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 happens through the action of bacteria, and beginning in 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 in these substances back into the atmosphere, completing the cycle.
In the form N2, nitrogen makes up about 80 percent of Earth's atmosphere. This form of nitrogen can't be used by plants or the animals that rely on them.. Bacteria are required to convert the 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 that all animal life depends on. A very small amount of fixed nitrogen is generated annually by lightning strikes and some non-living chemical processes.
To complete the nitrogen cycle, the organic matter of dead plants and animals is broken down by another class of bacteria. This process, which releases the fixed nitrogen, is called denitrification. The nitrogen re-enters the atmosphere in the original form of N2, or as ammonia.
Due to an important scientific discovery, humans no longer have to rely on the available nitrogen produced by bacteria. This resulted in synthetic nitrogen-based fertilizers, which are heavily relied upon in agriculture to feed a substantial number of the world's people. In this way, humans have become an important part of the nitrogen cycle. It has been estimated that as much as 50 percent of the fixed nitrogen present in the environment exists due to human activity.
Some plants and animals have a special relationship with nitrogen-fixing bacteria. The roots of some plants, notably legumes, have nodules on their roots where bacteria generate nitrogen that can be used directly by the plant. In return, the bacteria get organic substances from the plants, which they use as food. Some animals like cows and buffalo also host nitrogen-fixing bacteria in their digestive tracts, which produce a substantial amount of the nitrogen-containing compounds the animals need.