We often hear about how oxygen is an important gas that we cannot live without. While it is certainly vital (breathing would be difficult without this element), nitrogen is too. It is important because nitrogen is an essential nutrient needed to help sustain many living organisms. It is a building block for proteins and genetic material such as DNA and RNA. However, most of the nitrogen in the atmosphere is in an unusable format. That's where the importance of the nitrogen cycle comes in, this describes the process of converting nitrogen into different chemical forms. These nitrogen forms circulate between the terrestrial environment, marine environment and the atmosphere and are converted through biological and physical processes into a usable form.
The nitrogen cycle diagram
The main processes in the nitrogen cycle include nitrogen fixation, ammonification, nitrification and denitrification.
Different organisms can access different forms of nitrogen.
Fig. 1: The nitrogen cycle.
The steps in the nitrogen cycle
The nitrogen cycle is broken down into key steps:
Nitrogen fixation
Nitrogen fixation refers to the conversion of nitrogen gas (N2) to nitrogen-containing compounds. This process occurs naturally via lightning, artificially by the Haber-Bosch process, or specialised nitrogen-fixing microorganisms.
Nitrogen fixation by lightning
In the atmosphere, two nitrogen atoms in the nitrogen molecule are held together by a strong triple covalent bond, i.e., the atoms share three pairs of electrons. Lightning carries enough energy to break the covalent bonds of atmospheric nitrogen. When the nitrogen molecule splits, it bonds to atmospheric oxygen, forming nitrogen oxides (NOx).
'x' in NOx refers to the number of atoms.
Nitrogen fixation by the Haber-Bosch process
Human activities have become a large source of nitrogen fixation. Burning fossil fuels produces fixed nitrogen, while industrial nitrogen fixation is a byproduct of the Haber-Bosch process to produce ammonia. During the process, nitrogen gas combines with hydrogen gas in a high temperature and a high-pressure environment. The ammonia produced is widely used in synthetic fertilisers.
Nitrogen fixation by microorganisms
Free-living nitrogen-fixing bacteria can transform atmospheric nitrogen into inorganic nitrogen forms that plants can access. Plants will use inorganic nitrogen to make amino acids.
Mutualistic nitrogen-fixing bacteria, such as Rhizobium, live in plant nodes (e.g., peas and beans) and acquire carbohydrates from the plant. In turn, they provide amino acids to the plant. The relationship between these bacteria types and the plants is symbiotic or mutualistic.
Symbiotic or mutualistic relationships occur when two organisms live in close physical proximity, and both organisms provide advantages to one another, such as with nitrogen-fixing bacteria and their associated plant.
Nitrification
Nitrification is a two-step oxidation reaction process converting ammonium ions into nitrate ions. The process involves:
Oxidation of ammonium ions (NH4+) to nitrite ions (NO2-) by Nitrosomonas and Nitrococcus bacteria.
Oxidation of nitrate ions (NO2-) to nitrate ions (NO3-) by Nitrobacter bacteria.
Oxidation involves the loss of electrons or the gain of oxygen.
Nitrifying bacteria gain energy through these oxidation reactions. Both of the reactions require oxygen to occur; therefore, to increase productivity within agriculture, the soil needs to be kept well aerated.
Assimilation
Assimilation occurs when plants and animals use nitrate ions and ammonia to make amino acids and proteins. Nitrate ions and ammonia are formed by nitrogen fixation and nitrification, as we have just read.
Amino acids are the molecules that make up proteins.
Ammonification
Ammonification involves the conversion of organic nitrogen (nitrogen found in the cells of living organisms) into ammonia. Saprobiotic organisms (decomposers) are the key players in this process as they feed on organic matter, break it down and release ammonia. This ammonia now becomes available for nitrification and assimilation processes as it can be converted into ammonium ions.
Denitrification
Denitrification refers to reducing nitrate ions to nitrogen gas by anaerobic bacteria. This process can only happen in anaerobic conditions (no or very little oxygen available).
