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Which of the Main Gases in the Air Is Not Used Directly by Animals or Plants?

Although often mistakenly referred to solely as oxygen, the air all animals on Earth breathe is composed of 78 percent nitrogen, 21 percent oxygen and one percent other trace gases, including carbon dioxide and argon. Nitrogen, the most abundant gas in the atmosphere, is also one of the most critical elements required for life, however, its atmospheric form cannot be used directly by plants or animals. It is necessary for atmospheric nitrogen to be converted into a more useable form before it can be used.

Nitrogen Fixation
- Specialized bacteria found in the soil perform nitrogen fixation, which is the process of converting atmospheric N2, which is chemically inert, into usable forms of nitrogen, such as ammonium and nitrates. Nitrogen fixation also occurs as a result of lightning strikes. Nitrogen that has been fixed is stored in the soil where it can be used by plants.
Nitrification
- Additional bacteria in the soil are responsible for converting ammonia in the soil into nitrates in a process called nitrification. Ammonia is a different compound than ammonium and cannot be used by plants. This is why the bacteria in the soil must oxidize it to form nitrates, which the plants can use to form proteins.
Denitrification
- Denitrification is the reverse of nitrogen fixation and nitrification and it also occurs via bacteria in soil and seawater. The end result of denitrification is the release of molecules of N2 into the environment. The reason denitrification is necessary is that nitrogen is essential to the atmosphere, and if too much has been fixed or nitrified, there will not be enough in the atmosphere. Converting ammonium and nitrates back into atmospheric N2 ensures the levels are balanced.
Decay
- Decaying plant and animal life along with waste materials are broken down by bacteria and fungi in the soil. The products of the decay process are mainly proteins and urea. The decomposing bacteria and fungi convert the nitrogen in these compounds to ammonia and ammonium. Plants can utilize the ammonium ions immediately, while the ammonia molecules must go through nitrification before plants can use them to build proteins. In areas where animals and plants are not left to decay, such as on farmlands, natural or artificial fertilizers containing nitrates or ammonium are required for the plants to grow.

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