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12.22: The Nitrogen Cycle

Difficulty Level: At Grade Created by: CK-12
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What can bean plants do that most other plants can't?

No, they don't grow giant stalks to the clouds. Bean plants and other legumes (plants that have their seeds in pods) can use the nitrogen in the air to grow. It takes the help of special bacteria friends in the soil, and this relationship is unique to the legumes.

The Nitrogen Cycle

Like water and carbon, nitrogen is also repeatedly recycled through the biosphere. This process is called the nitrogen cycle. Nitrogen is one of the most common elements in living organisms. It is important for creating both proteins and nucleic acids, like DNA. The air that we breathe is mostly nitrogen gas (N2), but, unfortunately, animals and plants cannot use the nitrogen when it is a gas. In fact, plants often die from a lack of nitrogen even through they are surrounded by plenty of nitrogen gas. Nitrogen gas (N2) has two nitrogen atoms connected by a very strong triple bond. Most plants and animals cannot use the nitrogen in nitrogen gas because they cannot break that triple bond.

In order for plants to make use of nitrogen, it must be transformed into molecules they can use. This can be accomplished several different ways (Figure below).

  • Lightning: Nitrogen gas can be transformed into nitrate (NO3-) that plants can use when lightning strikes.
  • Nitrogen fixation: Special nitrogen-fixing bacteria can also transform nitrogen gas into useful forms. These bacteria live in the roots of plants in the pea family. They turn the nitrogen gas into ammonium (NH4+). In water environments, bacteria in the water can also fix nitrogen gas into ammonium. Ammonium can be used by aquatic plants as a source of nitrogen.
  • Nitrogen also is released to the environment by decaying organisms or decaying wastes. These wastes release nitrogen in the form of ammonium.

Ammonium in the soil can be turned into nitrate by a two-step process completed by two different types of bacteria. In the form of nitrate, nitrogen can be used by plants through the process of assimilation. It is then passed along to animals when they eat the plants.

Sending Nitrogen back to the Atmosphere

Turning nitrate back into nitrogen gas, the process of denitrification, happens through the work of denitrifying bacteria. These bacteria often live in swamps and lakes. They take in the nitrate and release it back to the atmosphere as nitrogen gas.

Just like the carbon cycle, human activities impact the nitrogen cycle. These human activities include the burning of fossil fuels, which release nitrogen oxide gasses into the atmosphere. Releasing nitrogen oxide back into the atmosphere leads to problems like acid rain.

The nitrogen cycle includes assimilation, when plants absorb nitrogen; nitrogen-fixing bacteria that make the nitrogen available to plants in the form of nitrates; decomposers that transform nitrogen in dead organisms into ammonium; nitrifying bacteria that turn ammonium into nitrates; and denitrifying bacteria that turn nitrates into gaseous nitrogen.


  • acid rain: Precipitation or deposits with a low (acidic) pH.
  • assimilation: Process of incorporating nitrogen from the soil into plants.
  • denitrification: Process of converting nitrates in soil back into nitrogen gas.
  • nitrogen cycle: Pathways through which nitrogen is recycled through the biosphere.
  • nitrogen fixation: Process of converting nitrogen gas in the air into nitrates in the soil.


  • Gaseous nitrogen is converted into forms that can be used by plants during the process of nitrogen fixation.
  • Denitrifying bacteria turn nitrate back into gaseous nitrogen.


Use the resources below to answer the questions that follow.

  1. What is the difference between nitrite and nitrate?
  2. What is the largest source of nitrogen on Earth? How does this nitrogen enter the food web?
  3. What kind of relationship exists between nitrogen-fixing bacteria and the plants whose roots they live around?
  4. What is assimilation? Describe the "loop" in the Nitrogen Cycle that involves assimilation by animals?
  5. At what step in the Nitrogen Cycle do bacteria assimilate nitrogen?
  1. What metabolic processes do organisms use nitrogen (N) for?
  2. What sorts of molecules are made using nitrogen? What other atoms go into making these molecules?
  3. What process increases when the free oxygen in soil is used up? Who carries out this process?


  1. What is nitrogen fixation? Describe how it happens.
  2. How is nitrate in the soil converted back to nitrogen gas?

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acid rain

Precipitation or deposits with a low (acidic) pH.


Process of incorporating nitrogen from the soil into plants.


Process of converting nitrates in soil back into nitrogen gas.

nitrogen cycle

Pathways through which nitrogen is recycled through the biosphere.

nitrogen fixation

Process of converting nitrogen gas in the air into nitrates in the soil.

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Difficulty Level:
At Grade
7 , 8
Date Created:
Nov 29, 2012
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