The Machanism of Nitrogen Fixation in Plants and Soil: Types and Examples

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Nitrogen fixation in plants primarily occurs through a process known as symbiotic nitrogen fixation, involving a special relationship between plants and certain nitrogen-fixing bacteria. This process commonly takes place in the root nodules of leguminous plants, such as soybeans, peas, and clover.

Here's a detailed explanation:

1. Nodulation: The process starts when the roots of the leguminous plants release specific chemical signals called flavonoids. These flavonoids attract certain species of soil bacteria, collectively known as rhizobia. These bacteria have the ability to convert atmospheric nitrogen (N₂) into a form that is usable by plants (ammonia, NH₃).

2. Infection and Nodule Formation: The rhizobia bacteria enter the root hairs of the plant through the zone of elongation. Once inside, they multiply and move towards the root cortex, forming an infection thread. This thread grows and eventually reaches the inner cells of the root cortex.

3. Nodule Initiation: In response to the bacterial infection, the plant forms small bumps or swellings on the root called nodules. These nodules are a result of the plant's immune response to contain the bacteria.

4. Bacteroids and Nitrogen Fixation: Within the nodules, the rhizobia differentiate into specialized forms called bacteroids. These bacteroids enter into a mutualistic relationship with the plant cells, where they provide fixed nitrogen, and the plant provides them with a suitable environment and nutrients.

5. Nitrogen Fixation: Inside the infected plant cells, the bacteroids convert atmospheric nitrogen (N₂) into ammonia (NH₃) through the enzyme nitrogenase. Ammonia is then converted into ammonium ions (NH₄⁺), which the plant can utilize as a nutrient to synthesize amino acids, proteins, and other nitrogen-containing compounds essential for growth and development.

6. Ammonium Assimilation: The plant cells actively transport ammonium ions from the nodules to other parts of the plant, where they are assimilated into various biomolecules.

7. Benefits of Nitrogen Fixation: Through this process, leguminous plants can utilize atmospheric nitrogen, which is otherwise unavailable to most plants. This ability enhances soil fertility, as the fixed nitrogen enriches the soil, benefiting other neighboring plants as well.

In plants, nitrogen fixation occurs mainly through two types:

1. Symbiotic Nitrogen Fixation: This type of nitrogen fixation involves a mutualistic relationship between certain plants and nitrogen-fixing bacteria. The bacteria reside in specialized structures called root nodules, which form on the roots of specific plants. These plants are typically legumes, such as soybeans, peas, clover, and others. The symbiotic bacteria, often belonging to the genus Rhizobium, convert atmospheric nitrogen (N₂) into ammonia (NH₃), which the plants can use as a nitrogen source for their growth and development.

2. Asymbiotic Nitrogen Fixation: In this type of nitrogen fixation, free-living nitrogen-fixing bacteria, such as Azotobacter and Azospirillum, exist in the soil. They are not in a symbiotic relationship with plants and do not form root nodules. Instead, they independently fix atmospheric nitrogen into ammonia (NH₃) and release it into the soil. Nearby plants can then take up this ammonia and utilize it as a nutrient source.

Here are some examples of leguminous plants that engage in symbiotic nitrogen fixation through nodules with rhizobia bacteria:

1. Soybeans (Glycine max): Soybeans are a widely cultivated leguminous crop, known for their high protein content and versatile uses in various food products, animal feed, and industrial applications.

2. Peas (Pisum sativum): Peas are a popular vegetable crop grown for their nutritious green pods and seeds. They are commonly used in salads, soups, and as a side dish.

3. Clover (Trifolium spp.): Clover is a common pasture plant used in animal husbandry due to its high nutritional value for grazing animals. It also contributes to soil improvement by fixing nitrogen.

4. Lentils (Lens culinaris): Lentils are a significant source of plant-based protein and are widely consumed in various cuisines around the world.

5. Chickpeas (Cicer arietinum): Chickpeas, also known as garbanzo beans, are a staple in many Middle Eastern, Mediterranean, and Indian dishes, and are rich in protein and fiber.

6. Alfalfa (Medicago sativa): Alfalfa is a forage crop extensively grown for livestock feed due to its high protein and nutrient content.

7. Fava Beans (Vicia faba): Fava beans are large, flat, green beans commonly used in cooking and are also known for their nitrogen-fixing capabilities.

Asymbiotic nitrogen fixation occurs through the activity of free-living nitrogen-fixing bacteria in the soil. Some examples of these bacteria and the plants that benefit from their nitrogen-fixing capabilities include:

1. Azotobacter: Azotobacter is a genus of free-living nitrogen-fixing bacteria commonly found in various soil types. They are known to fix nitrogen and release ammonia into the soil, which can be taken up by nearby plants. While they can associate with the roots of some plants, they are primarily considered as free-living nitrogen fixers.

2. Azospirillum: Azospirillum is another group of free-living nitrogen-fixing bacteria that promote plant growth and nitrogen fixation. They establish beneficial associations with the roots of several grasses, including cereals like wheat, rice, and maize, as well as non-leguminous plants.

3. Cyanobacteria: Some cyanobacteria, also known as blue-green algae, are capable of nitrogen fixation. They can form symbiotic relationships with certain plants, such as bryophytes (mosses), ferns, and cycads, and provide them with fixed nitrogen.

4. Frankia: Frankia is a genus of actinobacteria that forms symbiotic associations with several non-leguminous woody plants, including various trees and shrubs. These bacteria create specialized structures called actinorhizal nodules, where they fix atmospheric nitrogen and supply it to the host plants.

In conclusion, the process of nitrogen fixation in plant nodules involves a fascinating interaction between the plant and bacteria, enabling these plants to thrive in nitrogen-deficient soils and contribute to sustainable agricultural practices by reducing the need for synthetic fertilizers.

Thus, Both symbiotic and asymbiotic nitrogen fixation are essential for maintaining soil fertility and providing plants with an adequate supply of nitrogen, a crucial element for their growth, development, and production of proteins and other nitrogen-containing compounds. These processes play vital roles in sustaining agricultural productivity and ecosystem health.

Therefore, It's important to note that while these free-living nitrogen-fixing bacteria can contribute to soil fertility and provide nitrogen to plants, their overall contribution to nitrogen fixation is relatively smaller compared to symbiotic nitrogen fixation, especially from leguminous plants like soybeans, peas, and clover. Symbiotic nitrogen fixation in legumes remains the most significant natural process for supplying nitrogen to plants and enhancing soil fertility in various ecosystems.