Nitrogen Cycle – Definition, Steps, Importance

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  • The nitrogen cycle is arguably the second most critical cycle for living organisms, behind the carbon cycle.
  • Nitrogen is necessary for plant growth and hence contributes significantly to the human food chain, but its availability in the environment is heavily influenced by human activity.
  • Daniel Rutherford discovered nitrogen in 1772 and named the gas “noxious air.” Late in the eighteenth century, several chemists, including Scheele, Cavandish, Priestly, and Lavoisier, studied “dephlogisticated” air, the word for air lacking oxygen at the time.
  • By the early 20th century, the Haber–Bosch process was able to “fix” nitrogen from the atmosphere on an industrial scale. By the late 19th century, nitrogen’s critical significance as a plant nutrient was understood.
  • Nitrogen fixation affects the amount of available food in an ecosystem.
  • Prior to the industrial method of N generation, crop growth was perpetuated on the same land by recycling crop leftovers and manures.
  • By cultivating rice and legumes, or by mining guano and nitrate deposits, ‘new’ nitrogen was produced. In contrast, as the human population has grown, so has the demand for food and the reliance on inorganic fertilisers to sustain agriculture. This tendency has altered the global, national, and local nitrogen cycle.

Nitrogen Cycle Definition

The Nitrogen Cycle is a biogeochemical process that converts the inert nitrogen in the atmosphere into a form that is more accessible to living organisms.

  • Furthermore, nitrogen is an essential plant nutrient. However, plants and animals cannot directly utilise the copious nitrogen in the atmosphere.
  • Dinitrogen (N2) gas comprises approximately 79% of the Earth’s atmosphere and is inaccessible to living organisms. This reservoir is predicted to contain around 3,8 109 kg N, or roughly 90 percent of the world reservoir. Crustal reservoirs account for 10% of the remainder.
  • It involves multiple processes, including nitrogen fixation, nitrification, denitrification, decomposition, and putrefaction.
  • There are both organic and inorganic forms of nitrogen gas. Organic nitrogen is present in living species, and it is transferred along the food chain when other live organisms are consumed.
  • There are numerous inorganic forms of nitrogen in the atmosphere. Symbiotic bacteria convert the inert nitrogen into a form that plants can use, such as nitrites and nitrates.
  • Nitrogen undergoes numerous transformations to preserve the ecosystem’s equilibrium. The marine nitrogen cycle is among the most intricate biogeochemical cycles.
  • Nitrogen must be accessible in the form of inorganic formal ammonia (NH3), ammonium (NH4), nitrite, (NO2), or nitrate for plant growth (NO3). In the terrestrial nitrogen cycle, soil nitrogen cycling activities predominate, with most nitrogen inputs coming from surface application (fertiliser and manure).
  • Microbes decompose organic matter to provide the majority of the soil’s accessible nitrogen. It is then possible for mineralization/immobilization, nitrification, nitrate leaching, denitrification, and plant absorption to occur.
  • Nitrate is entirely soluble in water, and because it is not adsorbed to clay particles, it is susceptible to leaching by percolating rainfall or irrigation water.
  • Nitrogen can often migrate in one of three directions: (1) upward – crop absorption and gaseous loss, (2) downward – as leaching to groundwater, or (3) laterally – via surface and subsurface flow to surface waterways.
Nitrogen Cycle
Global Nitrogen Cycle

Stages of Nitrogen Cycle

Nitrogen fixation, nitrification, assimilation, ammonification, and denitrification are the steps in the Nitrogen Cycle. These processes occur in stages and are described in detail below:

