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The gas nitrogen (N2), composed of molecules of two nitrogen atoms, occupies 78% of the Earth's atmosphere. It is colorless, odorless, and tasteless. Nitrogen is as important as it is common. It's essential to the nutrition of plants and animals. Nitrogen is a constituent in all proteins and in the genetic material (DNA) in all organisms.

The low content of nitrogen in most soils exists in stark contrast to the abundance of nitrogen in air. This is because gaseous nitrogen molecules have very strong bonds linking the atoms together, making the gas chemically stable and unusable by most biological organisms. Some species of bacteria absorb nitrogen from the air and convert it into ammonium, which plants can use. This process, called nitrogen fixation, is the principal natural means by which atmospheric nitrogen is added to the soil. Legumes, such as beans, can fix nitrogen from the atmosphere. This is accomplished by nitrogen-fixing bacteria living in nodules on the plant roots.

Nitrogen molecules in the atmosphere can also be broken by the energy generated by lightning strikes and volcanic action. Whenever lightning flashes in the atmosphere, some nitrogen combines with oxygen and forms the gas nitric oxide (NO). This nitric oxide is converted to nitric acid, which is highly soluble in water and falls to the ground in rainwater, to be absorbed by soils. Globally, however, nitrogen-fixing bacteria are a far more significant source of fixed nitrogen.