Nitrogen is required for plants and animals to grow and to survive. Plants and animals need nitrogen for the production of amino acids that make up proteins and DNA.
Each nucleotide in DNA is made up of a nitrogen-containing base attached to a sugar and a phosphate group. While plants get nitrogen from water and soil, animals get nitrogen by consuming plants and other animals. Aquatic plants get nitrogen by absorbing nitrates and ammonium. Most of the nitrogen on Earth is in the atmosphere, and it is not in a form that animals and plants can use. According to Windows to the Universe, lightning, fires and certain bacteria can break down atmospheric nitrogen for animals and plants to use.
During the nitrogen cycle, plants and animals decompose, adding nitrogen to the soil or to the water. The bacteria in the soil convert the nitrogen into inorganic forms that living plants and animals use. Bacteria can also convert nitrogen in water so that the nitrogen can return to the atmosphere. Nitrogen can enter this cycle from the atmosphere and from human activities such as pollution. Factories that manufacture nitrogen fertilizers and municipal waste treatment plants that produce fertilizer from sewage sludge contribute to the amount of nitrates in the nitrogen cycle.