Stages of Nitrogen Cycle
Stages of Nitrogen Cycle

1. Nitrogen Fixation Process

  • This is the first phase of the nitrogen cycle. In this process, atmospheric nitrogen (N2), which is predominantly available in an inert form, is transformed to ammonia (NH3).
  • During the process of nitrogen fixation, the inert form of nitrogen gas is deposited into soils mostly through precipitation from the atmosphere and surface waters.
  • Symbiotic bacteria known as Diazotrophs carry out the entirety of the nitrogen fixation process. Additionally, Azotobacter and Rhizobium play a significant part in this process. These bacteria contain the nitrogenase enzyme, which can combine gaseous nitrogen with hydrogen to produce ammonia.
  • Fixation of nitrogen can occur either through atmospheric fixation involving lightning, or through industrial fixation involving the production of ammonia under high temperature and pressure conditions. This can also be remedied by human operations, specifically industrial processes that produce ammonia and nitrogen-rich fertilisers.

Nitrogen Fixation Process

Plants are the primary dietary source. The nutrients derived from plants are produced by plants using various components from the atmosphere and soil. This group of elements also contains nitrogen. Plants extract nitrogen from the soil for use in protein synthesis. Unlike carbon dioxide and oxygen, leaves cannot absorb air nitrogen through their stomata. As a result of the inability of plants to directly utilise atmospheric nitrogen. Certain microbes and natural phenomena contribute to the fixation of nitrogen.

  • Atmospheric Nitrogen Fixation: Due to the high temperature present during lightning, the inert nitrogen in the atmosphere is transformed into nitrous oxide with the aid of lightning. The nitrogen is reduced to nitrogen atoms, which react with oxygen to produce nitrous oxide, nitrogen peroxide, and nitric oxide. These molecules eventually dissolve in rainwater to create diluted nitric acid. When diluted nitric acid reaches the Earth’s surface, it combines with the current alkalies to generate nitrates that are readily absorbed by plants.
  • Biological Nitrogen Fixation: Certain bacteria or prokaryotes can transform atmospheric nitrogen into ammonia. The term for this process is biological nitrogen fixing. Nitrogenase is an enzyme that transforms dinitrogen to ammonia. Nitrogen-fixing bacteria may be symbiotic or free-living.
    • Free-Living Bacteria: Free-living nitrogen fixers include Azotobacter, Beijernickia, Rhodospirillum, cyanobacteria, and others. Rhizobium (in the root nodules of legumes) and Frankia (in the root nodules of non-leguminous plants) are symbiotic nitrogen fixers.
    • Symbiotic Bacteria: Rhizobium is a bacterial species that aids in nitrogen fixation. These bacteria reside in the roots of leguminous plants (e.g., pea and bean plants) and aid in fixing nitrogen in the soil by utilising specific enzymes. During this biological process, the non-absorbable form of nitrogen is converted into an absorbable form. This kind of nitrogen dissolves in the soil, and plants absorb the soil’s changed nitrogen. This is why farmers practise crop rotation, in which leguminous plants replenish nitrogen levels in the soil without the need for fertilisers. An example of the symbiotic association between Rhizobium and leguminous plants is nitrogen fixing by bacteria. While bacteria are responsible for fixing nitrogen in the soil, plants provide them with food.
  • Industrial Nitrogen Fixation: The Haber process converts atmospheric nitrogen into ammonia, which is then transformed into nitrates for use in various fertilisers.
  • Nitrogen Fixation by Lightning: Lightning is another activity that aids in nitrogen fixation. It is a natural phenomena in which the non-absorbable form of nitrogen is converted into an absorbable form by the energy of lightning. Even though lightning’s impact to nitrogen fixation is minimal, it protects plants from scarcity of vital nutrients. Nitrogen oxides, such as NO, N2O, and NO2, are also created by industrial operations, automotive exhausts, power plants, and forest fires.

2. Nitrification

  • Nitrification is a natural process that nitrifying bacteria perform. It is a crucial phase in the soil’s nitrogen cycle.
  • Nitrification is a natural process carried out by specialised autotrophic bacteria in the environment. This aerobic mechanism converts ammonia to nitrites (NO2-) and ultimately to nitrates (NO3-).
  • Nitrosomonas refers to a group of specialised, gram-negative, rod-shaped, chemoautotrophic bacteria. Nitrosomonas is a rod-shaped genus of chemoautotrophic, gram-negative bacteria. These bacteria are responsible for the conversion of ammonia to nitrates.

