Nitrogen fixation is essential for the creation of fundamental building components of plants, animals, and other living forms, such as nucleotides for DNA and RNA and amino acids for proteins and other biological substances. Despite the fact that nitrogen is the most prevalent gas in our environment, most organisms cannot directly utilise this gas in its molecular state.

This page will address questions like ‘What is the function of nitrogen on our planet?’, ‘Why do all living beings require it?’, nitrogen-fixing bacteria, and so on. Most importantly, we’ll understand what nitrogen fixation is, its types, and their role in plants. Read on to know more about nitrogen fixation in detail!

What is Nitrogen?

Before understanding the concept of nitrogen fixation, we need to understand one of the most important and most abundant gas of the atmosphere, ‘Nitrogen‘. Nitrogen is one of the primary nutrients vital for the survival of all living organisms. Although nitrogen constitutes about \(78\% \) of the Earth’s atmosphere, it is unavailable to the plants in the gaseous dinitrogen state. It needs to be converted into different forms like nitrites or nitrates through various processes. Atmospheric precipitation, geological sources, agricultural lands, livestock and poultry farms, mineralisation of organic matter, etc., are some sources of nitrogen and nitrogenous compounds.

Role of Nitrogen in Plants

1. Nitrogen is the key component of amino acids, which is the building block of proteins and enzymes.
2. It is an essential macronutrient for plant growth and function. 
3. As proteins make up the structural materials of all living matter, nitrogen plays a vital role in the structural makeup of all the cells of the living matter.
4. Major constituents of nucleic acids, vitamins and hormones.
5. Part of coenzymes, NAD, NADP and alkaloids and are required in large amounts by meristematic tissues.
6. Nitrogen is an important component of genetic and metabolic compounds in plant cells.

What is Nitrogen Fixation?

Nitrogen fixation is a process by which atmospheric nitrogen or molecular nitrogen is converted into related nitrogenous compounds like ammonia, nitrites and nitrates in the soil or aquatic systems. In a simpler way, nitrogen fixation is a process by which nitrogen gas is converted into inorganic nitrogen compounds. It is one of the important steps of the nitrogen cycle. The nitrogen cycle is a crucial nutrient cycle of the ecosystem. Following are some important pointers related to nitrogen fixation:

1. The process of nitrogen fixation is very important as the molecular form of nitrogen is of no use to plants and animals. 
2. It is beneficial for all living matter only when it is converted into a form which can be easily absorbed by the plants.
3. Nitrogen fixation occurs with the help of microorganisms as a part of the nitrogen cycle either by natural means or via industrial methods.
4. A very small amount of nitrogen is fixed by lightning or ultraviolet radiation, which reacts with the nitrogen gas in the atmosphere to produce nitric oxide.
5. Usually, nitrogen fixation takes in the root nodules of leguminous plants which inhabit certain bacteria (Rhizobium). Rhizobium facilitates the process of nitrogen fixation; these nitrogen-fixing bacterias live in symbiotic association with the plants.

Fig: Nitrogen Cycle

History of Nitrogen Fixation

1. The process of biological nitrogen fixation was discovered by the Dutch microbiologist Martinus Beijerinck and German agronomist Hermann Hellreigel.
2. In ancient times, during Roman civilisation, people had known that if the legumes were grown along with the non-leguminous plants, the growth is much higher than the usual growth. That is when crop rotation came into existence. But no one had known the factor that had brought about the increase in growth.
3. Hermann, for the first time, discovered that the roots of leguminous plants were the sites of nitrogen fixation where atmospheric nitrogen is converted into ammonium.
4. Later, Beijerinck discovered that there were certain bacteria that were responsible for fixing nitrogen and are known as nitrogen-fixing bacterias. He named them ‘rhizobia’. He also discovered the symbiotic relationship between the root nodules of leguminous plants and the bacteria.

What are the Types of Nitrogen Fixation?

Nitrogen fixation is carried by physicochemical and biological means. Only \(10\% \) of natural nitrogen fixation takes place by physicochemical means, whereas \(90\% \) is carried out by biological means. Thus, we can classify nitrogen fixation in following two types:

1. Non-biological or Physical nitrogen fixation
2. Biological nitrogen fixation

Fig: Flowchart for Nitrogen Fixation

Non-biological or Physical Nitrogen Fixation

Generally, physical nitrogen fixation takes place in the rainy season during lightning, thunderstorm and atmospheric pollution. This type of fixation can be discussed in further two divisions mentioned below:

