Nitrogen Fixation: Definition, Process, Examples, Types, & Facts

Importance Of Nitrogen

Nitrogen is a basic constituent of the makeup of all life on Earth. It is found in amino acids, proteins, enzymes, tissues and hormones. Nitrogen comprises about 78% of atmospheric gases; however, it needs to be fixed into a form usable by plants. This is a step necessary for the synthesis of proteins, DNA, and other important compounds.

The Nitrogen Cycle

The nitrogen cycle is the biogeochemical process by which nitrogen is changed into its various forms as it circulates through an ecosystem. This cycle maintains the nitrogen balance in the environment. The important stages of the nitrogen cycle involve:

• Nitrogen Fixation: The atmospheric nitrogen, N₂, is converted to ammonia, NH₃.

• Nitrification: The ammonia is transformed first into nitrites, NO₂, and then nitrates, NO₃.

• Nitrate Assimilation: Plants absorb nitrates to synthesize proteins.

• Ammonification: Organic nitrogen gets decomposed into ammonia.

• Denitrification: Nitrates are reduced back to atmospheric nitrogen in the air, completing the cycle.

What Is Nitrogen Fixation?

Nitrogen fixation is a chemical process of reformatting inert atmospheric nitrogen either into ammonia or related nitrogenous compounds, making them available to most living organisms. This process is important since nitrogen forms the basis for amino acids, proteins, and nucleic acids. While nitrogen makes up a huge part of the Earth's atmosphere, it needs to be changed into forms usable by plants and, indirectly, the animals that graze on them.

Mechanism Of Biological Nitrogen Fixation

Biological nitrogen fixation is done mainly by a few bacteria having the enzyme nitrogenase. This enzyme catalyzes the conversion of nitrogen gas to ammonia according to the following equation:

N₂ + 8H⁺+ 8e⁻ + 16ATP → 2NH₃ + H₂ + 16ADP + 16P_i

In this reaction, atmospheric nitrogen reacts with protons and electrons in the presence of ATP, which generates ammonia.

Types of Nitrogen-Fixing Bacteria

There are two major divisions regarding nitrogen-fixing bacteria:

Symbiotic Nitrogen-Fixing Bacteria

These bacteria are known to form symbiotic relationships with certain plants, especially legumes. Probably the most well-known example is Rhizobium, which infects the root hairs of leguminous plants and induces the formation of root nodules. Inside these nodules, the bacteria fix atmospheric nitrogen into ammonia, which is used by the plant for growth. In turn, the plant provides the bacteria with carbohydrates and a safe environment.

Non-Symbiotic Nitrogen-Fixing Bacteria

These bacteria do not associate with the plant and live freely in the soil. Examples include Azotobacter and Clostridium. They also fix atmospheric nitrogen and increase the amount of nitrogen available to plants in the soil.

Associative Nitrogen-Fixing Bacteria

These nitrogen fixing bacteria live in close association with the roots but they do not enter in the plant tissues. These bacteria, for example Azospirillum, colonise roots, fix atmospheric nitrogen and receive nutrients from the plants. This relationship is also called associative symbiosis.

Role of Legumes in Nitrogen Fixation

Leguminous plants have a symbiotic relationship with nitrogen-fixing bacteria. Hence, they are involved in nitrogen fixation. The formation of nodules on the roots allows for effective nitrogen uptake, hence the reason why legumes are included in crop rotation and sustainable agriculture. When legumes are planted, it enhances the natural amount of nitrogen in the soil, which means farmers do not need synthetic fertilizer to raise plants.

Factors Affecting Nitrogen Fixation

Several factors will determine how effective nitrogen fixation will be. Some of the factors include:

Applications Of Nitrogen Fixation

The applications of nitrogen fixation includes:

• Biofertilizers: Using bacteria like Rhizobium, Azospirillum and Anabaena in sustainable farming.

• Environmental Balance: Maintains the levels of nitrogen in the ecosystems.

• Agricultural Productivity: Reduces the use of chemical fertilizers.

• Soil Reclamation: Enhances the nitrogen levels in the soil and makes it suitable for growing crops.

Nitrogen Fixation NEET MCQs (With Answers & Explanations)

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

• Steps of the Nitrogen cycle

• Types of Nitrogen fixing bacteria

• Factors affecting Nitrogen fixation

Practice Questions for NEET

Q1. Which of the following organisms does not fix nitrogen?

1. Azotobacter

2. Nostoc

3. Spirogyra

4. Anabaena

Correct answer: 3) Spirogyra

Explanation:

Spirogyra is a genus of green algae that is not capable of fixing atmospheric nitrogen. Other than some cyanobacteria, other algae, for example, have the necessary enzyme, known as nitrogenase and the nif gene for fixation of nitrogen, Spirogyra lacks such. This places it on the disadvantageous side when it comes to contribution to the provision of nitrogen to an aquatic ecosystem, meaning it gets most of its nitrogen from external sources instead of fixing atmospheric nitrogen.

Hence, the correct answer is option 3) Spirogyra.

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. So the correct answer is "Azospirillum".

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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