Process of Nitrification

Nitrosomonas performs the microbiological process of nitrification. In this biological process, reduced nitrogen molecules are successively oxidised to nitrite and nitrate.

Primarily, two species of autotrophic nitrifying bacteria perform the nitrification process. Ammonia is predominantly converted to nitrate by soil-dwelling bacteria and other nitrifying bacteria.

Nitrosomonas, along with Nitrosococcus and Nitrosospira, is the most commonly recognised genus linked with this initial step of nitrification.

Step – 1

In the first stage of nitrification, ammonia is converted to nitrite by ammonia-oxidizing bacteria. The following is the response:

Ammonia (NH3) + Oxygen (O2) → Nitrogen Dioxide (NO2)- + 3 molecules of Hydrogen (3H+) + 2 electrons

Step – 2

In the presence of nitrite-oxidizing bacteria, nitrite is converted to nitrate during the second stage of nitrification. The following is the response:

Nitrogen Dioxide (NO2)- + Water (H2O) → Nitrate (NO3)- + 2 molecules of Hydrogen (2H+) + 2 electrons

Nitrobacter is the well-known genus that plays a crucial role in this second stage of nitrification. Other genera, including Nitrospina, Nitrospira, and Nitrococcus, are capable of autotrophic nitrite oxidation.

Factors Affecting the Nitrification Process

Multiple environmental conditions influence the rate of nitrification. These environmental variables consist of:

  • pH.
  • Temperature.
  • Transfer rate.
  • The type of media.
  • The level of the filter.
  • Dissolved oxygen.
  • Presence of inhibiting agents.
  • Wastewater BOD -Biochemical Oxygen Demand.

3. Assimilation

  • Primary producers – plants absorb nitrogen compounds from the soil with the aid of their roots. These nitrogen compounds are available in the form of ammonia, nitrite ions, nitrate ions, or ammonium ions and are used to make plant and animal proteins.
  • Thus, it enters the food system when plants are consumed by primary consumers.

Nitrogen Assimilation Process in Plants

  • Plants absorb nitrogen in the form of nitrates and ammonium ions from the soil. Plants absorb ammonium ions and nitrates via their respective transporters.
  • After absorption, nitrate is transferred to the leaves and converted to ammonia. In addition, ammonia is transformed into the amine groups of different amino acids.
  • The reduction of nitrate occurs in two phases. First, nitrate is converted to nitrite in the cytoplasm by nitrate reductase, and then nitrite is converted to ammonia in the chloroplasts by nitrite reductase. It is also found in the plastids of the roots.
  • This ammonia is combined with glutamate to produce glutamine. glutamine synthetase serves as the catalyst for the process. By transamination, similar amino acids to asparagine are produced. This pathway is known as glutamine synthetase-glutamate synthase (GS-GOGAT).
  • At the physiological pH, ammonia is protonated and transformed into ammonium ions. As ammonium ions are highly poisonous, plants cannot accumulate them. It is transformed into amino acids by a variety of methods.

Synthesis of amino acids from ammonium ion (NH4+) in plants

The production of amino acids from NH4+ occurs via two principal routes.

  1. Reductive Amination: The reaction between ammonium and 𝛂-ketoglutaric acid produces glutamate. The catalysing enzyme is glutamate dehydrogenase (GDH pathway).𝛂-ketoglutaric acid + NH4+ + NADPH → glutamate + H2O + NADP
  2. Transamination: In this process, the amino group of one amino acid is transferred to the other keto acid, resulting in the synthesis of another amino acid. The primary amino acid from which the transfer occurs is glutamic acid. The transaminase enzyme catalyses this process.

