Nitrogen-Fixing Bacteria - A TerraSoil Overview

What Are Nitrogen-Fixing Microbes?

Nitrogen-fixing microbes are bacteria and archaea capable of converting atmospheric nitrogen (N₂) into ammonia (NH₃), a form that plants can readily assimilate. This process, known as biological nitrogen fixation (BNF), is vital for plant nutrition and soil fertility.

How Much Nitrogen Do Fixers Supply to Plants?

Nitrogen-fixing bacteria can supply significant amounts of nitrogen to plants. On average, leguminous crops can receive between 50 to 300 kg of nitrogen per hectare per year through symbiotic nitrogen fixation that are found in the nodules in the roots. There are many species of free-living nitrogen fixers with various strengths. The current favourite species is Azospirillum Brasilense.

Top 10 Species of Nitrogen Fixers Used in Agriculture

1. Rhizobium: Forms symbiotic relationships with legumes.

2. Bradyrhizobium: Similar to Rhizobium, but with slower-growing strains.

3. Azospirillum: Associates with cereals and grasses.

4. Frankia: Forms symbiosis with actinorhizal plants.

5. Sinorhizobium: Effective in symbiosis with specific legumes like alfalfa.

6. Mesorhizobium: Involved with chickpeas and other legumes.

7. Anabaena: Cyanobacteria that fix nitrogen in association with aquatic plants.

8. Azotobacter: Free-living nitrogen fixers in the soil.

9. Klebsiella: Free-living nitrogen fixers, though not as significant as others.

10. Nostoc: Cyanobacteria that can also fix nitrogen independently or in symbiosis.

Additional Benefits Afforded to Plants by Using Nitrogen-Fixing Bacteria

1. Enhanced Growth and Yield: Increased nitrogen availability promotes robust plant growth and higher yields.

2. Improved Soil Structure: Nitrogen-fixing bacteria can enhance soil aggregation, improving aeration and water retention.

3. Disease Resistance: Some nitrogen fixers can induce systemic resistance in plants, reducing susceptibility to pathogens.

4. Reduced Need for Chemical Fertilizers: By naturally providing nitrogen, these bacteria reduce the dependence on synthetic fertilizers, lowering production costs and environmental impact.

Factors Affecting Successful Colonization of the Roots

1. Soil pH: Optimal pH for most nitrogen-fixing bacteria is between 6.0 and 7.0.

2. Soil Moisture: Adequate moisture levels are crucial for bacterial activity and root colonization.

3. Soil Nutrient Levels: Excessive nitrogen in the soil can inhibit nitrogen fixation, as plants will rely less on bacterial symbionts.

4. Host Plant Compatibility: Successful colonization depends on the compatibility between the plant and the bacterial strain.

Difference Between Free-Living and Symbiotic Species

1. Free-Living Nitrogen Fixers: These bacteria, like Azotobacter and Klebsiella, fix nitrogen independently in the soil. They are less efficient than symbiotic fixers because they do not receive direct carbohydrates from plants.

2. Symbiotic Nitrogen Fixers: These bacteria, such as Rhizobium and Frankia, form intimate associations with plant roots, receiving carbohydrates in exchange for fixed nitrogen. This symbiosis significantly enhances their nitrogen-fixing efficiency.

What Food Do Nitrogen Fixers Consume?

Nitrogen-fixing bacteria primarily consume carbohydrates provided by their plant hosts (in the case of symbiotic bacteria) or organic matter in the soil (for free-living bacteria). These carbohydrates fuel the energy-intensive process of nitrogen fixation.

What Kills Nitrogen Fixers?

1. Chemical Fertilizers: High levels of synthetic nitrogen fertilizers can inhibit nitrogen-fixing activity.

2. Pesticides and Herbicides: Certain agrochemicals can be toxic to nitrogen-fixing bacteria.

3. Soil Disturbance: Tillage and soil compaction can disrupt bacterial habitats and reduce their populations.

4. Extreme pH Levels: Very acidic or alkaline soils can adversely affect bacterial survival and activity.

Optimum Growing Conditions for Nitrogen Fixers

1. Temperature: Most nitrogen fixers thrive at temperatures between 20-30°C.

2. pH Level: A slightly acidic to neutral pH (6.0-7.0) is ideal.

3. Moisture: Adequate soil moisture is necessary, but waterlogged conditions can be detrimental.

4. Organic Matter: High levels of organic matter support the growth of nitrogen-fixing bacteria by providing necessary nutrients and energy sources.

Sustainability Benefits of Using Nitrogen-Fixing Bacteria

1. Reduced Chemical Inputs: By naturally providing nitrogen, these bacteria reduce the need for synthetic fertilizers, decreasing environmental pollution and production costs.

2. Improved Soil Health: Nitrogen-fixing bacteria enhance soil structure and fertility, promoting sustainable agricultural practices.

3. Increased Biodiversity: Supporting nitrogen-fixing bacteria fosters a diverse microbial community in the soil, which is crucial for ecosystem resilience.

How Nitrogen Fixers Are Produced and Stored

Nitrogen-fixing bacteria are typically produced through fermentation processes, where they are grown in large bioreactors under controlled conditions. The inoculum is then harvested, formulated into products like powders, granules, or liquids, and stored in cool, dry conditions to maintain viability until application.

Conclusion

Nitrogen-fixing bacteria are a cornerstone of sustainable agriculture, providing essential nutrients to plants, enhancing soil health, and reducing the need for synthetic fertilizers. Understanding their role, benefits, and the conditions that support their activity is crucial for optimizing their use in agricultural systems.

References

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