Nitrogen is an essential element for both plants and animals, as it is a component of proteins, nucleic acids, and other vital biomolecules. Plants take up nitrogen from the soil in the form of ammonium (NH4+) or nitrate (NO3-) ions, and use it to synthesize amino acids and other nitrogen-containing compounds. Animals obtain nitrogen by consuming plants or other animals that have already incorporated nitrogen into their tissues, and use it to build their own proteins and other essential biomolecules.
Most of the nitrogen on Earth occurs in the atmosphere, where it exists as a gas (N2). However, N2 is relatively inert and cannot be used directly by most organisms. Instead, nitrogen must be "fixed" into a more biologically useful form, such as ammonium or nitrate, before it can be taken up by plants and animals. The vast majority of biologically available nitrogen is found in soil and marine sediments, where it has been converted into forms that can be used by living organisms. In the soil, most nitrogen is found in organic matter or in the form of nitrate ions, while in the ocean, it exists mainly as dissolved organic matter or as ammonium or nitrate ions.