Aspartic acid and glutamic acid are modified by the addition of an amino group to produce asparagine and glutamine, respectively. These amides are transferred to other plant tissues via xylem arteries.

Some plants like soybean, transfer fixed nitrogen from nodules as ureides. It coincides with the rise of sap as a result of transpiration.

3. Ammonification

  • Plants are incapable of exploiting air nitrogen directly. A few bacteria aid in the transformation of atmospheric nitrogen into forms that plants can use.
  • Through their roots, plants absorb nitrates from the soil and convert them into proteins. When animals consume these plants, their bodies absorb the proteins.
  • The nitrogen contained in organic matter is returned to the soil when plants or animals die.
  • Decomposers, such as soil-dwelling bacteria or fungus, turn organic matter back into ammonium.
  • This breakdown process generates ammonia, which is then utilised in other biological activities.

Implications of Ammonification

  • Ammonification is the transformation of organic nitrogen into inorganic ammonia or ammonium ions. Organic nitrogen is the form of nitrogen found in living organisms’ components.
  • Ions, proteins, vitamin B, urea, and so forth are all examples of nitrogen-containing molecules found in living organisms.
  • In the process of ammonification, nitrogen is extracted from decomposing plant or animal matter and their waste products.
  • Nitrogen in an environment should exist in a form that can be utilised by living organisms. Here, ammonification plays a significant role, as it provides nitrogen to the soil in a way that allows plants to utilise nitrogen and pass it up the food chain.
  • Numerous plant species that thrive in acidic soils receive nitrogen most effectively by ammonification.
  • When fertilizers are added into the soil to increase ammonia levels, it might cause an overgrowth of algae, which results in toxicity of soil and imbalance in ecosystems.

Example of Ammonification

Bacillus, Proteus, Clostridium, Pseudomonas, and Streptomyces are examples of bacteria capable of producing ammonia.

4. Denitrification

  • The nitrogen cycle concludes with denitrification. The nitrogen cycle consists of living organisms fixing atmospheric nitrogen and then releasing it back into the atmosphere. Denitrification is the process of releasing nitrogen from living organisms into the atmosphere.
  • By converting nitrate (NO3-) to nitrogen gas, the nitrogen component is returned to the atmosphere through the process of denitrification (N).
  • Thiobacillus species and Pseudomonas bacteria present in the soil carry out the denitrification process in the absence of oxygen. Gram-negative bacteria breakdown nitrate molecules in soil and aquatic systems into nitrous oxide (N2O) and nitrogen gas, which are subsequently discharged into the atmosphere.
  • This process involves a wide variety of microorganisms; consequently, it is also known as the microbial process.
  • This biogeochemical process is one of the primary environmental responses to alterations in the oxygen (O2) concentration. Denitrification is a universal process that occurs naturally in regulated ecosystems – marine and freshwater habitats, tropical and temperate soils, wastewater treatment plants, aquifers, manure storage facilities, etc.
  • The final step of the nitrogen cycle is denitrification. It is a naturally occurring, microbially-mediated process in which denitrifiers utilise nitrate as a source of energy.
  • In this process, soil microorganisms transform plant-available soil nitrate (NO3–) into volatile nitrogen (N) gases. Denitrification generates many gases, including nitric oxide (NO), nitrous oxide (N2O), and dinitrogen (N2).
  • The process flowchart for denitrification is as follows: Nitrite  →  Nitric Oxide  →   Nitrous oxide  →  Nitrogen gas.

A succession of intermediate gaseous nitrogen oxide compounds are generated during this process, which is assisted by bacteria. Let’s gain a deeper understanding in the following areas:

  • Denitrification is the portion of respiration in which facultative anaerobes decrease the oxidised form of Nitrogen in reaction to the oxidation of an electron source, such as organic materials.
  • Nitrogen electron acceptors consist of nitrate, nitrite, nitric oxide, and nitrous oxide, which culminate in the formation of dinitrogen and the completion of the nitrogen cycle.
  • For energy, denitrifying microorganisms require a negligible amount of oxygen (less than 10 percent) and organic carbon.
  • The denitrification process is carried out by heterotrophic bacteria such as Paracoccus denitrificans and pseudomonas, as well as certain autotrophic denitrifiers such as Thiobacillus denitrificans. There is more than one enzymatic pathway involved in the reduction of nitrate to dinitrogen by multiple types of bacteria involved in denitrification.

