About 78% of nitrogen is present in the atmosphere. Nitrogen is not directly absorbed by the plants. Nitrates, nitrites, ammonium salt, and organic nitrogenous compounds act as source of nitrogen in the soil. These compounds are present in the soil in the form of proteins, amino acids, urea etc. Some plants absorb urea directly, but proteins and amino acids are not directly absorbed by plants.
The process of conversion of free nitrogen of the atmosphere into utilisable compounds of nitrogen like nitrate, ammonia, amino acids etc is called nitrogen fixation. There are three types of nitrogen fixation- physical, biological and industrial nitrogen fixation.
- Physical nitrogen fixation: During thunder and lightening, N2 and O2 of atmosphere react to form NO (nitric oxide).
NO (nitric oxide) on further oxidation form nitrogen dioxide.
The nitrogen oxides dissolve in water to form nitric acid and nitrous acid. They enter into the soil along with rain water and react with alkaline earth metals to form corresponding nitrate and nitrite salt.
K+HNO3→KNO3 (potassium nitrate)
Ca+HNO3→Ca (NO3)2 (calcium nitrate)
K+HNO2→KNO2 (potassium nitrite)
2.Biological nitrogen fixation: Conversation of atmospheric nitrogen into inorganic or organic usable forms through the agency of living organism is called biological nitrogen fixation. This process is mainly carried out by two types of nitrogen fixation depending on the microorganisms.
- Asymbiotic nitrogen fixation.
- Symbiotic nitrogen fixation.
Asymbiotic biological nitrogen fixation: Nitrogen is fixed asymbiotically in the soil by free living microorganism. Asymbiotic or free nitrogen fixers can be classified into three groups:
- Aerobic bacteria, for example, Azotobacter.
- Anaerobic bacteria, for example, Clostridium, Rhodospirillum and
- Blue green algae, or cyanobacteria for example, Anabaena, Nostoc, Aulosira, Cylindrospermum, Trichodesmium.
These microorganisms are abundant in the soil and contribute to the nitrogen content of the soil. All nitrogen fixing blue green algae (Cyanobacteria) are characterized by the presence of long, thick-walled colourless cells called heterocysts. Heterocysts are considered to be the site of nitrogen fixation.
Mechanism of asymbiotic biological nitrogen fixation: Nitrogen fixation is a reductive process for which the following are required.
- Enzyme- Nitrogenase.
- Reductant- Ferredoxin.
The process of asymbiotic biological nitrogen fixation is as follows:
Step 1: Pyruvic acid is broken down into acetyl-PO4 and hydrogen is released.
Step 2: Ferredoxin is reduced by H+ ion.
Step 3: Acetyl phosphate reacts with ADP to form ATP and acetate.
Step 4: In the presence of ATP and reduced ferredoxin, molecular N2 is adsorbed on the surface of enzyme nitrogenase.
Step 5: Reduction of N2 takes place until it is completely reduced to NH3.
Symbiotic nitrogen fixation: Symbiotic nitrogen fixers are those which fix nitrogen being associated symbiotically with other plants. The symbiotic fixers include Rhizobium, Nostoc, Anabaena and the plants include legumes (root nodules), cycas (coralloid roots) etc. The most common forms of symbiotic nitrogen fixing bacteria (e g., several species of Rhizobium) occur in the root of the members of Leguminosae. Besides these, many other vascular plants (e g., Casuarina, Myrica, Purshia, Cerococarpus etc) are also host to nitrogen fixing microorganisms. In some plants nodules also occur on stems (e g., Aeschynomene, Sesbania ) and leaves (for example., Psychotria, Azolla).
Biochemistry of symbiotic nitrogen fixation:
It is similar to that of asymbiotic nitrogen fixation. Here, an additional pigment leghemoglobin is present in the root nodule cells, which has the ability to combine rapidly with oxygen to maintain the activity of enzyme nitrogenase to release oxygen when required. This leghemoglobin provides oxygen to nitrogen fixing bacteroid and these are stimulated to produce ATP required for nitrogen fixation.
Symbiotic nitrogen fixers in leguminous plants inhabit small knob-like protuberance called nodules on the roots of the plant. These nodules vary considerably in their shape and size. They may be spherical, elongated and may have finger-like projections. They size vary from pin head to 1 cm in diameter.
Formation of nodules: At first, the root secrets certain glycoproteins, which attract specific Rhizobium species. The bacterial cells produce some extracellular hormones and enzymes that produce two effects:
- Cause the root hair to curl in a characteristic crook-shape.
- Help in the partial destruction of the cell wall of root hair.
The above is followed by the invasion of root hairs by the threads of mucilaginous substance in which bacterial cells are imbedded. Later, the thread moves into the cortex. This mucilaginous threads or infection threads stimulate the outer cortical cells to increase the DNA content of the nuclei, as a result of which, the chromosome compliment becomes polypoid. By the repeated division of these polypoid cells, nodules are formed. Bacteria in the nodules secrete a growth hormone called indole acetic acid (IAA). It stimulates the nodules to grow in size. As the nodules grow, they become effectively supplied with vascular tissue. Bacterial cells multiple rapidly inside infected host cell and are transferred into swollen form called bacteroides. The functional nodule contains a reddish pigment called leghemoglobin (hemoglobin of legumes). This pigment appears to be an oxygen carrier in the process of nitrogen fixation in nodules. It facilitates the diffusion of oxygen to the vigorously respiring bacteria within the cells of nodules, thereby stimulating the production of ATP. ATP is required for nitrogen fixation. Therefore, cells lacking leghemoglobin are not capable of fixing nitrogen.
- Industrial nitrogen fixation: In various fertilizer industries, the atmospheric N2 and H2 are compressed at a particular temperature and pressure to produce NH3 by Haber’s process. NH3 is then converted to different fertilizers including urea.