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.
*Lightning: When lightening strikes, nitrogen gas is transformed into nitrate (NO3-) that plants can use.
*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+) (a process called ammonification). 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.
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.
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.