Half Reactions in Denitrification

Denitrification often occurs by a mixture of the following half-reactions, with the enzymes involved being listed in brackets:

  • NO3− + 2 H+ + 2 e−→ NO₂− + H2O (Enzyme involved: Nitrate reductase)
  • NO₂− + 2 H+ + e− → NO + H2O (Enzyme involved: Nitrite reductase)
  • 2 NO + 2 H+ + 2 e− → N₂O + H2O (Enzyme involved: Nitric oxide reductase)
  • N₂O + 2 H+ + 2 e− → N₂ + H2O (Enzyme involved: Nitrous oxide reductase)

A net balanced redox reaction where nitrate (NO3) is completely reduced to dinitrogen (N2) can be expressed as follows:

2 NO3− + 10 e− + 12 H+ → N2 + 6 H2O

Factors affecting the denitrification process

The entire denitrification process is affected by the following variables:

The soil’s organic matter is the most influential factor in the denitrification process. The sole source of sustenance for the bacteria is the organic stuff present in the soil. Therefore, soil bacteria require a source of readily available organic matter, whether it comes from plants, the soil, or other sources.

Other variables include

  • Soil pH.
  • Soil texture.
  • Temperature.
  • Soil oxygen concentration.
  • The soil’s moisture content
  • Nitrate concentration in the soil.

Nitrogen Cycle in Marine Ecosystem

Similar to the nitrogen cycle in terrestrial ecosystems, the nitrogen cycle in marine ecosystems is same. The sole difference is that marine microorganisms are responsible for nitrogen fixing in the marine ecosystem. How the cycle operates:

  • Nitrogen from precipitation, surface runoff, and the atmosphere all contribute to its entry into the oceans.
  • The subsequent process performed by cyanobacteria is nitrogen fixation.
  • The phytoplankton consumes the cyanobacteria and excretes ammonia and urea into the water.
  • Because of waste mixing or sinking, ammonia is injected at lower depths.
  • Bacteria convert ammonia to nitrates and nitrites.
  • Nitrates rise because to vertical mixing and upwelling, and phytoplankton absorbs them. In consequence, the cycle continues.
  • Dinitrogen is returned to the atmosphere as a result of the process of denitrification.
Nitrogen Cycle in Marine Ecosystem
Nitrogen Cycle in Marine Ecosystem

Nitrogen as a limiting nutrient

Many activities in natural ecosystems, including primary production and breakdown, are constrained by the availability of nitrogen. In other words, nitrogen is frequently the limiting nutrient, i.e., the nutrient that is in the smallest supply and so restricts the growth of organisms or populations.

How do we determine if a nutrient is scarce? This is frequently evaluated as follows:

  • Adding more of a limiting nutrient will increase plant growth; for instance, it will lead plants to grow taller than if nothing were supplied.
  • If a non-limiting nutrient is added, it will have no effect; for example, plants will grow to the same height regardless of the presence or absence of the nutrient.

Adding nitrogen to half of the bean plants in a garden and observing that they grew higher than untreated plants would indicate that nitrogen was limiting. If, however, we did not observe a difference in growth in our experiment, it would indicate that a nutrient other than nitrogen is limiting.

Nitrogen and phosphorus are the two most prevalent limiting elements in both natural and agricultural ecosystems. If you examine a bag of fertiliser, you will notice that it includes a great deal of nitrogen and phosphate.

