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Climate science

The nitrogen cycle

Published on
September 8, 2021
Quick summary
  • Nitrogen is essential to life and is responsible for plant health and the formation of DNA and RNA
  • The nitrogen cycle is a process by which nitrogen moves through living and non-living things, changing its forms into other nitrogen compounds while doing so
  • Humans disrupt the nitrogen cycle by burning fossil fuels that release nitric oxides into the air and have 300x the warming effect of carbon dioxide
  • The Nitrogen Cycle: 1. Ammonification turns ammonia into ammonium to turn dead matter into a form plants can absorb. 2. Nitrification involves bacteria converting the form of nitrogen into nitrates which are used by plants. 3. Denitrification completes the cycle with nitrogen being sent into the atmosphere where it is turned into nitrogen gas. 


Have you ever heard of the nitrogen cycle? Although it may seem like the less popular cousin of the carbon cycle, the nitrogen cycle is another essential ingredient to life on our planet. Nitrogen is a key element in the production of amino acids and proteins. Plants need as much nitrogen as they do carbon. When they lack this particular element, they turn yellow and their growth is stunted. Even in humans, nitrogen is responsible for the formation of DNA and RNA - the actual essence of us as people! [1] However, as impressive and beneficial this element is to us, humans have been disrupting the nitrogen cycle with our repeated use of, you guessed it, fossil fuels. Nitrous oxide, a greenhouse gas that is 300 times more damaging than carbon dioxide, combines with other elements and as a result, produces things like acid rain and toxic smog [2]. To understand the importance of nitrogen to life on earth, we will be going through each step of the cycle to hopefully give a better understanding of why we need to preserve it. 


Step 1: Nitrogen Fixation and Ammonification

The majority of the nitrogen is only available in the earth’s atmosphere. So how exactly does this get to the ground? Well, that is where nitrogen fixation comes in - a process by which this element is transformed into a form which plants can absorb through their roots. This happens when nitrogen becomes either ammonia, nitrates or nitrites through the use of light energy and ultraviolet rays. This process then produces two things: nitrogen oxide and nitrogen dioxide - two compounds that are brought to the ground via snow and rain. [3]  But, nitrogen can also be transformed into ammonia through the broken down remains of dead plants and animals. Microorganisms feed on the decomposed matter and as a result ammonia is released. This is called ammonification.[4]


Step 2: Nitrification

This stage has two steps which involve the transformation of ammonia and ammonium into other forms of nitrogen available for plants and animals. First, the soil bacteria called Nitrosomonas and Nitrococcus convert ammonia into nitrites. This is not so useful to plants so it is once again transformed into nitrate by another bacteria called Nitrobacter. These changes can only happen when oxygen is present. This whole process is important as it produces an extra stash of nitrogen that is absorbed by the plants through their roots.[4]


Step 3: Assimilation

Once the nitrogen has settled into the soil and transformed into forms which plants can absorb, these compounds are then soaked up by the plants through a process called assimilation [4]. This is an especially important step as this is the point where nitrogen enters the plant and becomes possible to be passed down the food chain. 


Step 4: Denitrification

The final stage of the nitrogen cycle entails nitrates being sent into the atmosphere once again and converted into atmospheric nitrogen, as it once was. This happens through denitrification - a process where nitrate is converted into nitrogen gas which removes the bioavailable nitrogen and thus, sends it into the atmosphere instead. From here, the cycle is repeated once again. [5]

Sources for reference.

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