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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 ).

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 may use up so much oxygen in the water that nothing else can grow. Soon, even the algae die out. Decomposers break down the dead tissue and 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.

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: http://www.youtube.com/watch?v=pdY4I-EaqJA (5:08).


  • dead zone : Zone in an ocean or lake where organisms cannot live because there is little or no oxygen.
  • nitrogen cycle : Movement of nitrogen through the atmosphere, soil, and biosphere.


  • 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.


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 nitrates?
  5. How is nitrogen released from the soil?


  1. Describe how nitrogen is fixed.
  2. How do dead zones come about?
  3. Why does the Gulf of Mexico contain such a large dead zone?

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