Nitrogen is used by living organisms to produce a number of complex organic molecules like amino acids , proteins , and nucleic acids . The atmospheric store is about one million times larger than the total nitrogen contained in living organisms. Other major stores of nitrogen include organic matter in soil and the oceans. Despite its abundance in the atmosphere, nitrogen is often the most limiting nutrient for plant growth. Most plants obtain the nitrogen they need as inorganic nitrate from the soil solution .
Ammonium is used less by plants for uptake because in large concentrations it is extremely toxic.
Animals receive the required nitrogen they need for metabolism , growth, and reproduction by the consumption of living or dead organic matter containing molecules composed partially of nitrogen. Within in most ecosystems nitrogen is primarily stored in living and dead organic matter . This organic nitrogen is converted into inorganic forms when it re-enters the biogeochemical cycle via decomposition . This process is known as mineralization and it is carried out by a variety of bacteria , actinomycetes , and fungi . Nitrogen in the form of ammonium can be absorbed onto the surfaces of clay particles in the soil.