Nitrogen Cycle

Nitrogen is a critically important nutrient for organisms, being one of the most abundant elements in their tissues, and an integral component of many biochemicals, including amino acids, proteins, and nucleic acids.

The availability of biologically useful forms of nitrogen is a common limiting factor in the productivity of plants. This is especially true of plants growing in terrestrial and marine environments, and to a somewhat lesser degree in freshwater. Consequently, plants in many ecosystems will be more productive if the supply of available nitrogen is increased through fertilization. This is why fertilization is such a common practice in agriculture, and why nitrogen is by far the most commonly applied nutrient.

Most plants obtain their nitrogen by assimilating it from their environment, mostly as nitrate or ammonium dissolved in soil water that is taken up by roots, or as gaseous nitrogen oxides that are taken up by plant leaves from the atmosphere. However, some plants live in a symbiotic relationship with microorganisms that have the ability to fix atmospheric nitrogen (properly called dinitrogen) into ammonia, and these plants benefit greatly from access to an increased supply of nitrogen.

Almost all animals obtain the nitrogen they require by eating plants and assimilating the plant's organic forms of nitrogen, which are then broken down metabolically and used by the animal to synthesize their own necessary biochemicals. A few animals, however, can utilize inorganic sources of nitrogen. For example, ruminants such as the cow can utilize urea or ammonia, because microorganisms that live in their forestomachs can assimilate these inorganic chemicals and synthesize amino acids and proteins, which the cattle can then use.