Human activity affects cycling of nitrogen

  • We cannot fix nitrogen organically, but we do so industrially! Approximately 450 million metric tonnes of fixed nitrogen are produced annually using the Haber-Bosch process, in which N2 reacts with hydrogen at high temperatures.
  • The majority of this fixed nitrogen is used to create fertilisers for lawns, gardens, and agricultural areas.
  • Human activity mostly releases nitrogen into the environment through the combustion of fossil fuels and the use of nitrogen-containing fertilisers in agriculture. Both processes enhance atmospheric concentrations of nitrogen-containing molecules. The formation of acid rain (as nitric acid, HNO3) and contributions to the greenhouse effect (as nitrous oxide, N2O) are negative impacts linked with high amounts of atmospheric nitrogen other than N2.
  • In addition, when artificial fertilisers containing nitrogen and phosphorus are employed in agriculture, surplus fertiliser can be dumped into lakes, streams, and rivers through surface runoff. The principal effect of fertiliser runoff is eutrophication of both saltwater and freshwater. In this process, fertiliser runoff causes algae or other microorganisms to overgrow, or “bloom.” Nitrogen and phosphorus availability inhibited their growth in the absence of fertiliser runoff.
  • At night, eutrophication can lower the amount of oxygen in the water because algae and microbes that feed on them consume enormous quantities of oxygen for cellular respiration. This can result in low-oxygen, species-depleted areas known as dead zones, which can lead to the demise of other animals, such as fish and shrimp, inhabiting the affected ecosystems.

Bacteria play a key role in the nitrogen cycle.

  • Bacteria and other unicellular prokaryotes transform atmospheric nitrogen (N2) into physiologically useful forms through a process known as nitrogen fixing. Some nitrogen-fixing bacteria species are free-living in soil or water, whereas others are beneficial plant symbionts.
  • Nitrogen-fixing microbes sequester atmospheric nitrogen by converting it to ammonia (NH3), which plants can use to produce organic molecules. When plants are consumed by animals, the nitrogen-containing molecules are transferred to the animal’s system. They may be assimilated into the animal’s body or broken down and ejected as waste, such the urea in urine.
  • Nitrogen does not persist in the bodies of living beings indefinitely. Instead, microorganisms turn the organic nitrogen back into N2 gas. In terrestrial—land—ecosystems, this process usually involves multiple steps. Bacteria transform nitrogenous substances from dead organisms or wastes into ammonia (NH3), which is then turned into nitrites and nitrates. Prokaryotes transform nitrates into N2 gas by denitrifying them.

The Danger of Too Much Nitrogen

  • While the necessity of nitrogen to plant and animal life may make it seem as though there is no such thing as too much, there are in fact risks associated with adding excessive nitrates to the soil.
  • Nitrogen compounds can be harmful in excessive doses, just like any other substance. Just as excessive oxygen is poisonous to air-breathing organisms, excessive nitrogen can be damaging to plants.

In Humans

  • Nitrates can also be directly toxic to humans – when consumed in large quantities in food or water, nitrates can increase cancer risks and interfere with blood chemistry, leaving blood unable to properly carry oxygen.
  • Humans that consume high levels of nitrates in their food or water may have “blue baby syndrome.”

Within Ecosystems

  • The risk of destabilising ecosystems is a further significant concern. Some species may utilise nitrogen molecules to develop quicker than others; therefore, when there is an abundance of nitrogen in the environment, these organisms can grow so quickly that they harm other organisms.
  • Concerns have been expressed concerning the use of artificial nitrate fertiliser because, if it enters rivers, lakes, or even the ocean, it can lead to an explosion of plant growth.
  • More plant life may seem like a positive thing, but aquatic plants such as algae can block sunlight and oxygen from reaching other aquatic organisms and generate toxins that make humans and other animals ill.
  • Nitrate fertiliser in water systems has been implicated for some blooms of “red tides,” “brown tides,” and Pfiesteria bacteria, all of which create toxins that can make humans and other animals sick or even kill them.
  • How to maintain the fertility of fields without the use of nitrate fertilisers is still being researched by scientists. It is believed that, one day, farmers may be able to achieve high crop yields without adding large quantities of synthetic nitrates to the soil by employing sustainable procedures involving natural or genetically modified nitrogen-fixing plants.

