The nitrogen cycle converts atmospheric nitrogen into a usable form for plants and animals through the action of bacteria. Humans have become an important part of the cycle through synthetic nitrogen fertilizers. Some plants and animals have a special relationship with nitrogen-fixing bacteria.
The nitrogen cycle is a process in which nitrogen from the atmosphere is converted into a form that can be used by plants and animals. This occurs through the action of bacteria and, since the 20th century, human activity. When nitrogen is converted into its usable form, it is said to be fixed, and plants and algae incorporate the nitrogen into amino acids, proteins, and deoxyribonucleic acid (DNA). Animals obtain nitrogen-containing compounds from plants, making the nitrogen cycle essential for all life on earth. When living creatures die, other types of bacteria release the nitrogen from these substances into the atmosphere, completing the cycle.
In the N2 form, nitrogen makes up about 80% of the Earth’s atmosphere. This form of nitrogen cannot be used by plants or animals that rely on it. Bacteria are needed to convert N2 into ammonia (NH3) and ammonium ions (NH4+). In a process called nitrification, soil bacteria convert ammonia into the nitrate ion (NH3). This part of the nitrogen cycle, known as nitrogen fixation, allows plants to produce the amino acids and other nitrogen-containing compounds on which all animal life depends. A very small amount of fixed nitrogen is generated each year by lightning and some non-living chemical processes.
To complete the nitrogen cycle, organic matter from dead plants and animals is broken down by another class of bacteria. This process, which releases the fixed nitrogen, is called denitrification. Nitrogen re-enters the atmosphere in its original N2 form, ie as ammonia.
Thanks to an important scientific discovery, humans no longer have to rely on the available nitrogen produced by bacteria. This has led to synthetic nitrogen fertilizers, which are heavily relied upon in agriculture to feed substantial numbers of people around the world. In this way, humans have become an important part of the nitrogen cycle. It has been estimated that up to 50 percent of fixed nitrogen in the environment exists as a result of human activity.
Some plants and animals have a special relationship with nitrogen-fixing bacteria. The roots of some plants, especially legumes, have nodules on the roots where bacteria generate nitrogen which can be used directly by the plant. In return, the bacteria obtain organic matter from plants, which they use for food. Some animals such as cows and buffalo also harbor nitrogen-fixing bacteria in their digestive tracts, which produce a substantial amount of the nitrogen-containing compounds that animals need.
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