Many essential nutrients recycle through ecosystems. The nitrogen cycle is one example of this process. All organisms require nitrogen for proteins and nucleic acids. How does this element enter living systems? The atmosphere is about 80% nitrogen gas. Nitrogen is unavailable to plants and animals in this form. In terrestrial communities, nitrogen-fixing bacteria live in the soil or in nodules on the roots of some groups of plants. These bacteria convert nitrogen gas into ammonia, which they use to synthesize the nitrogenous compounds they require. Excess ammonia is released into the soil where nitrifying bacteria convert it into nitrite and nitrate. Nitrate is absorbed by plants and used to produce protein and other important molecules. Now the nitrogen can move through the food chain as plants are consumed by animals. When plants and animals produce wastes or die, the process of decomposition releases nitrogen into the soil as ammonia. There are also denitrifying bacteria. They convert nitrate into nitrogen gas, thus completing the cycle.