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18.5: Nitrogen Cycle in Ecosystems

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    Why are these fish dead?

    In a dead zone, there is so little oxygen that fish can't live. What causes the oxygen to disappear? Indirectly, it's nitrogen.

    The Nitrogen Cycle

    Living things need nitrogen. Nitrogen is a key element in proteins. Like carbon, nitrogen cycles through ecosystems. The nitrogen cycle is pictured below (Figure below).

    Diagram of the nitrogen cycle

    The nitrogen cycle includes air, soil, and living things.

    Fixing Nitrogen

    Air is about 78 percent nitrogen. Decomposers release nitrogen into the air from dead organisms and their wastes. However, producers such as plants can’t use these forms of nitrogen. Nitrogen must combine with other elements before producers can use it. This is done by certain bacteria in the soil. It’s called “fixing” nitrogen.

    Human Actions and the Nitrogen Cycle

    Nitrogen is one of the most important nutrients needed by plants. That’s why most plant fertilizers contain nitrogen. Adding fertilizer to soil allows more plants to grow. As a result, a given amount of land can produce more food. So far, so good. But what happens next?

    Rain dissolves fertilizer in the soil. Runoff carries it away. The fertilizer ends up in bodies of water, from ponds to oceans. Nitrogen is a fertilizer in the water. Since there is a lot of nitrogen, it causes algae to grow out of control. Pictured below is a pond covered with algae (Figure below). Algae use up carbon dioxide in the water. After the algae die, decomposers break down the dead tissue. The decomposers use up all the oxygen in the water. This creates a dead zone. A dead zone is an area in a body of water where nothing grows because there is too little oxygen. There is a large dead zone in the Gulf of Mexico (Figure below). The U.S. states outlined on the map have rivers that drain into the Gulf of Mexico. The rivers drain vast agricultural lands. The water carries fertilizer from these areas into the Gulf.

    Pond covered with algae, and a dead zone in the Gulf of Mexico due to too much nitrogen

    The pond on the left is covered with algae because there is too much nitrogen in the water. The red-shaded area in the map on the right is a dead zone in the Gulf of Mexico. It’s called the hypoxic (“low oxygen”) zone in the figure.

    This very thorough video on the nitrogen cycle with an aquatic perspective was created by high school students.


    • Nitrogen is an essential part of many molecules needed by living organisms.
    • Nitrogen is fixed when it is changed into a form that organisms can use.
    • Dead zones come about when excess nitrogen in the water causes algae to grow out of control. Decomposers use oxygen to decompose the algae when they die. The lack of oxygen makes it impossible for other organisms to live in that zone.


    1. What is nitrogen fixing?
    2. Why is nitrogen fixing important?
    3. How do dead zones come about?
    4. Why does the Gulf of Mexico contain such a large dead zone?

    Explore More

    Use the resource below to answer the questions that follow.

    1. What percentage of the air is nitrogen?
    2. How is nitrogen removed from the air?
    3. What contributes nitrogen to the soil?
    4. What happens to soil nitrites?
    5. How is nitrogen released from the soil?
    6. Why is this called the nitrogen cycle?

    This page titled 18.5: Nitrogen Cycle in Ecosystems is shared under a CK-12 license and was authored, remixed, and/or curated by CK-12 Foundation via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.

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