Nitrogen Cycle: Definition, Diagram, Steps And Chart

What Is The Nitrogen Cycle?

A process in which nitrogen is converted into its forms of chemicals, allowing for movement of through the atmosphere, terrestrial, and marine ecosystems. It is key in nature to the sustenance of balance of nitrogen.

Nitrogen is critical because it is essential for all living organisms and has a basic existence within every living organism as part of amino acids, proteins, nucleic acids, and other cellular constituents. Nitrogen plays a very important role in the growth, reproduction and proper functioning of plants and animals.

There are many processes in the nitrogen cycle before nitrogen ultimately reaches back into the atmosphere: nitrogen fixation, nitrification, assimilation, ammonification, and denitrification. All these processes facilitate living things to convert nitrogen into usable forms and recycle it back into the atmosphere.

Role Of Nitrogen In Living Organisms

Nitrogen is an element of life that holds a very pivotal role in the formation of proteins, DNA, and chlorophyll. It is imperative for plant growth and agriculture as it is the most essential of amino and nucleic acids, important building blocks of life.

Nitrogen in plants is very important during photosynthesis and the transfer of energy. Nitrogen deficiency can lead to retarded growth, yellowing of leaves, and reduction in the yield of crops. This, in turn, can have massive effects on both food production and ecosystem health.

Steps Of The Nitrogen Cycle

The nitrogen cycle is the key process in the ecosystem that includes the transformation and movement of nitrogen in many different chemical forms. The nitrogen cycle includes nitrogen fixation, in which atmospheric nitrogen (N₂) is converted in the form of ammonia (NH₃) by biological agents, for example, legumes and Rhizobia, or through atmospheric processes such as lightning and industrial techniques.

The ammonia is then transformed into nitrites (NO₂^-) and nitrates (NO₃^-) having nitrifying bacteria of genera Nitrosomonas and Nitrobacter, which is termed as a process of nitrification.

Nitrogen Fixation

• Nitrogen fixation can be defined as the process of converting atmosphere-bound or free atmospheric nitrogen (N₂) into ammonia (NH₃) usable by plants.

• This is biologically done through the symbiotic function of the bacteria Rhizobia with legumes, which induces nodulation on plant roots.

• Atmospheric nitrogen fixation also occurs through lightning, while human activities include industrial processes such as the Haber-Bosch method.

• Fixation is a very important first step for making nitrogen available for use in living things. It sustains fertility in the soil and growth in plants.

Nitrification

• Nitrification is the biological oxidation of ammonia to nitrite, which at times is followed by the oxidation of nitrite to nitrate.

• Nitrification is mediated by nitrifying bacteria, mainly Nitrosomonas and Nitrobacter.

• Nitrification is a critical step in the nitrogen cycle because it takes ammonia, which is harmful to plants at high levels, to nitrates, forms taken up and used by plants with ease for growth and development.

Assimilation

• Assimilation involves plant roots taking up nitrates (NO₃^-) from the soil, later converting these forms of nitrogen into plant proteins and nucleic acids.

• The conversion is a type of process through which the activity of inorganic nitrogen occurs into organic molecules and helps the plants grow and develop.

• It is also an important process for allowing nitrogen to enter the food web and its entry into several trophic levels, and sum up to the productivity of the entire ecosystem.

Ammonification

• Ammonification is the breakdown of organic matter by decomposers, primarily bacteria or fungi, inside dead plants or animals, and then converting it back into ammonia (NH₃) or ammonium ions (NH₄⁺).

• The recycling of nitrogen in the ecosystem and again taken up by this process, ensures nitrogen enters the soil in useful form and it will continue to make the soil healthier and fertile.

Denitrification

• Denitrification is the step of the microbiological reduction of nitrates back to nitrogen gas and then releasing it into the atmosphere.

• It will be carried out by anaerobic denitrifying bacteria, such as Pseudomonas and Clostridium.

• This step completes the nitrogen cycle, returning nitrogen to the atmosphere and maintaining a balance of nitrogen levels in the soil so that excessive nitrates do not accumulate in the soil and thereby result in the pollution of soil and water.

Nitrogen Cycle In Marine Ecosystem

The nitrogen cycle in marine ecosystems includes:

Nitrogen Fixation

• It is performed by marine cyanobacteria like Trichodesmium.

• It covers most of the diazotroph activity.

• In the process, atmospheric nitrogen that we breathe in is reduced to ammonia or ammonium.

Ammonification

• Bacterial breakdown of organic matter.

• Converts organic nitrogen to ammonia.

Nitrification

• Ammonia is oxidized into nitrite.

• Chloride and sulphate reduction leading to the formation of hydrogen sulphide gas.

• Nitrate and ammonium assimilation by the phytoplankton and marine plants.

• Assimilated into amino acids and proteins.

Denitrification

• Nitrate NO₃^- is reduced into nitrogen gas N₂ by denitrifying bacteria.

