Essay on the Nitrogen Cycle

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,

2. Ammonification,

3. Nitrification,

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.

Ammonification:

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.

Nitrification:

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.

Denitrification:

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.