1. Natural Nitrogen Fixation: Due to the influence of lightning (electrical discharge in the clouds) and thunder, the \({{\rm{N}}_2}\) and \({{\rm{O}}_2}\) in the atmosphere react to form nitric oxide \(\left( {{\rm{NO}}} \right){\rm{.}}\) Nitric oxide then oxidises again to form nitrogen peroxide \(\left( {{\rm{N}}{{\rm{O}}_{\rm{2}}}} \right){\rm{.}}\)
   The reactions involved are:

\({{\rm{N}}_2} + {{\rm{O}}_2}{\rm{(Lightning)}} \to {\rm{2NO\;(NitricOxide)}}\)

\(2{\rm{NO}} + {{\rm{O}}_2} \to 2{\rm{N}}{{\rm{O}}_2}\,{\rm{Oxidation\;(Nitrogenperoxide)}}\)

During rainfall, \({\rm{N}}{{\rm{O}}_2}\) in the atmosphere reacts with the rain water to form nitrous acid \({\rm{HN}}{{\rm{O}}_3}\) and nitric acid \({\rm{HN}}{{\rm{O}}_2}.\) When this acid rain falls in the ground, it is leached into the soil and reacts with alkaline radicals to form nitrates \(\left( {{\rm{N}}{{\rm{O}}^{3 – }}} \right)\) and nitrites \(\left( {{\rm{N}}{{\rm{O}}^{2 – }}} \right)\) which is directly used by the plants. Following the chemical reaction involved in the process:

\(2{\rm{N}}{{\rm{O}}_2} + {{\rm{H}}_2}{\rm{O}} \to {\rm{HN}}{{\rm{O}}_2} + {\rm{HN}}{{\rm{O}}_3}\)

\({\rm{HN}}{{\rm{O}}_3} + {\rm{Ca}}\,{\rm{or}}\,{\rm{K}}\,{\rm{salts}}\, \to {\rm{Ca}}\,{\rm{or}}\,{\rm{K}}\,{\rm{nitrates}}\)

Industrial Nitrogen Fixation: In the industrial process, ammonia is produced by the reaction of nitrogen which is produced as a result of certain reactions, with the hydrogen of water. The ammonia produced in this process is later used in the preparation of fertilisers.

Biological Nitrogen Fixation  

The process of conversion of atmospheric nitrogen into nitrogenous compounds by microorganisms such as bacteria, fungi and algae is known as Biological Nitrogen Fixation (BNF) or diazotrophy. Some prokaryotes such as bacteria and cyanobacteria that can fix atmospheric nitrogen are called nitrogen fixers or diazotrophs.

The atmospheric nitrogen is reduced to ammonia in the presence of a catalyst known as nitrogenase. This enzyme is found naturally in certain microorganisms like symbiotic (Rhizobium and Frankia) and non-symbiotic or-living (Azospirillum, Azotobacter and BGA).

Biological nitrogen fixation is carried out by two types of microorganisms, those which live in close symbiotic association with other plants and ones that are non-symbiotic or living. Hence, biological nitrogen fixation can be classified into two types based on the type of microorganism involved in the process. 

1. Symbiotic Nitrogen Fixation
2. Non-symbiotic Nitrogen Fixation

Let’s learn about these types in further detail.

Symbiotic Nitrogen Fixation

Symbiotic nitrogen fixation is an example of a mutualistic relationship between plants and microbes. Here, plants provide a niche and fixed carbon to microbes, and microbial partners fix atmospheric nitrogen. Such a relationship between microbe and host is mutually beneficial to both organisms.

The various types of biological symbiotic nitrogen fixation can be grouped under the following three categories: 

1. Nitrogen Fixation through Nodule formation 
2. Nitrogen Fixation through Non-nodulation

A. Nitrogen Fixation through Nodule Formation 

Let us first discuss nitrogen fixation through nodule formation in leguminous plants:

1. Symbiotic nitrogen fixers are found in a large number of legume plants, mainly the genus Rhizobium. 

2. They settle themselves inside specialised structures in the roots of the plants called root nodules. 

3. These bacterias can fix nitrogen only when they are present inside the nodules. 

4. Such association is regarded as symbiotic as the host plant supplies the nodule bacteria with the required organic carbon (carbohydrates). In return, microorganisms provide fixed nitrogen to the host plant.

5. Bradyrhizobium japonicum is a slow-growing symbiont of Soybeans. Azorhizobium caulinodans is a stem nodule forming symbiont in Sesbania species.

B. Nitrogen Fixation through Nodule Formation in Non-Leguminous Plants:

1. Many plant species from families other than Leguminosae are also known to produce root nodules. 

2. Some of the important examples of non-leguminous plants that produce root nodules and fix nitrogen include:

a. Genus Frankia forms root nodules in association with Alnus sp., Casuarina equisetifolia, Myrica gale, etc.

b. There is a single example of rhizobia nodulating a non-legume genus Parasponia, which is a woody member of the Ulmaceae family.

