Process diagram: The nitrogen cycle

The diagram shows how nitrogen circulates through the air, soil and living things in the nitrogen cycle.

Overall, the cycle has seven stages. Nitrogen from the air is made available to plants by bacteria, passes through animals, and is eventually returned to the atmosphere as nitrogen gas, so the cycle has no fixed beginning or end.

The cycle can be described as beginning with nitrogen fixation, in which atmospheric nitrogen is turned into ammonia by bacteria. Other bacteria then convert the ammonia into nitrates in a process called nitrification. These nitrates are absorbed from the soil by the roots of plants, which is known as plant uptake. When animals eat the plants, nitrogen is passed to herbivores and predators.

Nitrogen is returned to the soil in a different way. Through excretion and decay, waste and dead organisms release nitrogen. Decomposers then release ammonia in the stage of ammonification. Finally, bacteria convert nitrates back into nitrogen gas in denitrification, and the gas returns to the air, where the cycle starts again.