Three Processes That Fix Atmospheric Nitrogen: Lesson & Quiz

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Wendy McDougal

Wendy has taught high school Biology and has a master's degree in education.

There are three major ways in which atmospheric nitrogen is fixed into compounds that plants and animals can use. Learn about these processes and take a quiz at the end.

We also recommend watching Nitrogen Fixation: Significance to Plants and Humans and The Nitrogen Cycle, Acid Rain and Fossil Fuels

Important Atmospheric Gases

When was the last time you thought about the earth's atmosphere? It may not be on your mind very much. We can't really see the atmosphere. We breathe in air from the atmosphere because our brains tell our bodies to do so without us even realizing it. But we probably don't think much about the important gases out there. So, let's take a moment to consider them.

We know there is oxygen floating around out there, since we must breathe it in to survive. There is also carbon dioxide, which is a gas that we exhale as a waste product. But do you know which gas makes up almost 80% of the earth's atmosphere? It may come as a surprise to you that the answer is nitrogen gas, or N2.

Nitrogen is an element that is essential to living organisms. Most of the nitrogen on earth is found in the air. However, unlike oxygen gas, most living creatures cannot take nitrogen gas from the atmosphere and utilize it in their bodies as needed. Luckily, there is a process called nitrogen fixation that takes care of this process. In this lesson, we will look at the three ways that nitrogen from the atmosphere is 'fixed' so that plants and animals can use this very important element.

What Is Nitrogen Used For?

Before we look at the specifics of nitrogen fixation, let's take a look at why living organisms need nitrogen in the first place. Nitrogen is an important building block for every living creature. Amino acids, which are compounds that make up proteins, contain nitrogen as one of their molecular components. Nitrogen is also a key factor in DNA and RNA, which are the genetic blueprints of every living organism on the planet. Life as we know it simply could not exist without the presence of nitrogen.

With so much nitrogen floating around in our atmosphere, it would seem that we should be able to simply breathe it in for our bodies to use, like oxygen. But this is not the case. As mentioned previously, nitrogen is found in our atmosphere in the form of gas, or N2. This means that there are two nitrogen atoms bonded together. The bond between those atoms is very strong, and organisms don't have the means to break that bond in order to use individual nitrogen atoms. This is where nitrogen fixation comes in.

An Overview of Nitrogen Fixation

Nitrogen fixation is a fancy way of saying that nitrogen atoms in N2 are broken apart and then converted into other compounds that can be utilized by living organisms. Let's look at a common example of some other elements that must be changed in order for us to use them. Table salt, NaCl, is made from sodium (Na) and chlorine (Cl). By themselves, these would never be elements that we would ingest. Sodium is a soft metal, and chlorine is a highly poisonous gas. However, when combined into the compound NaCl, they make a substance that we eat on a daily basis.

In order for organisms to use nitrogen, it must first be incorporated into compounds that are usable. These include ammonia (NH3) and nitrates (NO3). Once converted, plants are able to take up these nitrogenous compounds through their roots and use them in their stems and leaves. Then animals eat the plants, and they are able to utilize the nitrogen within their bodies. How does this nitrogen fixation happen?

Three Forms of Nitrogen Fixation

Fixation by Lightning

There are three major ways in which atmospheric nitrogen is fixed. The first that we will discuss is fixation by lightning. Now, this may seem like something out of a science fiction movie. But in reality, a flash of lightning barreling through the atmosphere has a profound effect on nitrogen gas. The massive energy provided by the lightning flash blasts apart the N2, allowing nitrogen atoms to recombine with atmospheric oxygen. Nitrates (NO3) are formed, which then dissolve in rainwater. It falls to the ground and is released into soil to be readily used by plants and micro-organisms.

Industrial Fixation

The second way that atmospheric nitrogen is fixed is through industrial fixation. This is human-caused nitrogen fixation. Nitrogen can be fixed in a laboratory by combining N2 with hydrogen (H) at high temperatures and under great pressure. The bond between the nitrogen atoms is broken and the atoms recombine with hydrogen to make ammonia. This ammonia is then used as a key ingredient in fertilizer, which is used on crops.

Biological Fixation

The last process of nitrogen fixation is biological fixation. This happens with the help of microorganisms in the soil such as bacteria. This process accounts for about 60% of all nitrogen fixation. Certain bacteria living in soil possess an important enzyme called nitrogenase. Nitrogenase has the ability to convert N2 into ammonia (NH3) by adding hydrogen. You may recall that breaking up that N2 molecule requires a lot of energy. The bacteria uses an energy source called ATP to make that happen. Within the soil, an extra hydrogen is added to ammonia to make ammonium (NH4), which is taken up by the plants.

There is another unique form of biological nitrogen fixation that happens with members of the legume family. Legumes are plants such as beans, peas, and soybeans. They have roots with specialized growths on them called nodules. Nitrogen-fixing bacteria actually make their home inside these nodules. Like a tenant, the bacteria lives inside the nodule and pays 'rent' by fixing nitrogen for the plant. The plant supplies room and board, including food sources for the bacteria. This symbiotic relationship works out very well for both.

Root Nodules of Legume Plant
Nodules on Roots


Nitrogen fixation is the process by which nitrogen gas from the atmosphere is converted into different compounds that can be used by plants and animals. There are three major ways in which this happens: first, by lightning; second, by industrial methods; finally, by bacteria living in the soil. Nitrogen is taken up by roots of plants, and organisms that eat the plants are able to use the nitrogen to build amino acids and proteins.

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