3. Bacteria such as Klebsiella in genus Psychotria and bacteria Burkholderia in genus Pavetta zimermanniana form leaf nodules.

Fig: Nitrogen Fixation Through Nodule Formation

Nitrogen Fixation through Non-nodulation:

There are few plants in which symbiotic nitrogen fixation takes place, but nodules are not formed. This type of association is known as Pseudo symbiotic or Pseudo symbiosis. Following are some examples:

a. Lichens, an association with fungi and algae (e.g., cyanobacteria or green algae)

b. Anthoceros (Bryophyte) associated with Nostoc.

c. Azolla (fern) in association with Anabaena.

d. Cycas, a Gymnosperm, in association with Anabaena or Nostoc, has blue-green algae in its coralloid roots.

e. Roots of Digitaria, Sorghum and Maise associated with Spirillum.

Fig: Lichens

Non-Symbiotic Nitrogen Fixation

Biological nitrogen fixation by microorganisms livingly or staying without direct contact with plant cells is called non-symbiotic biological nitrogen fixation. Soil contains several-living nitrogen-fixing microorganisms. These involve many aerobic and anaerobic bacteria and blue-green algae. 

Asymbiotic-living nitrogen-fixing microbes are pretty primitive. This fixation is a reduction process independent of respiration. This type of microorganism fixes nitrogen more actively under poor aeration in the absence of hydrogen gas.

The symbiotic nitrogen fixers can be classified into the following categories:

1. Free-living aerobic nitrogen-fixing bacteria
2. Free-living anaerobic nitrogen-fixing bacteria
3. Free-living chemosynthetic bacteria
4. Cyanobacteria or Blue-green algae
5. Free-living fungi 

Examples of these microorganisms are given below:

Type of Organism	Examples
Free-living aerobic nitrogen-fixing bacteria	Photosynthetic bacteria like Chlorobium, Chromatium. Non-photosynthetic bacteria like Azotobacter, Azomonas, Derxia, Beijerinckia, etc.
Free-living anaerobic nitrogen-fixing bacteria	Photosynthetic bacteria like Rhodospirillum, Non-photosynthetic bacteria like Clostridium.
Free-living chemosynthetic bacteria	Heterotrophic like Desulfovibro.
Cyanobacteria or Blue-green algae	Heterocyst bearing microbes like Nostoc, Anabaena, Rivularia, Calothrix.  Non-Heterocyst bearing microbes like Oscillatoria, Gloeocapsa, Lyngbya, Plectonema, etc.
living fungi	Yeasts and Pullularia

Summary

Nitrogen needs to be converted into different forms like ammonia, nitrites or nitrates through nitrogen fixation via various means as the molecular form of nitrogen is of no use to plants and animals. Nitrogen plays a vital role in the life of living organisms. Hence nitrogen fixation is a very necessary and crucial process. Nitrogen fixation is carried by physicochemical (only \(10\% \) and biological \(90\% \) means. Biological nitrogen fixation is mainly performed by a set of microorganisms such as bacteria, fungi, cyanobacteria, etc.

They convert atmospheric nitrogen into various nitrogenous compounds that can be readily used by plants. Plants are the source of nitrogen to animals when they consume them. Nitrogen is recycled continuously in its various forms through the nitrogen cycle in the atmosphere. The nitrogen cycle is one of the very important nutrient cycles; it ensures to balance of the amount of nitrogen in the atmosphere.

FAQs on Nitrogen Fixation

Q.1: What are the three types of nitrogen fixation?
Ans: 1. Biological nitrogen fixation
2. Atmospheric nitrogen fixation
3. Industrial nitrogen fixation

Q.2: What are the 7 steps of the nitrogen cycle?
Ans: 1. Nitrogen fixation.
2. Assimilation
3. Ammonification
4. Nitrification
5. Denitrification
6. Dissimilatory nitrate reduction to ammonium
7. Anaerobic ammonia oxidation

Q.3: How nitrogen fixation takes place in the soil?
Ans: Nitrogen fixation is carried out naturally in soil by microorganisms that are termed diazotrophs. These include bacteria such as Azotobacter and Archaea. Some nitrogen-fixing bacteria like Rhizobia have symbiotic relationships with plant groups, such as legumes.

Q.4: What is nitrogen fixation, and why is it important?
Ans: The process of fixing atmospheric nitrogen into the soil through various means such as lightning or by certain organisms is called nitrogen fixation. It is a very important phenomenon because nitrogen is the primary element required by all living organisms.

Q.5: What is an example of nitrogen fixation?
Ans: Nitrogen fixation in the root nodules of leguminous plants or by the symbiotic association of Azolla and Anabaena in the paddy fields are two examples of nitrogen fixation.

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