Here is an essay on the nitrogen cycle.
Nitrogen is one of the important elements in biological compounds, mainly of nucleic acid and protein and, therefore, it is essential for life. Atmosphere contains about 78% nitrogen, but free nitrogen cannot be utilised by most of the organisms, except a few blue green algae and some bacteria.
The fixation of nitrogen takes place by physical, chemical and biological means. Small amounts of N2 are converted to ammonia by electrical discharges in atmosphere i.e., by physical force, and settle down on earth by rain. About 30 million metric tonnes of N2 are produced by industry however about 200 million metric tonnes of nitrogen are fixed every year by biological organisms.
The biological N2 fixation takes place by a few organisms like:
1. Symbiotic Bacteria:
Rhizobium, Brady- rhizobium, Frankia.
2. Free-Living Bacteria:
Azotobacter, Azomonas, Dersia etc.
3. Blue Green Algae:
Nostoc, Anabaena, etc., fix nitrogen inside heterocyst where oxygenic photosynthesis does not occur.
In nitrogen cycle, free N2 gas of atmosphere is converted into ammonia or oxidised to nitrate at different stages. Blue green algae and N2 fixing bacteria play a significant role in converting the atmospheric gaseous nitrogen into organic nitrogenous compounds and, finally, to nitrate, which is soluble in water.
Nitrates are absorbed and utilised by the plants for the synthesis of amino acids vis-a-vis proteins. The plants are consumed by herbivores. When the plants and animals die, they are decomposed by, bacteria, thus the N2 becomes released in the atmosphere.
The N2 cycle (Fig. 2.34) thus consists of the following steps:
1. Nitrogen fixation,
4. Denitrification, and finally, and
5. Release of gases in the atmosphere.
Biological Nitrogen Fixation:
The major share of nitrogen fixation is occupied by biological N2 fixation. The biological N2 fixation of atmospheric nitrogen depends on the nitrogenase enzyme system, composed of nitrogenase and nitrogenase reductase. Nitrogenase is very much sensitive to O2 and it becomes irreversibly inactivated on exposure even at low concentration of O2. In leguminous plants, N2 fixation takes place in root nodules, where the enzyme is protected by red pigment leghemoglobin.
The plants are eaten by herbivores and fixed nitrogen goes into their body and after their death, it becomes decomposed and mixed with the soil.
The nitrogen may be fixed in the soil by the following means:
(a) Physical fixation takes place by lightning and thereby N2 comes down in soil through rain.
(b) There is huge industrial production of N2 fertiliser, these are mixed with the soil during cultivation.
(c) Farmyard manure is one of the major sources of N2.
(d) Major share of N2 is contributed by the biological fixers.
In this process nitrogen in organic matter of dead plants and animals is converted to ammonia and amino acids.
Urea is applied in the field as additional nitrogen source. The microbial decomposition of urea causes the release of ammonia which is returned to atmosphere or may go to neutral aqueous environments as ammonium ions.
This is the process of conversion of ammonia to nitrate. It takes place in two steps. Ammonia is first oxidised to nitrous acid by Nitrosomonas, Nitrospira and Nitrococcus. The molecular O2 acts as electron acceptor in this step. In the second step, nitrous acid becomes oxidised and converted into nitric acid, mostly by Nitrobacter. Proper aeration in the soil is essential for the availability of oxygen.
During this process, nitrate is converted to molecular nitrogen by the different bacteria like Bacillus cereus, Pseudomonas aeruginosa etc. and thus, N2 goes back into the atmosphere. This is called dissimilatory nitrate reduction. This takes place during depletion of oxygen in the medium.
In assimilatory nitrate reduction process nitrite is converted into ammonia.
The nitrate added to the soil is reduced to NH3 by plants and fermentative bacteria rather than to nitrogen by denitrifying bacteria.