Nitrogen Cycle: What, When, Where, and Why

Whenever you take a breath, what do you inhale? Oxygen, of course. You also inhale nitrogen. In fact, 80% of the air in the atmosphere is made of nitrogen. The nitrogen cycle, or n cycle, on Earth, is very important as it provides every living thing with what they need to grow. Humans, animals, even plants.

Nitrogen is the most abundant source in the atmosphere. It is also the building block of proteins, nucleic acids like DNA, and a very important component of all life. The cycle itself is a complex biochemical system where nitrogen is changed from its natural molecule N2 into a form that is useful for the biological process. Most life can not use nitrogen in its natural form. It has to be converted down.


What is The Nitrogen Cycle

As said before, the cycle is a very complex process. Nitrogen needs to be broken down into a form that can be used by plants, animals, humans, anything and everything. This next part will describe the nitrogen cycle steps in a scientific way, followed by a way that is fairly easy to understand.


Nitrogen Fixation

First thing that happens in the nitrogen cycle is nitrogen is deposited by precipitation, rain, into the soil and surface waters. As soon as the nitrogen is in the soil or water, it starts to undergo some changes. The nitrogen molecule is made up of two nitrogen atoms (N2). The nitrogen atoms split and merge with the hydrogen molecules to make ammonia (NH4+). To do this requires three types of microorganisms. Bacteria living off of certain plants, free anaerobic bacteria, and algae.

There are a few plants that can be used to help with nitrogen depletion, like alfalfa and beans. Nitrogen can also be fixed in man-made ways. Most commonly an industrial process that creates ammonia and nitrogen-rich fertilizers. Farmers use these fertilizers to help plants from stronger and more quickly.


Nitrification Cycle

The next step of the nitrogen cycle is to turn the ammonia into nitrite. Other bacteria, known as nitrifying bacteria, take the ammonia that was created in the fixation stage and converts it twice. Once into nitrite (NO2-) and then into nitrite (NO3-). This is so the nitrites can be properly oxidized. The ammonia is much too toxic for most organisms to use, with the exception of some plants. Also, gaseous nitrogen also can not be used. Therefore, these bacteria remove the nitrogen from the atmosphere and change it into something that can be.



This stage of the nitrogen cycle is when the various nitrogen compounds, nitrate (NO2-), nitrate (NO3-), ammonia, and ammonium, are used by the plants. This is done by the roots plants and trees. These compounds are then used in the formation of plant and animal proteins. These proteins include amino acids, nucleic acids, and chlorophyll.



This stage in the nitrogen cycle happens when plants and animals die, or when, to put it bluntly, animals poop. The nitrogen in the organic matter is released back into the soil where it goes through a stage of decay by even more bacteria known as decomposers. During this decay, ammonia is produced which is then available for the nitrification and other biological processes.



Here in the nitrogen cycle, nitrogen makes its way back into the atmosphere. Nitrite (NO3-) is changed back into the gas nitrogen (N2). This happens mostly in wet soils where the water makes it fairly difficult for the microorganisms to get oxygen. The organisms, known as denitrifying bacteria, process the nitrite to use their oxygen, creating free nitrogen gas as its byproduct.


Nitrogen Cycle for Kids

It is very important for everyone to understand as it is a very important part of life. Here are the same steps that were just explained, but in a more simple way.


Fixation- Also known as nitrogen “fixing”. This first step of the cycle is making nitrogen able to be used by plants. Bacteria will change nitrogen into ammonium.


Nitrification-  This is when the ammonium gets changed into nitrites by bacteria. Nitrites are what plants can use.


Assimilation- This is when plants get the nitrogen. They absorb the nitrites through their roots. It is then used in a number of ways that are important for life to continue.


Ammonification- When an animal dies, fungi and bacteria turn the nitrogen back into ammonium so it can be used in a new cycle.


Denitrification- Any extra nitrogen in the soil is returned to the atmosphere. As usual, there special bacteria that do this.

Quick Facts About Atmospheric Nitrogen

  • Human activities have increased the deposition of nitrogen in many ways.


  • Burning of fossil fuels and forests. This releases nitrogen into the atmosphere.


  • Fertilizing crops with nitrogen rich fertilizers, as said before.


  • Ranching. Livestock waste releases ammonia into the soil and water.


  • Releasing sewage and septic tanks into the streams, rivers and groundwater.


  • Nitrous oxide is a greenhouse gas


  • Nitrogen has no color, smell or taste


  • It is used in many explosives *BOOM*


  • Your total body weight is almost 3% nitrogen


  • The largest nitrogen reservoirs on Earth is the Air.


Nitrogen Deposition and the Environment

The earth has all of the nitrogen that it needs. The increase of nitrogen deposition into our atmosphere actually causes more harm than good. The effects can be seen all over the Earth’s system.



In the ecosystem, the addition of extra nitrogen in the soil can create changes that favor weeds over other plants. This can reduce species diversity and actually can change the whole ecosystem. Nitrogen level is directly linked to changes in grassland plants from mosses to grasses and flowers.



