🌿 Biological Nitrogen Fixation
Learn how nitrogen-fixing microorganisms convert atmospheric nitrogen into ammonia and why biological nitrogen fixation is vital in agriculture.
The atmosphere contains a vast reservoir of nitrogen, but plants cannot directly use molecular nitrogen. Biological nitrogen fixation solves this problem. It is one of the most agriculturally important microbial processes because it converts inert atmospheric nitrogen into ammonia through living microorganisms.
Meaning of biological nitrogen fixation
Biological nitrogen fixation is the microbial reduction of atmospheric nitrogen gas to ammonia.
Only certain prokaryotes possess this ability. These organisms are called diazotrophs.
Biological nitrogen fixation converts atmospheric N2 into ammonia through the action of nitrogen-fixing microorganisms.
Agricultural importance of BNF
Biological nitrogen fixation is crucial because it:
- adds new nitrogen to the biosphere
- reduces dependence on chemical nitrogen fertilizers
- supports legume productivity
- improves soil fertility in cropping systems
In low-input agriculture, BNF is especially important as a natural source of nitrogen.
Types of nitrogen-fixing microorganisms
Nitrogen-fixing organisms may be grouped by their mode of association.
Free-living nitrogen fixers
- Azotobacter
- Beijerinckia
- Clostridium
- some cyanobacteria
Associative nitrogen fixers
- Azospirillum
- Herbaspirillum
These live closely associated with roots but do not form true nodules.
Endophytic nitrogen fixers
- Gluconacetobacter diazotrophicus
Symbiotic nitrogen fixers
- Rhizobium
- Bradyrhizobium
- Azorhizobium
- Frankia
- Anabaena azollae
Rhizobium-legume symbiosis
The best-known symbiotic nitrogen-fixing association is between Rhizobium and leguminous plants.
General features
- bacteria infect young roots
- root nodules are formed
- bacteria live inside nodules
- fixed nitrogen is supplied to the plant
- the plant provides carbohydrates and a favorable environment
This is one of the most efficient natural nitrogen-supplying systems in agriculture.
Cross inoculation concept
Different rhizobial groups nodulate different host legumes. This is the basis of cross inoculation groups, which helps in selecting the right inoculant for a crop.
Effective nodulation depends on compatibility between the legume host and the correct rhizobial strain.
Nitrogenase enzyme
The reduction of atmospheric nitrogen is carried out by the enzyme complex nitrogenase.
Components of nitrogenase
- Mo-Fe protein
- Fe protein
Nitrogenase requires:
- ATP
- reducing power
- suitable electron carriers
Overall significance
It is the key enzyme that makes biological nitrogen fixation possible.
Energy requirement and oxygen sensitivity
Nitrogen fixation is an energy-intensive process. A large amount of ATP is required to reduce one molecule of nitrogen gas to ammonia.
Nitrogenase is also highly sensitive to oxygen. Nitrogen-fixing systems therefore need protective mechanisms.
Examples of protection
- Leghaemoglobin in legume nodules
- high respiratory rate in Azotobacter
- heterocyst formation in some cyanobacteria
- microaerophilic behavior in some associative fixers
Nitrogenase is oxygen-labile, so all nitrogen-fixing systems require some protection mechanism.
Formation of root nodules
In symbiotic fixation, nodule formation involves coordinated interaction between the bacterium and plant root.
Broad sequence
- rhizobia multiply near the root
- root hair infection begins
- infection thread develops
- cortical cells divide
- nodule is formed
- bacteria differentiate into nitrogen-fixing forms
This makes nodulation both a microbiological and plant-developmental process.
Factors affecting nitrogen fixation
Biological nitrogen fixation is influenced by:
- availability of combined nitrogen
- soil pH
- moisture
- temperature
- phosphorus supply
- molybdenum and cobalt availability
- energy supply from photosynthesis or carbon substrates
High levels of nitrate or ammonium often suppress active nitrogen fixation.
Agricultural significance of BNF systems
Important associations
- Rhizobium with legumes
- Bradyrhizobium with soybean
- Azolla-Anabaena in rice systems
- Frankia with actinorhizal plants
- associative fixation near cereal roots
These systems are widely relevant in sustainable agriculture, green manuring, pulse production, and integrated nutrient management.
Summary Cheat Sheet
- Biological nitrogen fixation is the microbial conversion of atmospheric N2 into ammonia.
- Only certain prokaryotes called diazotrophs can fix nitrogen.
- Nitrogen fixers may be free-living, associative, endophytic, or symbiotic.
- Rhizobium in legume root nodules is the classic symbiotic nitrogen fixer.
- Nitrogenase is the key enzyme for nitrogen fixation.
- Nitrogen fixation requires high energy and is sensitive to oxygen.
- Leghaemoglobin helps protect nitrogenase in legume nodules.
- Soil pH, moisture, temperature, carbon supply, and mineral nutrients affect BNF.
- BNF is vital for soil fertility and reduced dependence on nitrogen fertilizer.
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