The nitrogen cycle represents the circulation and recycling of the chemical element nitrogen in nature.

The nitrogen cycle depends on the action of some specialized bacteria. Bacteria of the soil called nitrogen-fixing bacteria present in plant roots absorb molecular nitrogen from the air and liberate nitrogen under the form of ammonia. The decomposition of organic material also produces ammonia. In the soil and roots (mainly of leguminous), a first group of chemosynthetic bacteria called nitrifying bacteria, the nitrosomonas, produces energy consuming ammonia and releasing nitrite (NO2). The second group of nitrifying bacteria, the nitrobacteria, uses nitrite in chemosynthesis releasing nitrate (NO3). In the form of nitrate, nitrogen is then incorporated by the plants to be used as constituent of proteins and nucleic acids and the element then follows along the food chain. Nitrogen returns to the atmosphere by the action of denitrifying bacteria that use nitrogen-containing compounds from the soil and release nitrogen gas (molecular nitrogen).

Image Diversity: the nitrogen cycle