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The Nitrogen Cycle


Nitrogen is important to organisms. Nitrogen is required to make amino acids, the building blocks of proteins. The air that we breathe is 78% nitrogen. Most organisms, including humans, can't use the free gaseous form of nitrogen found in the air. We need other organisms and natural processes to "fix" nitrogen into a solid form that plants and animals can use.

Most of the nitrogen plants and animals use is made by bacteria. Bacteria in the soil can change nitrogen in the atmosphere (N2) into ammonia (NH3), a form that plants can use. Other types of bacteria form large lumps, or nodules, around the roots of peas, beans and clover to take in nitrogen from the atmosphere (N2) and change it into nitrites and nitrates. These compounds contain nitrogen and oxygen and are commonly used in fertilizers.


Animals receive the nitrogen they need from the consumption of plants. When plants, animals and other organisms die, decomposers return nitrogen to the soil.

What effect do humans have on the nitrogen cycle? As mentioned above, humans use fertilizers on plants to get them to grow better; and nitrogen is a major component of most fertilizers. Is this a concern? It can be.

Nitrogen additions in excessive amounts pollute our ecosystems. This can change the functioning of the ecosystem and the communities that are supported by it.

Human activity has also increased the global concentrations of nitrous oxide (N2O), a strong greenhouse gas. (It is also used to make trips to your local dentist less stressful.) The amount of nitrogen transported in rivers and groundwater has increased. This can accumulate in the final destination and can lead to eutrophication of lakes, which basically kills most of the wildlife living there.

Other changes we have made in the natural nitrogen cycle might include an increased loss of biodiversity, especially with plants that have adapted to the low levels of nitrogen in the soil. This also affects the animals and microbes that depend on this vegetation.

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Updated October 24, 2008 by: Glen Westbroek

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