Nitrogen Cycle

Nitrogen (N) can occur in many chemical forms in the environment. Organic nitrogen refers to a very diverse array of nitrogen-containing organic molecules, ranging from simple amino acids through proteins and nucleic acids, to large and complex molecules such as humic substances in soil and water. A smaller number of inorganic forms of nitrogen occur in the environment, but these are very important ecologically. The most prominent of the inorganic molecules of nitrogen are:

1. Dinitrogen (N₂). Sometimes this molecule is less accurately called "nitrogen," although that term should properly be restricted to nitrogen atoms. Dinitrogen is a gas, and it is very unreactive because its two nitrogen atoms are held together by a relatively strong, triple bond. About 79% of the volume of Earth's atmosphere consists of dinitrogen, but because of its almost inert character few organisms can directly use this gas in their nutrition. As is explained below, N₂ must be "fixed" into other forms before it can be assimilated by most organisms.
2. Nitrate (NO 3 ). This is a negatively charged ion (or anion), and is highly soluble in water. Nitrate is the preferred form of nitrogen nutrition for most species of plants. If nitrate is not assimilated by plants or microorganisms, it is readily leached from soils.
3. Ammonia (NH₃) and ammonium (NH ⁺ 4 ). Ammonia usually occurs as a gas, vapor, or liquid, while ammonium is a positively charged ion (or cation), formed by the addition of a hydrogen ion (H⁺) to ammonia. Ammonium is soluble in water, although it is also electrochemically attracted to negatively charged surfaces associated with clays and organic matter in soil, and is therefore not as mobile as nitrate. Plants that are adapted to acidic soils can utilize ammonium as their source of nitrogen nutrition, although most species of non-acidic soils can only utilize nitrate.
4. Nitric oxide (NO). This gas is emitted to the atmosphere mostly as a result of combustions. The NO of combustions is generated by two mechanisms: (a) oxidation of the organic nitrogen of the fuel of biomass, oil, or coal, and (b) combination of atmospheric dinitrogen with oxygen under conditions of high temperature and pressure, as occurs, for example, in the internal combustion engine of automobiles. NO is produced in large quantities by both of these reactions, and it is typically emitted into the atmosphere.
5. Nitrogen dioxide (NO₂) and its products. In the atmosphere, emitted NO is oxidized to NO₂, which is therefore considered to be a secondary, non-emitted, gas. NO and NO₂ are often considered together, as NO_x. In the atmosphere, gaseous NO_x may be directly taken in by plant foliage, or it may be oxidized to nitrate. The nitrate may combine with atmospheric cations, such as ammonium, to form an ammonium nitrate salt. Fine aerosols of ammonium nitrate particulates may directly deposit in ecosystems, or they may serve as condensation nuclei for the formation of ice crystals, which may eventually become snowflakes or raindrops that are delivered to ecosystems as precipitation. By these mechanisms, biologically useful forms of fixed nitrogen are deposited from the atmosphere. It should also be mentioned that atmospheric nitrate can also combine with hydrogen ion, and can occur as a dilute solution of nitric acid. This is the reason why nitrate derived from NO_x is such an important contributor to acidic precipitation (see entry on acid rain).
6. Nitrous oxide (N₂O). The major source of atmospheric N₂O is denitrification occurring in wet, nitrate-rich soils. N₂O is a relatively unreactive and persistent gas in the atmosphere. However, N₂O is very slowly oxidized to NO_xgases under the influence of sunlight and atmospheric catalysts.

Depending upon environmental and biological conditions, all of these various organic and inorganic compounds containing nitrogen can be variously transformed. The diverse manners in which the movements and transformations of these chemicals occur is known as the nitrogen cycle.