Nitrogen cycle

Nitrogen cycle

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The nitrogen cycle is a process that changes nitrogen into different chemical forms. These forms of nitrogen move between the land, oceans and air. There are four main steps in the nitrogen cycle: nitrogen fixation, ammonification, nitrification and denitrification (see Figure 1). Different living things can use different forms of nitrogen.

The nitrogen cycle
The nitrogen cycle

The steps in the nitrogen cycle

The nitrogen cycle is broken down into key steps: Nitrogen fixationNitrification Assimilation Ammonification Denitrification Anaerobic ammonia oxidation

Nitrogen fixation

Nitrogen fixation is when nitrogen gas (N2) changes into nitrogen-containing compounds. This happens naturally through lightning, or through specialised microorganisms, and human activities. Nitrogen has a strong triple covalent bond in the atmosphere, and breaking this bond is challenging. However, lightning can split the nitrogen molecule, which bonds with atmospheric oxygen to create nitrogen oxides (NOx).

Human activities, such as burning fossil fuels and industrial nitrogen fixation, have become a significant source of nitrogen fixation. The Haber-Bosch process to produce ammonia uses nitrogen gas and hydrogen gas in a high-pressure, high-temperature environment. This process is widely used in synthetic fertilisers.

Free-living nitrogen-fixing bacteria can transform atmospheric nitrogen into inorganic nitrogen that plants can use to make amino acids. Meanwhile, mutualistic nitrogen-fixing bacteria, such as Rhizobium, live in plant nodes and acquire carbohydrates from the plant. In return, they provide amino acids to the plant. This relationship between nitrogen-fixing bacteria and plants is symbiotic or mutualistic.

Symbiotic or mutualistic relationships occur when two organisms live close together, and both organisms benefit from one another. Examples include nitrogen-fixing bacteria and associated plants like peas and beans.


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

Oxidation of ammonium ions  (NH4+) to nitrite ions (NO2-) by Nitrosomonas and Nitrococcus bacteria.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 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 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 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). 

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?

The nitrogen cycle is vital for the balance of ecosystems, and any disruption can lead to imbalances. For instance, excess nitrogen in soil can lead to low pH levels, making the soil too acidic. Soil with a low pH can be harmful to bacteria and microorganisms that cannot survive in such conditions. This can affect the nutrient levels in the soil, leading to a lack of nutrients for plants and other organisms.

In aquatic environments, the accumulation of nitrogen run-offizers or other sources canrophication levels become excessive water bodies. Eutrophication can lead to overgrowth of algae and other aquatic plants, which in turn can lead to a depletion of oxygen levels in the water. This can be harmful to fish and other animals that need oxygen to survive.

The nitrogen cycle is an essential process that needs to be maintained to keep ecosystems in balance. Any disruption to this cycle can have significant impacts on the environment, including soil and water quality, plant growth, and animal health. Therefore, it is crucial to manage the use of nitrogen fertilisers and other sources of nitrogen to prevent imbalances in the nitrogen cycle.

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 are commonly used to increase plant growth and overall crop. However, the excessive use of nitrogen-containing fertilisers can create imbalances in the soil's nutrient levels, leading to a range of negative impacts. One of the significant impacts of excessive fertiliser use is reduced species diversity. Nitrogen-rich soils favour the growth of grasses and other rapidly growing vegetation, which outcompete other species and reduce biodiversity. Another impact of excessive fertiliser use is leaching. Nutrients from fertilisers can enter streams, rivers, and oceans, leading to pollution and harm aquatic life. E is another of leaching as the excessive accumulation of nutrients harms microorganisms and wildlife in water bodies. This can lead to the overgrowth of algae and other aquatic plants, which can deplete oxygen levels in the water and harm fish and other animals that depend on oxygen to survive. To prevent these harmful impacts, it is essential to manage the use of fertilisers carefully, ensuring that they are used in the right quantities and at the right times. This can help to maintain the balance of nutrients in the soil and prevent negative impacts on the environment.

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 harmful impacts of human activity on the nitrogen cycle, individuals and companies can take several steps. One crucial step is to be mindful of their nitrogen footprint. This involves making conscious choices to reduce the use of nitrogen-containing fertilisers, choosing renewable energy sources, and reducing meat consumption.

Another approach that can help to mitigate the negative impacts of human activity on the nitrogen cycle is nitrogen monitoring. This involves carefully monitoring nutrient levels in soil and taking steps to minimise the toxic effects of ammonia build-up and nutrient run-off.

Overall, it is essential to recognise the value of the nitrogen cycle and take steps to protect and maintain it. With careful management and responsible use of nitrogen-containing products, we can help to preserve this critical nutrient cycle and ensure the health and well-being of ecosystems and the planet.

Nitrogen cycle

What are the 7 steps of the nitrogen cycle? 

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

What is the ammonification step in the nitrogen cycle?

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

Why is the nitrogen cycle important to the plants?

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.

What is the nitrogen cycle?

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.

How can human impact on the nitrogen cycle be prevented?

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).

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