The nitrogen cycle is the biogeochemical process by which nitrogen is converted into various chemical forms as it circulates among the atmosphere, terrestrial, and marine ecosystems. Although nitrogen gas (N2) makes up about 78 percent of the atmosphere, most organisms cannot use it in this form. It must first be 'fixed' into a usable form like ammonia through a process called nitrogen fixation. This is primarily carried out by specialized bacteria living in the soil or in the root nodules of certain plants like legumes. Once fixed, the nitrogen is taken up by plants and incorporated into proteins and DNA. When animals eat these plants, the nitrogen is transferred to them. Eventually, through a process called denitrification, other bacteria convert nitrates back into nitrogen gas, which is then released back into the atmosphere, completing the cycle.