• Generative habitable conditions with no oxygen provision

• Usually occurring in the sediments of most marine

Anammox (Anaerobic Ammonium Oxidation)

• The process by which ammonium NH₄⁺ is directly converted into nitrogen gas N₂ by anaerobic bacteria.

Importance Of Nitrogen Cycle

The nitrogen cycle is important for the following reasons:

Primary Production

• Nitrogen forms an essential nutrient for phytoplankton, which are the base of the marine food web.

• Supports the production of phytoplankton, which photosynthesize, produce oxygen, and are the beginning of the marine food chain.

Nutrient Cycling

• Provides a steady supply of nitrogen to the forms utilized by different marine organisms.

• This will result in the overall health and productivity of the marine ecosystems.

Biodiversity

• Sustain various marine life, where the necessary food supply comes from various organisms.

• An elaborate food chain is sustained and also many ecological niches.

Carbon Sequestration

• Enhance the capacity of the ocean as a carbon dioxide sink by an increase in phytoplankton growth.

• Part of the biological carbon pump that cycles carbon from the surface to the deep ocean.

Ecosystem Stability

• Regulates the levels of nutrients from spiraling out of control, maintaining a balance so as not to promote unmodified harmful algal blooms.

• Nitrogen fixation, nitrification, and denitrification processes are balanced.

Climate Regulation

• Nitrogen oxides are controlled for their amounts in this cycle to regulate atmospheric greenhouse gas levels.

• Regulates atmospheric nitrogen levels through ocean-atmosphere interplay.

Detoxification

• Sea areas are detoxified enough of excess nitrate by the nitrogen cycle's processes of denitrification among others.

• Open the likelihood of eutrophication and dead zones, which are disastrous conditions for most of life.

Sustainable Fisheries

• Highlight on fisheries production. This is by providing the nutrient reservoir required to support growth and reproduction.

• Maintains fish stock and any other marine resources.

Marine Sediment Health

• It plays a critical role in sediment health in that it recycles nutrients and organic matter.

• Sustains life of benthic organism and hence supports sediment stability.

Human Impact Mitigation

• It minimizes the human impacts of nitrogen pollution as it is a natural attribute of the nitrogen cycling system.

Human Impact on the Nitrogen Cycle

The human activities have several effects on the nitrogen cycle:

• Runoff from agriculture and sewage are the types of pollution that increase nitrogen

• Nitrogen is a cause of eutrophication and dead zones, leading to disturbance

• The marine nitrogen cycle is linked to terrestrial and atmospheric nitrogen cycles.

• Supply and demand of nitrogen compounds through ocean-atmosphere exchange influence global nitrogen dynamics.

Ways to Maintain Nitrogen Balance

Nitrogen can be balanced by the following steps:

• Promoting biofertilizers and crop rotation.

• Reduced use of chemical fertilizers.

• Using sustainable agriculture practices.

• Encourage wetland restoration to filter nitrates naturally.

Nitrogen Cycle NEET MCQs (With Answers & Explanations)

The key concepts to be covered under this topic for different exams are:

• Steps of the Nitrogen cycle

• Importance of Nitrogen cycle

Practice Questions for NEET

Q1. One of the free-living anaerobic nitrogen-fixer is

1. Azotobacter

2. Nostoc

3. Rhodospirillum

4. Rhizobium

Correct answer: 3) Rhodospirillum

Explanation:

Rhodospirillum is a free-living nitrogen-fixing bacterium. It is a facultative anaerobe, meaning it can adapt its metabolic processes based on the oxygen levels in its environment. In low-oxygen conditions, it uses alcoholic fermentation, while in high-oxygen conditions, it performs aerobic respiration. This metabolic flexibility allows Rhodospirillum to flourish in diverse environmental conditions and contribute to nitrogen fixation.

Hence, the correct answer is option 3) Rhodospirillum.

Q2. Minerals involved in carbohydrate translocation, maintaining ribosome structure and activation of nitrogenase respectively are

1. Mn, B, Ca

2. Ca, Mg, Mo

3. B, Mn, Mo

4. Cu, mg, B

Correct answer: 3) B, Mn, Mo

Explanation:

Minerals involved in carbohydrate translocation, maintaining ribosome structure, and activation of nitrogenase respectively are boron, manganese, and molybdenum.

Hence, the correct answer is option 3) B, Mn, Mo.

Q3. The common nitrogen-fixer in paddy fields is

1. Frankia

2. Rhizobium

3. Azospirillum

4. Oscillatoria

Correct answer: 3) Azospirillum

Explanation:

Plants cannot use nitrogen from the atmosphere. There are nitrogen-fixing bacteria that convert atmospheric nitrogen into ammonium compounds and supply them to plants. Examples include Azotobacter, Frankia, Rhizobium, and Azospirillum. It can be seen that Azospirillum is widely used in rice fields as a nitrogen-fixing organism. It is used as a bio-fertilizer in rice fields. It promotes plant growth.

Hence, the correct answer is option 3) Azospirillum.

Also Read:

• MCQs on Nitrogen Fixation and Nitrogen Metabolism

• Fate of Ammonia

• Essential Mineral Elements

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