You may have studied the water cycle or the rock cycle in school. These are just two examples of cycles.
Very simply, when scientists talk about cycles, they are talking about sequences of events that repeat themselves. Some cycles are very simple. For example, the seasons of the year represent a cycle in that they always repeat – Winter, Spring, Summer, Fall, and then back to Winter!
The nitrogen and carbon cycles are very complex cycles. They fit into the category called biogeochemical cycles.