Nitrogen is a major component of amino acids, which are the building block molecules of proteins and enzymes. Since molecular nitrogen contains a triple bond between the atoms, it is a very stable molecule and therefore, biologically inert. A large amount of energy is required to break the triple bond, and lightning is responsible for converting some of the atmospheric nitrogen into forms that organisms can use. The process of converting atmospheric nitrogen into forms that organisms can use is called nitrogen fixation . Other bacteria convert ammonia to nitrates through nitrification .
Most plants use nitrogen in the form of nitrates.
Nitrogen is also fixed by symbiotic bacteria that live in and among the root cells of several types of plants, most notably, the legume plants such as beans, peanuts, and peas. Other plants such as alfalfa, locust, and alders also have root nodules. They contribute to the nitrogen-fixing process as well. Once nitrogen is absorbed by plants and built into the plant molecules, the nitrogen can be passed to consumers and to decomposer organisms through the food chain. Nitrogen can be mineralized and converted to organic compounds that enter the soil or water upon their death, or enter as waste through their digestive tracts.