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What is the Nitrogen Cycle?

B. Schreiber
B. Schreiber

The nitrogen cycle is a process in which nitrogen from the atmosphere is converted into a form that can be used by plants and animals. This happens through the action of bacteria, and beginning in the 20th century, human activity. When nitrogen is converted into its usable form, it is said to be fixed, and plants and algae incorporate the nitrogen into amino acids, proteins, and deoxyribonucleic acid (DNA). Animals obtain nitrogen-containing compounds from plants, making the nitrogen cycle essential for all life on earth. When living creatures die, other types of bacteria release the nitrogen in these substances back into the atmosphere, completing the cycle.

In the form N2, nitrogen makes up about 80 percent of Earth's atmosphere. This form of nitrogen can't be used by plants or the animals that rely on them.. Bacteria are required to convert the N2 into ammonia (NH3) and ammonium ions (NH4+). In a process called nitrification, soil bacteria convert ammonia into the nitrate ion (NH3). This part of the nitrogen cycle, known as nitrogen fixation, allows plants to produce the amino acids and other nitrogen-containing compounds that all animal life depends on. A very small amount of fixed nitrogen is generated annually by lightning strikes and some non-living chemical processes.

The nitrogen cycle, including the steps that make up nitrification, is a crucial process that takes place within soil.
The nitrogen cycle, including the steps that make up nitrification, is a crucial process that takes place within soil.

To complete the nitrogen cycle, the organic matter of dead plants and animals is broken down by another class of bacteria. This process, which releases the fixed nitrogen, is called denitrification. The nitrogen re-enters the atmosphere in the original form of N2, or as ammonia.

Due to an important scientific discovery, humans no longer have to rely on the available nitrogen produced by bacteria. This resulted in synthetic nitrogen-based fertilizers, which are heavily relied upon in agriculture to feed a substantial number of the world's people. In this way, humans have become an important part of the nitrogen cycle. It has been estimated that as much as 50 percent of the fixed nitrogen present in the environment exists due to human activity.

Some plants and animals have a special relationship with nitrogen-fixing bacteria. The roots of some plants, notably legumes, have nodules on their roots where bacteria generate nitrogen that can be used directly by the plant. In return, the bacteria get organic substances from the plants, which they use as food. Some animals like cows and buffalo also host nitrogen-fixing bacteria in their digestive tracts, which produce a substantial amount of the nitrogen-containing compounds the animals need.

Discussion Comments


What happens to young corn when you put too much fertilizer on it and then there is a big rain?


@matthewc23 - There is a lot of confusion about legumes fixing nitrogen. The person you heard was right. Not all of them are able to do it.

One of the most planted crops besides corn is soybeans. Luckily, this is one of the legumes than can fix nitrogen in the soil, so farmers don't have to put nearly as much fertilizer on the soil, if any. After harvesting some crops, farmers often plant alfalfa. It is another plant that fixes nitrogen.

I am from West Virginia, and there is a lot of surface mining here. Whenever they finish with a mine, they plant a tree called black locust that can fix nitrogen. I'm pretty sure it's a legume, too.

I would be curious to know if there are any more plants besides legumes that can fix nitrogen. Maybe someone else will know.


@Emilski - That's very interesting. I never knew about the no till farming. I see that a lot around where I live, but didn't know that is why they did it.

I was wondering about the legumes and fixing nitrogen. I have heard people say that not all legumes are N fixers. Is that true? I was also wondering what types of legumes are the best nitrogen fixers and how they are used. Also, are there any other plants that can fix nitrogen besides legumes?


@kentuckycat - Good questions. My uncle used to have a small farm where he grew corn and soybeans, so I know a little bit about the basics. I doubt you will cover most of this in your class, but it is good to know, especially if you ever end up in a job where you work outdoors.

From what I know, all nitrogen fertilizer is sold as either urea or ammonia depending on where you live. Urea is a solid and ammonia is a gas that gets "pumped" into the ground. It's the same stuff people steal to use for meth production. I'm not sure how they make it, though.

Runoff can be a big problem in some places. It runs off with rainwater and lowers the oxygen concentration in streams, which is obviously a problem for fish and other things. The problem is that a lot of the nitrogen never gets incorporated into the soil, so farmers have to put extra on to make sure they get the highest production possible.

One thing they are doing now to try to stop it is called no till farming where you leave the stems and roots from the last year's crop intact. It holds together soil and cuts down on runoff.


We just started learning about the nitrogen cycle in my biology class. It is a lot more complicated than I expected! We just finished up with the carbon cycle and it was pretty easy.

Whenever farmers put nitrogen on their soil, what form do they use, and where does it come from? How do they produce the nitrogen in a lab?

I have heard that nitrogen runoff from corn fields can be a big problem. Why is that? Why doesn't the nitrogen stay in the field, and what happens when it does end up getting into rivers and streams? Is there any way to better control it?

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    • The nitrogen cycle, including the steps that make up nitrification, is a crucial process that takes place within soil.
      By: KoiQuestion
      The nitrogen cycle, including the steps that make up nitrification, is a crucial process that takes place within soil.