In addition to the land, excess nitrogen can also affect the water quality. In rivers, lakes, and coastal systems, a condition called eutrophication can occur. Eutrophication is when excessive nutrient conditions create algae that deplete oxygen which kills fish, other organisms, and ruins water quality. In the Gulf of Mexico, parts are so polluted with fertilizer that the water gets clogged with algae, killing fish and marine life.



Remember the ozone layer of the atmosphere? Hardly ever talked about anymore. Well, excess amounts of nitrogen oxides are a forerunner to a tropospheric ozone. This is proven to be damaging to lungs and even decrease plant production.



Talking about changes to the air, nitrogen oxides can react with the water in clouds to form nitric acid. Nitric acid and sulfur dioxide are a major component to acid rain. Everyone knows acid rain is not to be messed with. Not only can it damage your property, it also can damage and kill water life and plants.


As much as nitrogen is of great use to the planet, there can always be too much of a good thing. For nitrogen, the results could be drastic and hard to reverse.



Why is the Nitrogen Cycle Important

As said before, nitrogen is important because it is one of the main building blocks of all life. Nitrogen is found in all living organisms like amino acids, which in turn make up proteins, nucleic acids, adenosine triphosphate, and many others. Actually, nitrogen is the basis of many parts of DNA including thymine, cytosine, adenine, and guanine. In RNA, the nitrogenous bases are the same with the exception of uracil, which is not present in DNA.


Simply, we could not exist without nitrogen. Since it makes up much of our atmosphere, is the base of many parts of DNA and RNA, life would unravel and nothing would be able to survive.


Nutrient Cycle

Now, we heard about the very important nitrogen cycle. However, there are other cycles that are just as, if not more important to the Earth’s ecosystem. All of these cycles fall under the name nutrient cycle. The nutrient cycle is the use, movement and recycling of nutrients in the environment. Elements like carbon, oxygen, hydrogen, and phosphorus, are extremely important to life and must be recycled for the many organisms on Earth, including humans, to exist. They are called Biogeochemical cycles because they involve biological, geological and chemical processes.


Biogeochemical Cycles

These cycles involve two types, global and local cycles. Global cycles is when elements like carbon, nitrogen, oxygen, and hydrogen are recycled through abiotic environments such as the atmosphere, water and soil. The atmosphere, being the main abiotic environment where these elements are found, making their cycles a global event. Local cycles are the recycling of phosphorus, calcium, and potassium in the abiotic soil environment which is obviously found over a local region. Hence the name local cycle.


The Element Carbon 

Carbon (CO2), like nitrogen, is another one of the most important cycles because, like nitrogen, every organism alive needs it. CO2 serves as the backbone for all organic polymers like carbohydrates. The carbon cycle most commonly happens through photosynthesis and respiration. There are two cycles that carbon can go through. One is the fact carbon cycle, which is when carbon moves through biotic elements like plants, animals, and decomposers. The other cycle is the slow carbon cycle. This is when carbon moves through abiotic elements like rocks, soil, and oceans. This is much slower than moving through biotic elements because it can actually take up to 200 million years for the carbon to complete the cycle.

The Cycle

This cycle is a smaller circle than the nitrogen cycle. Basically, organisms that use photosynthesis, like plants, remove carbon (CO2) from the atmosphere. The CO2 is used to create organic molecules and build biological mass. Animals will consume those organisms (cow eating plants) and gain the carbon that is stored within the plants. The release of carbon happens in a number of ways.

The first way to release carbon is through respiration. Breathe in oxygen, exhale carbon. The second way is when decomposers break down dead or decaying organic matter and the carbon stored in that matter is then released. Similar to how nitrogen is released. The third way is by burning organic matter, like forest fires. Trees that store carbon, release it into the atmosphere when they are burned. Finally, rocks and fossils that store carbon, return it to the atmosphere during erosion, volcanic eruptions, or the cause of the increased amount of carbon in our atmosphere which is the main reason behind global warming, fossil fuel combustion.


Honorable Mentions

The other elements mentioned above have cycles similar to that of carbon and nitrogen. They are just as important as carbon and nitrogen.  Most of the oxygen that you find in the atmosphere, photosynthesis creates. The same process that is found in the carbon cycle. Photosynthetic organisms take the carbon from the atmosphere and turn it into oxygen for use by animals and humans. Animals and humans then remove the oxygen from the atmosphere and change it back into carbon, and the cycle continues.


One of the other very important elements that is mentioned is phosphorus. Phosphorus is a big ingredient in biological molecules, like RNA, DNA, phospholipids, and adenosine triphosphate or ATP. ATP is produced by cell respiration and fermentation. Phosphorus is mainly found in rocks, soil, and water. It is added to those environments by runoff from the breakdown of rocks. Like the other elements, phosphorus is then absorbed by plants and animals eat those plants. Then, of course, returned to the soil by means of decomposition. Yeah, all of these elements seem to follow the same pattern.



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Patrick Sands

Hey, I'm Pat. I am a Millersville grad with a Bachelors of Arts in English. I love to write, play video games, watch movies and TV, basically be a total nerd whenever I can. Green and Growing is important to me because it allows me to help others be as green and eco-friendly as possible. With Climate Change being what it is, it is even more important for people to get educated about their environment. This website allows me to do my part in that. Also, I'm a huge goof who tries to add some humor into anything I write. Stay Excellent out there!

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