Earth atmosphere contains about 80% of nitrogen. It's an essential element for living beings.
Big amount of nitrogen is constantly being produced by the living creatures and an equal amount is being returned. This continuous circulation of nitrogen among the water, air, soil and living organisms is called as the nitrogen cycle.
The nitrogen cycle is a biogeochemical process that involves the conversion of nitrogen into various forms that can be used by living organisms. It is an essential process for life on Earth, as nitrogen is a key component of proteins and nucleic acids.
The nitrogen cycle can be divided into several stages:
Nitrogen fixation: This is the process by which atmospheric nitrogen (N2) is converted into ammonia (NH3) or nitrate (NO3-) by certain bacteria. Some bacteria live in the soil, while others live in the roots of certain plants (such as legumes) and form a symbiotic relationship with them. Lightning can also fix nitrogen by breaking N2 molecules apart and allowing them to react with oxygen (O2) in the air.
Nitrification: Once ammonia is produced through nitrogen fixation, it can be converted into nitrite (NO2-) and then nitrate (NO3-) by other bacteria. This process is called nitrification and occurs in two steps: first, ammonia is converted into nitrite by bacteria called Nitrosomonas; then, nitrite is converted into nitrate by bacteria called Nitrobacter.
Assimilation: Plants and animals can take up nitrate from the soil and use it to build proteins and nucleic acids.
Ammonification: When plants and animals die, the nitrogen in their tissues is converted back into ammonia by bacteria and fungi. This process is called ammonification.
Denitrification: Finally, denitrification is the process by which bacteria convert nitrate back into atmospheric nitrogen (N2), completing the nitrogen cycle. This process occurs in anaerobic (oxygen-poor) environments, such as wetlands, and removes nitrogen from the ecosystem.
The nitrogen cycle is crucial for maintaining healthy ecosystems and agriculture. Excessive use of nitrogen fertilizers can disrupt the natural balance of the nitrogen cycle and lead to environmental problems such as eutrophication (excessive growth of algae and other aquatic plants) and contamination of water sources.