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, in turn, maintains the nitrogen balance in the environment. Precisely, 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+16Pi

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:

1. 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.

2. 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, too, fix atmospheric nitrogen and increase the amount of nitrogen available to plants in the soil.

The 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 that Affect Nitrogen Fixation

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

1. Soil pH: This should be within ideal levels, usually between 6.0 and 7.5, to ensure optimum activity of the bacteria.

2. Moisture Levels: It must have adequate moisture to help the bacteria thrive and the biochemical process ensues accordingly.

3. Temperature: Nitrogen-fixing bacteria need specific temperatures to attain their optimum activity.

4. Nutrient Availability: Availability of other nutrients can impact the whole health status of the plant or its ability to support nitrogen-fixing bacteria.

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