Nitrogen fixation is the very first process of the nitrogen cycle that causes the conversion of nitrogen gas in the atmosphere into ammonia and then to the related nitrogenous compounds like nitrates, nitrites, etc. in the soil.
In very simple words, nitrogen fixation is the way of removal of the atmospheric nitrogen from the air and bring it to the soil so that, nitrogen can be easily utilized further in the nitrogen cycle.
The nitrogen in the atmospheric air is relatively inert in the form of dinitrogen (N2) gas and, this can’t be accepted by the plants directly from the air.
So, the process of nitrogen fixation is very very much important to convert the inert nitrogen into plant consumable form.
This conversion of inert N2 gas into a form utilized by most organisms is the second most important biological process on Earth after photosynthesis.
Nitrogen fixation can be done in various types. The different types of nitrogen fixation processes are Atmospheric Nitrogen Fixation, Industrial Nitrogen Fixation, and Biological Nitrogen Fixation.
Here, in this post, we will talk about the Biological Nitrogen Fixation process only.
Types of Biological Nitrogen Fixation
There are 3 different types of Biological Nitrogen Fixation. These are:
- Free-Living Nitrogen Fixation (FLNF)
- Associative Nitrogen Fixation (ANF)
- Symbiotic Nitrogen Fixation (SNF)
1. Free-Living Nitrogen Fixation (FLNF)
Free-Living Nitrogen Fixation (FLNF) is a type of nitrogen fixation that is carried on by the free-living heterotrophic bacteria that live in the soil without showing any interactions with other plants.
These bacteria stay on or near the root surfaces of the plants i.e. the rhizosphere but don’t show any type of association.
They are found everywhere and are a very significant source of Nitrogen in some terrestrial systems.
These types of bacteria are mostly anaerobic in nature, and if the aerobic type they have a special type of mechanism to protect the nitrogenase enzyme from oxygen which is beneficial for nitrogen fixation.
Free-Living nitrogen-fixing bacteria include species like Anabaena, Nostoc, Azotobacter, Bacillus, Clostridium, Klebsiella, etc. These bacteria with the help of the nitrogenase enzyme, directly treat the nitrogen gas in the soil and convert it to ammonia.
These organisms get their source of energy typically by oxidizing organic molecules released by other organisms, decomposition, or by utilizing other inorganic compounds.
Various soil slurry experiments have shown that the main factors limiting Nitrogen fixation in the forest soil are the soil’s low temperature, low pH, and shortage of carbon sources.
Therefore, if all of these requirements are matched then the rate of Nitrogen fixation increases by these free-living bacteria.
Experimentally, it is seen that the pH of 6 is the most optimum and best acidity level for the soil under which the amount of nitrogen fixation is just doubled.
2. Associative Nitrogen Fixation (ANF)
Associative Nitrogen fixation (ANF) is the process by which dinitrogen gas of the atmosphere is converted to ammonia by the associative bacteria that stay only in casual association with plants.
Associative bacteria are those Nitrogen-fixing bacteria that live in loose associations with roots and so are also referred to as the called ‘associative’ nitrogen fixers.
The associative nitrogen-fixing bacteria grow on the surface of roots and may also colonize the outer layers of a root by entering between epidermal cells only.
You can simply refer to this associative type of bacteria as an intermediate form between the free-living bacteria (which don’t show any close association with plants) and the symbiotic bacteria (which show close symbiotic association with plants).
An example of some of the Associative Nitrogen fixation bacteria is the species belonging to the genus Azospirillum.
Species of Azospirillum are able to form close associations with several members of the Poaceae (grasses), including agronomically important cereal crops, such as rice, wheat, corn, oats, and barley.
It is seen that these types of bacteria are not very effective nitrogen fixers like the free-living and symbiotic ones. It’s because, although they are extremely fast in converting nitrogen to ammonia but, they are the slowest ones in releasing ammonia to the soil from their body.
This results in the slow availability of ammonia and its derivatives for consumption by the plants.
Experimentally, it was seen that it is only after the bacteria dies that the ammonia stored in their body is released back to the soil due to the breakdown of the cell wall.
3. Symbiotic Nitrogen Fixation (SNF)
Symbiotic Nitrogen Fixation (SNF) is the most common and well-studied type of biological nitrogen fixation. This nitrogen fixation follows the phenomenon of symbiosis which is the interaction between two different organisms living in close physical association, typically to the advantage of both.
In Symbiotic Nitrogen Fixation, various bacteria stay in close association in the root nodules of the plants. These bacteria help in providing nitrogen to the plant and in return, they get food and a place to infect in the roots.
One common example of such a type is the Rhizobium bacteria that infect the root nodules of legume plants. The plant supplies the rhizobia with energy in the form of amino acids and the rhizobia fix nitrogen from the atmosphere for plant uptake.
In legumes like alfalfa, beans, clover, cowpeas, lupines, peanut, soybean, etc. the ammonia that is produced by the Rhizobium bacteria is directly inputted in the plant’s root cells to produce biomolecules like amino acids, etc. without any further conversion to nitrite and nitrate.
Another example is of this type of nitrogen fixation is the water fern Azolla’s symbiosis with a cyanobacterium Anabaena azollae. Anabaena colonization forms cavities at the base of Azolla fronds where it fixes significant amounts of nitrogen in specialized cells called heterocysts.
These types of bacteria use nitrogenases enzyme to convert nitrogen to ammonia. This enzyme is very sensitive to oxygen and is rapidly degraded by oxygen.
For this reason, the aerobic ones like Rhizobium bacteria cease the production of the enzyme in the presence of oxygen, and they have built an awesome mechanism for doing so.
Many symbiotic nitrogen-fixing organisms like the cyanobacterium Anabaena azollae exist only in anaerobic conditions while respiring to draw down oxygen levels, or binding the oxygen with a protein such as leghemoglobin.
Biological Nitrogen (N2) fixation is the reduction of atmospheric nitrogen gas to ammonia.
The equation of Biological Nitrogen Fixation is: N2 + 8 H+ + 8 e− → 2 NH3 + H2
The reaction is mediated by an oxygen-sensitive enzyme nitrogenase and requires energy by the consumption of ATP.
Nitrogenases are present in all types of nitrogen-fixing bacteria. All biological nitrogen fixation is effected by enzymes called nitrogenases. These enzymes contain iron, and often with molybdenum and vanadium as well.
In the free-living heterotrophic nitrogen fixation, the bacteria stay freely in the soil, mainly in the rhizosphere region of the tree. Nitrogen fixation is mostly done by anaerobic heterotrophic bacteria.
While, in the symbiotic nitrogen fixation type, the bacteria stay in close association inside the root nodules of the legume plants. Nitrogen fixation is mostly done by aerobic photosynthetic bacteria.
Talking about the Associative Nitrogen fixation, it’s like an intermediate form between the free-living and the symbiotic nitrogen fixation type. Nitrogen fixation is mostly performed by microaerophilic bacteria as studied to date.