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What is the Nitrogen Cycle?

Nitrogen is all around us. We commonly think of the air that we breathe as being mostly oxygen. In fact it’s actually a mix of gasses and nearly 80% of that mix is nitrogen. Nitrogen in the atmosphere comes in a chemical shape called diatomic, meaning that two nitrogen atoms are bonded together. This bond is one of the strongest on Earth. Think of the two molecules as powerful magnets. When aligned in the right shape they stick together and it’s very hard to pull them apart.

Nitrogen, a Cornerstone of Life

Nitrogen is also one of the building blocks of life on this planet. It is one of the major components of amino acids the building blocks of protein. Without protein our bodies could not make muscle. Nitrogen also helps form nucleic acid a vital component of DNA. DNA is the genetic code that creates the blueprint to make the many different types of cells our bodies needs to survive. From the tiniest insect to the largest Blue Whale, DNA guides the formation and growth of these creatures. Without nitrogen life as we know it couldn’t exist.

What is the Nitrogen Cycle?

We mentioned in an earlier segment of this webpage about the power of the diatomic bond between two nitrogen molecules and how it was one of the strongest bonds on Earth. But somehow, nitrogen has to get from the atmosphere into a form that our body can use. Life on this planet cannot use nitrogen in its most common N2 form. Through one of the most important biogeochemical cycles on the planet called The Nitrogen Cycle this important nutrient is “fixed”.

nitrogen cycle

Nitrogen is all around us. We commonly think of the air that we breathe as being mostly oxygen. In fact it’s actually a mix of gasses and nearly 80% of that mix is nitrogen.


The nitrogen molecule is very inert. The molecules are happy in their N2 form and would like to stay that way. So breaking them apart requires a lot of energy, in a process called fixation. This energy occurs on Earth in these main places: the atmosphere, in tiny microbial bacteria, or with the help of industry.

Atmospheric Fixation

Atmospheric fixation is probably the most exciting of the three processes. It occurs when the enormous energy of lightning breaks apart the nitrogen molecules allowing them to bond with oxygen in the air forming nitrogen oxides. These chemicals dissolve easily into rain that is then carried down to the Earth.

Biological Fixation

Only a very select few bacteria and archaea (think of these as very primitive bacteria) have the ability to fix nitrogen. Some of these bacteria live symbiotically with plants like soybeans and alfalfa. Cyanobacteria, sometimes called blue green algae, fix nitrogen in aquatic environments. Others might establish a relationship with trees like alders. Still others live inside the bodies of insects like termites or live freely in the soil.

These critters contain a complex set of enzymes called nitrogenase, the only known biological elements capable of breaking the powerful triple bond of the nitrogen molecule. Imagine this enzyme like a lock where only a certain key fits. Nitrogenase has two keyholes, one for nitrogen and one for hydrogen. When the two elements fit into the keyholes, the nitrogenase enzymes break the bond between the two nitrogen molecules and connect them with hydrogen. Using tremendous amount of energy called ATP, nitrogenase squeezes together one nitrogen molecule and three hydrogen molecules forming two weak ammonia molecules (NH3).


Although ammonia can be used by some plants, most plants use nitrite (NO2) or into Nitrate (NO3). Bacteria species like Nitrobacter and Nitrosomas bacteria – generally called nitrifying bacteria – convert ammonia in the soil into this more usable form.

Nitrogen Assimilation

Plants take up nitrogen either as ammonia, nitrate, or nitrite via their roots. Animals get their nitrogen from the food they consume, passing the nitrogen farther and farther up the food chain. Ammonia is also a byproduct of metabolism, the process by which creatures generate energy from food. Ammonia is toxic and if too much builds up in the body it can be fatal. So creatures have developed a clever way to remove nitrogen from their bodies. In mammals and other animals, the kidneys produce a chemical called urea, made up of carbon dioxide, nitrogen and hydrogen. For most birds and reptiles, ammonia is excreted in the form of uric acid.


Nitrogen is also returned to the system by decay otherwise called nitrogen mineralization. As creatures eat food, some of the nitrogen is deposited as waste which is broken down by microbes. When plants and animals die their nitrogen is returned to the soil as ammonia by microbes and bacteria through the process of decay.


Not all the nitrogen that gets fixed is used by plants and animals some of it is returned to the atmosphere through a process called denitrification. This process occurs on land thanks to bacteria called Pseudomonas and Clostridium. In anaerobic conditions, better known as the absence of oxygen, these bacteria use nitrate as an electron acceptor in place of the oxygen during respiration. Effectively, they breathe nitrate and exhale nitrogen gas back into the atmosphere.


Nitrogen is naturally contained in the soil and rocks. Rain, wind, exposure to the atmosphere, and friction slowly degrade these rocks through a process called weathering. The nitrogen is then carried by wind, rain, or another mechanism to other places where it can be used by plants and animals alike.


Leaching is the process by which nitrogen that is contained in the soils is removed via water. Nitrates, nitrites and ammonia are all water soluble meaning that they easily dissolve into water. Remember back to the earlier segment on atmospheric fixation. The nitrogen oxides created by lightening dissolved in rain, well the same process occurs in the soil. Nitrogen trapped in the soil dissolves into the ground or rain water and is washed into larger bodies of water. When too much nitrogen leaches into a water-body like Barnegat Bay, the systems natural balance is disrupted. Eutrophication can occur.

Industrial Fixation
Industrial fixation occurs with the help of some very intense machines. In a blast furnace heated between 400-500 degrees centigrade and under extreme pressure nitrogen and hydrogen are combined to form ammonia, the principal component in fertilizer and explosives.

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