Biological Nitrogen Fixation (BNF)

Biological nitrogen fixation (BNF) is the process by which certain microorganisms convert atmospheric nitrogen (N₂) into ammonia (NH₃), which plants can use for growth. This process is essential because nitrogen is a key nutrient for plant growth but is often limiting in soils. Nitrogen fixation helps provide a natural source of nitrogen, reducing the need for synthetic fertilizers.

There are three main types of biological nitrogen fixation based on the type of association between the nitrogen-fixing organisms and their host plants or environment:

1. Symbiotic Nitrogen Fixation
2. Associative Nitrogen Fixation
3. Asymbiotic Nitrogen Fixation

1. Symbiotic Nitrogen Fixation

Symbiotic nitrogen fixation occurs when nitrogen-fixing bacteria live in a mutualistic relationship with specific plants. In this relationship, both the plant and the bacteria benefit from the interaction.

Key Features:

• Organisms Involved: Typically, leguminous plants (such as peas, beans, clover, and alfalfa) and rhizobia bacteria (such as Rhizobium, Bradyrhizobium, and Sinorhizobium).
• Process:
  • The plant roots exude specific compounds that attract rhizobia.
  • The rhizobia infect the plant roots, forming nodules where the bacteria live and fix nitrogen.
  • The bacteria convert atmospheric nitrogen (N₂) into ammonia (NH₃) using the enzyme nitrogenase.
  • The plant benefits by receiving nitrogen in a form it can use, while the bacteria receive carbohydrates and other nutrients from the plant as their food source.
• Example: Legumes such as beans and peas form nodules on their roots where rhizobia bacteria fix nitrogen.

Significance:

• Symbiotic nitrogen fixation is highly efficient, as the bacteria receive continuous nourishment from the plant, and the plant gets a steady supply of nitrogen.
• This process reduces the need for chemical nitrogen fertilizers in agriculture and promotes soil fertility.

2. Associative Nitrogen Fixation

Associative nitrogen fixation occurs when nitrogen-fixing bacteria live in the rhizosphere (soil surrounding the plant roots) or on the plant roots but do not form specialized nodules. The bacteria fix nitrogen in the soil and make it available to the plant.

Key Features:

• Organisms Involved: Various types of nitrogen-fixing bacteria, including Azospirillum, Azotobacter, Beijerinckia, and Herbaspirillum.
• Process:
  • These bacteria do not infect plant tissues or form nodules but still colonize the plant’s rhizosphere or root surface.
  • The bacteria fix nitrogen from the atmosphere and release ammonia, which is then available to the plant.
  • The plant benefits from the fixed nitrogen, but the relationship is less direct than in symbiosis.
  • Some bacteria can also produce growth-promoting substances like plant hormones (e.g., auxins) that enhance plant growth.
• Example: Azospirillum on the roots of grasses (such as maize or rice) can fix nitrogen in the rhizosphere and provide nitrogen to the plant.

Significance:

• Associative nitrogen fixation is important for a wide range of crops, particularly cereals and grasses, which do not typically form symbiotic relationships with nitrogen-fixing bacteria.
• It improves soil nitrogen availability and enhances plant growth in certain agricultural systems, especially in nitrogen-deficient soils.

3. Asymbiotic Nitrogen Fixation

Asymbiotic nitrogen fixation occurs when free-living nitrogen-fixing bacteria fix nitrogen independently in the soil or other environments without forming any association with plant roots.

Key Features:

• Organisms Involved: Free-living bacteria, such as Azotobacter, Clostridium, Beijerinckia, and Cyanobacteria (blue-green algae).
• Process:
  • These bacteria are not associated with plants but can still fix nitrogen when in the soil, water, or other suitable environments.
  • They fix atmospheric nitrogen into ammonia, which then becomes available for plant uptake either directly from the soil or after mineralization into forms like nitrates.
  • The bacteria are independent and do not form a close relationship with any particular plant.
• Example: Azotobacter is a well-known free-living nitrogen-fixing bacterium that operates in soil environments, contributing to nitrogen fixation without forming symbiosis with plants.

Significance:

• Asymbiotic nitrogen fixation contributes to soil nitrogen levels, particularly in non-crop areas like natural ecosystems.
• Although not as efficient as symbiotic nitrogen fixation, it still plays a crucial role in maintaining soil fertility in certain environments, especially in low-nitrogen conditions.

Comparison of the Three Types of Nitrogen Fixation

Importance of Biological Nitrogen Fixation

1. Sustainability: BNF reduces the reliance on synthetic nitrogen fertilizers, which can be costly and environmentally damaging.
2. Soil Fertility: It helps maintain soil nitrogen levels, improving soil health and reducing the need for chemical inputs.
3. Environmental Impact: BNF reduces the environmental impact of agriculture by decreasing the use of nitrogen fertilizers that can leach into waterways and cause eutrophication.
4. Agricultural Productivity: Legumes, which are involved in symbiotic nitrogen fixation, improve soil nitrogen content for subsequent crops in rotation, enhancing agricultural productivity.