An important chemical cycle, consisting of a complex series of processes that circulate nitrogen between the atmosphere and animals, plants, and soil. Nitrogen in the atmosphere may be converted into nitrogen oxides by lightning discharges; these are taken up by the soil and form nitrates that are then absorbed by plants and eventually by animals. In addition, nitrogen-fixing bacteria associated with the roots of certain plants convert the element into a form that may be absorbed. Other bacteria break down nitrogen compounds in dead animals and plants, or their wastes, and return it to the soil or, in some cases, to the atmosphere. See also carbon cycle.