Nitrogen is an important constituent of organic molecules, such as amino acids, proteins and nucleic acids. Nitrogen is recycled in nature by a natural phenomenon called the nitrogen cycle.
Nitrogen is mainly found in the atmosphere as a gas; other stores include organic matter in soil and water. Plants obtain nitrogen as inorganic nitrate (ammonium ion and nitrate ion) from the soil solution. Animals obtain nitrogen by consuming living or dead organic matter.
Nitrogen that enters plants and animals is known as organic nitrogen. Decomposition turns the nitrogen back into an inorganic form; decomposers in the upper layer of soil, such as bacteria and fungi, chemically modify the nitrogen found in organic matter, returning it to soil.
Once decomposed, the nitrogen seeps into clay particles of the soil (as ammonium), before undergoing a chemical alteration by autotrophic bacteria. The ammonium is then converted to nitrite, then to nitrate, in a process known as nitrification.
Nitrate is very soluble, and is lost from the soil as it flows to the oceans. The leached nitrate is then returned to the atmosphere by denitrification. Heterotrophic bacteria in anaerobic soil can also return nitrogen to the atmosphere by denitrification, by converting it into gases.
Humans have altered the nitrogen cycle by activities such as applying nitrogen fertilizer to crops, burning fossil fuels and ranching livestock.