogen found in the cells of living organisms) into ammonia. Saprobiotic organisms (decomposers) are the key players in this process as they feed on organic matter, break it down and release ammonia. This ammonia now becomes available for nitrification and assimilation processes as it can be converted into ammonium ions.

Denitrification

Denitrification refers to reducing nitrate ions to nitrogen gas by anaerobic bacteria. This process can only happen in anaerobic conditions (no or very little oxygen available). For example, in wetlands, the soil tends to be waterlogged, which sets up the perfect environment for anaerobic bacteria as little oxygen is found in the soil.

Reduction involves the removal of oxygen.

Dissimilatory nitrate reduction to ammonium (DNRA) is another step in the nitrogen cycle. This process involves the production of ammonium ions (NH4+), catalysed by enzymes in anaerobic bacteria.

The formula for DNRA is:

Anaerobic ammonia oxidation

During this process, ammonium ions (NH4+) and nitrite (NO-2) are converted into atmospheric nitrogen (anammox reaction). This step is a significant process in oceans and needs to occur in anaerobic conditions (absence of oxygen).

The formula for this is:

Why is the nitrogen cycle important to plants and animals?

Nitrogen is one of the main elements used to build a life. It is a component of many cells and is a building block in amino acids, proteins and nucleic acids. In plants, nitrogen is also used to make chlorophyll, a critical pigment involved in trapping light for photosynthesis.

Nucleic acids include DNA and RNA. These are involved with the storage and transfer of genetic material.

What is the human impact on the nitrogen cycle?

Disruption of the nitrogen cycle will lead to imbalances in the ecosystems. For example, soil containing excess nitrogen is in danger of having low pH, meaning the soil will be too acidic. Soil with a low pH is harmful to bacteria and microorganisms that cannot survive in these conditions. In the oceans and rivers, the accumulation of nitrogen run-off will lead to eutrophication. Eutrophication is the process whereby nutrient levels become excessive in water bodies. You can learn more about this in our article Fertilisers.

Burning of the fossil fuels

The burning of fossil fuels will release nitrogen and nitric oxides into the atmosphere. These actions affect air quality, leading to the production of acid rain and smog production.

Use of nitrogen-containing fertilisers

Fertilisers increase plant growth and overall crop productivity. However, the excessive use of nitrogen-containing fertilisers creates a harmful imbalance of the soil's nutrient levels. These harmful effects can lead to:

  • Reduced species diversity. Nitrogen-rich soils will favour the growth of grasses and other rapidly growing vegetation. Rapidly growing vegetation will outcompete other species and reduce biodiversity.
  • Leaching. Nutrients will enter streams, rivers and oceans.
  • Eutrophication. This is a consequence of leaching as the excessive accumulation of nutrients harms microorganisms and wildlife in water bodies.

Removal of vegetation

Leaching is the removal of nutrients from the soil. For example, removing plants and trees will remove nutrients with them. In addition to this, the soluble nutrients left, such as nitrate ions, will dissolve in rainwater. These dissolved nutrients will be carried deeper into the soil and eventually, the nutrients will travel too deep into the soil for the plant to reach them.

Reducing the human impact

To reduce the adverse effects of human activity on the nitrogen cycle, individuals and companies need to be mindful of their nitrogen footprint. Changes such as choosing renewable sources can lead to the gradual reduction of the amount of fossil fuels burned. Even choosing to eat less red meat can have a positive impact as cattle farming requires a large amount of feed, which is grown with fertilisers.

Nitrogen monitoring is another approach that the agriculture sector could benefit from. This describes the careful monitoring of nutrient levels in soil, minimising the toxic effects of ammonia build-up and, therefore, the effect of eutrophication from nutrient run-off.

Nitrogen cycle - Key takeaways

  • The nitrogen cycle is a nutrient cycle that converts nitrogen into multiple organic and inorganic compound forms. Nitrogen will cycle through the atmosphere and within the terrestrial and aquatic environments.

  • The main steps in the nitrogen cycle include nitrogen fixation, nitrification, denitrification, assimilation, ammonification, DNRA and anaerobic ammonia oxidation.

  • Nitrogen is a component of many cells and processes in living organisms. Nitrogen is a building block in amino acids, proteins and DNA.

  • Human impacts cause a disruption in the nitrogen cycle. These effects include leaching, eutrophication and reduced species diversity.

Nitrogen cycle

Nitrogen fixation, nitrification, denitrification, assimilation, ammonification, DNRA and anaerobic ammonia oxidation.

During the ammonification process, organic nitrogen is converted to ammonia. Ammonia produced is excreted as waste into the environment.

Nitrogen is used to make amino acids, proteins and DNA. It is also a crucial element in photosynthesis as nitrogen is used to make chlorophyll, which is an important pigment involved in photosynthesis.