Examples of the Nitrogen Cycle

The Story of Thanksgiving

  • According to the legend of the first Thanksgiving, the pilgrims celebrated their first harvest in the New World by feasting with the natives. But why was this crop significant enough to warrant a celebration? And why was it crucial that the Native Americans and European settlers ate together?
  • When European settlers arrived in the Americas, they had little knowledge on how to subsist. Having toiled on farms in England for centuries, the pilgrims expected farming in America would be similar. It found out that was not the case. It was difficult for the pilgrims to cultivate or locate enough food to last them through the winter.
  • One of the reasons for this was that the soil where the pilgrims landed contained little nitrogen. Their crops did not fix nitrogen, and they had not imported any large animals.
  • This was a significant issue, as dung was a common source of fertiliser in the ancient world. After unsuccessfully attempting to cultivate crops in American soil, the American Indians taught the Europeans how to fix their issues.
  • By burying dead fish in their agriculture fields, the pilgrims replenished the soil with nitrogen from the fish’s proteins and nucleotides. As a result, their crops grew, and the first European settlers learnt how to survive in the New World from the American Indians.

The Three Sisters

  • Some Native American tribes customarily cultivate corn, beans, and squash together.
  • This crop combination, sometimes known as “the three sisters,” is ingenious for multiple reasons. The combination of these three plants supplies humans with proteins containing all of the required amino acids.
  • For another, it contains beans, a nitrogen-fixing plant.
  • The roots of the beans include Rhizobium nodules, which contain bacteria that transform atmospheric nitrogen into a form that soil microorganisms and, ultimately, plants can utilise.
  • Similar to burying fish in the fields, cultivating beans with maize and squash prevents the soil from becoming too exhausted to support fresh plant growth. Even a single crop of maize or squash can benefit from nitrogen-fixing beans’ Rhizobium bacterium, which enriches the surrounding soil!

Artificial Fertilizer

  • Humans initially began fertilising crops with natural resources containing nitrogen, such as dead fish and animal excrement. These animal byproducts comprised proteins, amino acids, and nucleotides that were essential for the growth of soil microorganisms and plants.
  • Today, humans have developed industrial techniques that convert ammonia into nitrates in a manner identical to that of soil bacteria. These nitrates can be used directly by plants, and they can be produced in enormous amounts by human industry.
  • Unfortunately, human influence on the nitrogen cycle results in environmental changes that can have unforeseen repercussions. Artificial nitrates can foster the growth of “bad” plants and algae that create toxins and outcompete other types of life.
  • This is especially hazardous when precipitation carries artificial fertilisers from farms and lawns into rivers and lakes. The result may be the formation of poisonous algae that can suffocate wetland habitats and contaminate drinking water.

Importance of Nitrogen Cycle

This is the significance of the nitrogen cycle:

  • Assists plants in producing chlorophyll from nitrogen molecules.
  • Through a metabolic mechanism, aids in the transformation of inert nitrogen gas into a form plants can use.
  • In the process of ammonification, microorganisms aid in the decomposition of animal and plant debris, which indirectly contributes to environmental cleanup.
  • Nitrates and nitrites are released into the soil, which enriches the soil with the essential nutrients for plant growth.
  • Nitrogen is an essential component of the cell and is a component of numerous essential chemicals and biomolecules.

Human actions such as the combustion of fuels and the usage of nitrogen fertilisers also contribute to the nitrogen cycle. These mechanisms enhance the concentration of molecules containing nitrogen in the atmosphere. The nitrogen-containing fertilisers are washed into lakes and rivers, resulting in eutrophication.


