The diagram below shows the nitrogen cycle.

The diagram illustrates the sequence of nitrogen. In general, the process includes a series of stages like a natural circle. We can see that nitrogen moves in a cycle through the air, plants and animals, and the soil, and back into the air. The natural cycle of nitrogen begins through the air. After that, the action of lightning and free-living nitrogen-fixing bacteria carries the nitrogen into the soil. At the same time, nitrogen-fixing bacteria nodules of leguminous plants take the nitrogen directly from the air. The nitrogen in the soil is taken up by non-leguminous green plants, while, leguminous plants derive nitrogen directly from the bacteria in nodules. Both types of plants are consumed by animals. Then, when animals and plants die, or the animals excreta their food, the nitrogen is returned to the soil, where it is acted on by dentrifying bacteria. This cause the nitrogen is returned to the air, and the cycle continues.