The nitrogen cycle is the biogeochemical cycle in which nitrogen is converted through both biological and physical processes into multiple forms as it circulates among the atmosphere, terrestrial ecosystems, and marine ecosystems. Important processes in this cycle include fixation, ammonification, nitrification, and denitrification. The earth's atmosphere consists of 78 percent nitrogen gas, but such nitrogen, which is not locked in compounds with other elements, has limited availability for biological use, thus contributing to a scarcity of usable nitrogen in many types of ecosystems.
Nitrogen fixation is the process in which nitrogen gas from the atmosphere is converted into compounds such as ammonia, nitrites and nitrates that can be used by plants. The main way in which it occurs is by the action of specialized bacteria in the soil and in water bodies. It is also caused by lightning and ultraviolet radiation breaking the bonds of atmospheric nitrogen molecules.
Human activity, particularly the burning of fossil fuels, the use of artificial nitrogen fertilizers, and the release of nitrogen in wastewater, all on a massive scale, has greatly altered the nitrogen cycle globally. Such alteration can negatively affect both the natural environment and human health.