The nitrogen cycle is a nutrient cycle describing the conversion of nitrogen into different chemical forms. Nitrogen circulates between the terrestrial, marine environments and the atmosphere.

  • Choosing renewable energy sources over fossil fuels will help to reduce the amount of fossil fuels being burned.
  • Reduced consumption of red meat. Growing cattle will require a large amount of crop (which requires a large amount of fertiliser).

Final Nitrogen cycle Quiz

Question

What is meant by the nitrogen cycle and why is it important?

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Answer

The nitrogen cycle is a biogeochemical cycle during which organic and inorganic nitrogen in different chemical forms will circle through the atmosphere, terrestrial and marine environments. Nitrogen cycle is important because nitrogen is an important building block in all living organisms. Nitrogen is used to make amino acids, proteins and DNA.

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Question

What is the difference between nitrification and denitrification?

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Answer

Nitrification refers to the oxidation of nitrogen compounds. Denitrification is  the reduction of oxidised nitrogen compounds.

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Question

What group of the living organisms can access nitrogen in its gaseous form (N2)?


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Answer

Nitrogen gas is only available to nitrogen-fixing microorganisms.

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Question

Name a primary producer in the marine nitrogen cycle.


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Answer

Phytoplankton.

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Question

Fill in the blanks about splitting of the nitrogen molecule during lightning. Nitrogen atoms in the atmospheric nitrogen molecule are held by ________ bonds. This means that the atoms share three pairs of _______. These bonds can be _____ by lightning which carries enough energy to split the atoms. When the nitrogen molecule splits, it bonds to the _______ atoms, forming nitrogen oxides (NOx).

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Answer

Nitrogen atoms in the atmospheric nitrogen molecule are held by covalent bonds. This means that the atoms share three pairs of electrons. These bonds can be broken by lightning which carries enough energy to split the atoms. When the nitrogen molecule splits, it bonds to the oxygen atoms, forming nitrogen oxides (NOx).

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Question

What happens during the Haber-Bosch process?


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Answer

The Haber-Bosch process is an artificial way to produce ammonia. During the process, nitrogen gas is combined with hydrogen gas at a high temperature and a high-pressure environment.

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Question

Mutualistic nitrogen-fixing bacteria will form a symbiotic relationship with the plant roots to exchange material. True or False?


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Answer

True.

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Question

 Assimilation can be defined as the process where plants and animals use nitrate ions and ammonia formed by denitrification to make amino acids. True or False?


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Answer

False.

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Question

Fill in the blanks about the denitrification step in the nitrogen cycle. Denitrification refers to the removal of _______ from the nitrate ions to ________ gas by anaerobic bacteria. Denitrification happens in _______ conditions. Environments such as wetlands are ________, they are important in reducing excess _______ levels.

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Answer

Denitrification refers to the removal of oxygen from the nitrate ions to nitrogen gas by anaerobic bacteria. Denitrification happens in anoxic/anaerobic conditions. Environments such as wetlands are waterlogged, they are important in reducing excess nitrogen levels.

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Question

Oxygen is an electron acceptor in aerobic respiration. What is the electron acceptor in anaerobic respiration of specialised microbes? What is this process called?


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Answer

Microbes respiring anaerobically will use organic nitrate as electron acceptors. This is the DNRA (nitrate/nitrite ammonification) process.

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Question

Nitrobacter bacteria facilitates the oxidation of the ammonium ions. True or False?


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Answer

False.

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Question

Name three human impacts on the nitrogen cycle.


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Answer

Leaching, eutrophication and reduced species diversity.

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Question

Why is nitrogen monitoring important?


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Answer

In agriculture, nutrient levels need to be monitored to maximise soil productivity. It is also important to monitor excess ammonia and nitrites in the soil, as these in excess amounts are toxic. Monitoring can also prevent nutrient runoff into the water bodies. This will prevent eutrophication.

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Question

How can eating less meat contribute to a reduced nitrogen footprint?


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Answer

A large amount of crop feed is used to feed cattle. The crop feed will require large amounts of fertiliser to grow which could lead to soil imbalances and have other negative effects on the environment. 

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Question

Fill in the blanks about the nitrogen cycle disruption. Disruption of the nitrogen cycle leads to the _______ within the ecosystem. Soil containing excess nitrogen will have a _______ pH compared to the soil that has a normal concentration of nitrogen. Too much nitrogen in the oceans will lead to _______. _______ is one of the main causes of nutrient ______ into the water bodies.


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Answer

Disruption of the nitrogen cycle leads to the imbalances within the ecosystem. Soil containing excess nitrogen will have a lower pH compared to the soil that has a normal concentration of nitrogen. Too much nitrogen in the oceans will lead to eutrophication. Leaching is one of the main causes of nutrient runoff into the water bodies.

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