  • Nitrogen is plentiful in the atmosphere, but plants and animals cannot utilise it until it is transformed into nitrogen compounds.
  • Nitrogen-fixing bacteria play a critical role in converting atmospheric nitrogen into plant-usable nitrogen compounds.
  • Through their roots, plants absorb useable nitrogen molecules from the soil. These nitrogen molecules are then utilised by the plant cell to produce proteins and other chemicals.
  • Animals absorb nitrogen through ingesting nitrogen-containing plants and other animals. Proteins from these plants and animals are consumed by humans. The nitrogen is then assimilated into our body’s system.
  • In the last phases of the nitrogen cycle, bacteria and fungi aid in the decomposition of organic matter, resulting in the dissolution of nitrogenous compounds into the soil, which are then reabsorbed by plants.
  • Some microorganisms in the soil subsequently transform these nitrogenous substances into nitrogen gas. It eventually returns to the atmosphere.
  • The continual repetition of these sets of activities maintains the nitrogen concentration in the atmosphere.


Where does denitrification occur?

Denitrification is a microbiological process that removes beneficial nitrogen from soil and releases the greenhouse gas nitrous oxide (N2O) and tropospheric pollutant nitric oxide (NO). The biological cycle of denitrification involves an enzyme cascade that converts nitrate to dinitrogen.

Why does denitrification occur?

When the soil’s oxygen (O2) supply becomes depleted, a variety of microorganisms use oxygen for respiration instead of nitrate. Denitrification happens most frequently in wet, moist, or flooded soil when the oxygen supply for respiration is limited or restricted. Some fungi are capable of denitrification, however they are regarded insignificant.

When does the Denitrification process occur?

Denitrification is more active in locations where the percentage of water-filled soil pores reaches 60 percent. The end-product gas is contingent on the conditions of the soil and the microbial community. As the oxygen deficiency grows, bacteria fulfil their jobs by converting an increasing amount of nitrate to dinitrogen (N2) gas. Denitrification occurs in the loss of valuable nitrogen (N) for the purposes of nutrient management, but the effects on the atmosphere will vary.

What is nitrification?

The process by which bacteria convert atmospheric nitrogen into nitrites and nitrates is known as nitrogen fixation. It is utilised by plants, which then incorporates it into the food chain.

Name some denitrifying microorganisms?

Microorganisms that denitrify include Thiobacillus denitrificans and Micrococcus denitrificans.

What is the effect of denitrification?

Denitrification decreases the quantity of soil nitrogen that is fixed.

Why is nitrogen important for life?

Nitrogen constitutes several cellular components and is vital in many biological functions. For instance, the amino acids include nitrogen and serve as the building blocks for hair, tissues, and muscles, among other components of the human body.

Why do plants need nitrogen?

Nitrogen is required by plants since it is an essential component of chlorophyll. As a result, chlorophyll is essential for photosynthesis, and a lack of nitrogen can result in deficiency illnesses such stunted growth and other abnormalities.

What are the steps of Nitrogen Cycle?

  1. Nitrogen Fixation
  2. Assimilation
  3. Ammonification
  4. Nitrification
  5. Denitrification

What is Ammonification?

During the degradation of organic matter, ammonifying bacteria convert organic nitrogen into inorganic compounds such as ammonia and ammonium ions.

What is Nitrification?

Nitrification is the process by which bacteria convert ammonia to nitrate. The bacteria Nitrosomonas, Nitrococcus, etc., convert ammonia to nitrite (NO2), which is then transformed to nitrate (NO3–) by Nitrobacter.

What is Denitrification?

Denitrification is the process by which bacteria such as Pseudomonas, Thiobacillus, Bacillus subtilis, etc. transform nitrate into molecular nitrogen.

What is the function of nitrifying bacteria?

Nitrifying bacteria are a tiny category of aerobic bacteria primarily responsible for the transformation of ammonia into nitrates.

Which part of the plant is involved in nitrogen fixation?

Nitrogen fixation occurs spontaneously in the root nodules of the plant in the soil.


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