Nitrogen Cycle

Nitrogen is an element essential to the survival of living things; where it can be found in land-based plants, terrestrial soils, oceans, rock and marine sediment, and the atmosphere. Nitrogen availability, however, is often limited within ecosystems, and consequently needs to be heavily recycled.

Due to primary producers being unable to utilise nitrogen in its gaseous state, bacteria takes nitrogen from the air and turns it into ammonia – this is called ‘nitrogen fixing’, and is performed by bacteria. Ammonium sometimes from animal waste, can also be turned into nitrates and nitrites, through a process called ‘nitrification’  conducted by nitrifying bacteria. Plants then take these nitrites and nitrates up through their roots to be used to produce chlorophyll, which is crucial in the process of photosynthesis. Excess nitrites and nitrates undergo denitrification by bacteria, and returned to the atmosphere as nitrogen. See Figure 1 for a visual representation.

With increased usage of fossil fuels and fertilisers there have been increasing alterations in ecosystems due to increased nitrogen levels. In terrestrial systems this has lead to nutrient imbalances in trees, changes in forests and decreased biodiversity, as well as affecting aquatic systems, with increased ecosystem crashes through a process called eutrophication.

Figure 1: A visual representation of each process within the nitrogen cycle

nitrogen cycle .png

“Untitled”,2018, Graphic, Online Biology Notes. Source: (Accessed 04/12/2018)


Key terms:

Nitrogen Fixation – The chemical process that turns nitrogen into ammonia.

Nitrification – The chemical process of turning ammonium into nitrite, and then nitrite into nitrate.

Denitrification – The chemical process of turning nitrate into nitrogen.

Chlorophyll – Green pigment in a plant which absorbs sunlight to be used in photosynthesis.

Photosynthesis – The process of plants using energy from sunlight to break down water and carbon dioxide, producing oxygen as a by-product.

Biodiversity – The diversity of organisms within an ecosystem.

Eutrophication – The excessive richness of nutrients in a water body causing a damagingly dense growth of plant life.

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