For example, in wetlands, the soil tends to be waterlogged, which sets up the perfect environment for anaerobic bacteria as little oxygen is found in the soil.
Reduction involves the removal of oxygen or the gain of electrons.
Dissimilatory nitrate reduction to ammonium (DNRA) is another step in the nitrogen cycle. This process involves the production of ammonium ions (NH4+), catalysed by enzymes in anaerobic bacteria.
The formula for DNRA is:
Anaerobic ammonia oxidation
During this process, ammonium ions (NH4+) and nitrite (NO-2) are converted into atmospheric nitrogen (anammox reaction). This step is a significant process in oceans and needs to occur in anaerobic conditions (absence of oxygen).
The formula for this is:
Why is the nitrogen cycle important to plants and animals?
Nitrogen is one of the main elements used to build a life. It is a component of many cells and is a building block in amino acids, proteins and nucleic acids. In plants, nitrogen is also used to make chlorophyll, a critical pigment involved in trapping light for photosynthesis.
What is the human impact on the nitrogen cycle?
Disruption of the nitrogen cycle will lead to imbalances in the ecosystems.
For example, soil containing excess nitrogen is in danger of having low pH, meaning the soil will be too acidic. Soil with a low pH is harmful to bacteria and microorganisms that cannot survive in these conditions.
In the oceans and rivers, the accumulation of nitrogen run-off will lead to eutrophication. Eutrophication is the process whereby nutrient levels become excessive in water bodies.
You can learn more about this in our article Fertilisers.
Burning of fossil fuels
The burning of fossil fuels will release nitrogen and nitric oxides into the atmosphere. These actions affect air quality, leading to the production of acid rain and smog.
Use of nitrogen-containing fertilisers
Fertilisers increase plant growth and overall crop productivity. However, the excessive use of nitrogen-containing fertilisers creates a harmful imbalance of the soil's nutrient levels. These harmful effects can lead to:
Reduced species diversity. Nitrogen-rich soils will favour the growth of grasses and other rapidly growing vegetation. Rapidly growing vegetation will outcompete other species and reduce biodiversity.
Leaching. Nutrients will enter streams, rivers and oceans.
Eutrophication. This is a consequence of leaching as the excessive accumulation of nutrients harms microorganisms and wildlife in water bodies.
Removal of vegetation
Leaching is the removal of nutrients from the soil.
For example, removing plants and trees will remove nutrients with them.
In addition to this, the soluble nutrients left, such as nitrate ions, will dissolve in rainwater. These dissolved nutrients will be carried deeper into the soil and eventually, the nutrients will travel too deep into the soil for the plant to reach them.
Reducing the human impact
To reduce the adverse effects of human activity on the nitrogen cycle, individuals and companies need to be mindful of their nitrogen footprint. Changes such as choosing renewable sources can lead to the gradual reduction of the number of fossil fuels burned. Even choosing to eat less red meat can have a positive impact as cattle farming requires a large amount of feed, which is grown with fertilisers.
Nitrogen monitoring is another approach that the agriculture sector could benefit from. This describes the careful monitoring of nutrient levels in soil, minimising the toxic effects of ammonia build-up and, therefore, the effect of eutrophication from nutrient run-off.
Nitrogen cycle - Key takeaways
The nitrogen cycle is a nutrient cycle that converts nitrogen into multiple organic and inorganic compound forms. Nitrogen will cycle through the atmosphere and within the terrestrial and aquatic environments.
The main steps in the nitrogen cycle include nitrogen fixation, nitrification, denitrification, assimilation, ammonification, DNRA and anaerobic ammonia oxidation.
Nitrogen is a component of many cells and processes in living organisms. Nitrogen is a building block in amino acids, proteins and nucleic acids (DNA and RNA).
Human impacts cause a disruption in the nitrogen cycle. These effects include leaching, eutrophication and reduced species diversity.
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