The document summarizes the nitrogen cycle, which is a process by which nitrogen is converted between its various chemical forms and circulated among air, land, water, and living organisms. Nitrogen is essential for all life and comprises 70-71% of the atmosphere. It enters organisms through eating, drinking, and breathing and is converted to forms such as ammonium, nitrates, and urea that can be used by plants and animals. The nitrogen cycle is linked to the water cycle as nitrogen is absorbed by plants from the soil and returns to the atmosphere.