In this essay we will discuss about the nitrogen cycle with the help of a diagram.
Importance of Nitrogen:
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Nitrogen is an essential constituent of protoplasm. Nitrogen is the component of amino acids, proteins, enzymes, nucleotides and nucleic acids.
Sources of Nitrogen:
Like all other major and minor elements, it flows in nature in a cyclic manner. Nitrogen is picked up as inorganic compound and is changed into organic form by plants and some prokaryotes. Though atmosphere contains 78.62% of nitrogen (about 4/5th of air) in gaseous state, yet animals cannot use it directly. Animals can use nitrogen either in inorganic forms as ammonia, nitrites and nitrates or in organic form such as urea, proteins and nucleic acids.
So nitrogen is present both in inorganic (as N,, NH3, N,0, NO, NO,, NO,” and NO -) and organic form. So the nitrogen reservoir is atmosphere. Nitrogen cycle unlike carbon is very complex gaseous type of cycle and is a perfect cycle.
The nitrogen cycle can be conveniently discussed under the following heads:
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A. Nitrogen Fixation.
B. Ammonification.
C. Nitrification.
D. Denitrification.
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A. Nitrogen Fixation:
According to Pratt (1977), the total supply of nitrogen is about 237 million metric tonnes per year. Of this, about 149 million metric tonnes (i.e., 63%) is made available through biological nitrogen fixation.
1. Atmospheric (Abiological) Nitrogen Fixation:
Photochemical and electrochemical reactions, oxygen combines with nitrogen to form oxides of nitrogen. Now they get dissolved in water to form nitrous acid and nitric acid which combine with other salts to produce nitrates.
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It can be summarized as under:
N2 + O2 → 2NO
2NO + O2 → 2NO2
2NO2 + H2O → HNO2 + HNO3
3NO2 + H2O → 2NHO3 + NO
In the tropics where thunderstorms and lightning are common, large quantities of nitrogen (i.e. 35 mg N2/m2/year) are converted into nitrogen.
2. Biological Nitrogen Fixation:
The biological nitrogen fixation involves the transformation of atmospheric N2 into nitrites and nitrates by living organisms. About 140- 170 mg of N2/m2 year is fixed by all the agencies, out of which not more than 35 mg of N2/m2 year is fixed by physical methods. This shows that biological N2 fixation is very efficient.
It is further of three types:
(a) Symbiotic Nitrogen Fixation:
It is brought about by certain bacteria like:
i. Rhizobium leguminosarum (Bacillus radicicola) in the root nodules of legumes (pea family);
ii. Frankia (of Actinomycetes) in the root nodules of Alnus, Casuarina, etc.;
iii. Klebsiella in the leaf nodules Of Dioscorea. —Rhizobium sesbania in the stem nodules of Aeschynomene.
iv. Nostoc and Anabaena (cyanobacteria) in the coralloids roots of Cycas.
v. Anabaena as an endophyte in the leaf of Azolla.
vi. Nostoc in the thallus of Anthoceros.
Rhizobia are gram negative, non-sporous, aerobic, bacilli bacteria. These show species-specific symbiotic relationship with the members of Leguminosea e.g. R. meliloti (alfalfa group), R. trifoli (clover group), R. leguminosarum (pea group), R. phaseoli (bean group) and R. japonicum (soyabean group). Phytoagglutinins (lecithins) are considered as recognition chemical substances between the two symbionts.
It is believed that Rhizobiun has a nitrogen fixing (nif) gene which controls the synthesis of nitrogenase. These bacteria form the nodules on the secondary roots of the legumes. The nodules contain a red coloured pigment called leghaemoglobin (LHb). This pigment acts as an oxygen scavenger and keeps the level of molecular oxygen low inside the nodules which activates the enzyme nitrogenase which reduces N^ to ammonia.
(b) Free Living (Asymbiotic) N2-Fixing Organisms:
These are primitive nitrogen fixers and fix the nitrogen as ammonia actively under poor aeration by the reductional process. Oscillatoria (blue-green alga) account for one-fourth of nitrogen input to the world’s ocean.
These include:
i. Obligatory aerobes e.g. Azotobactor, Beijerinkia.
ii. Facultative aerobes e.g. Escherichia, Bacillus, Enterobactor, Klebsiella.
iii. Anaerobic e.g. Closteridium.
iv. Photosynthetic e.g. Chromatium, Rhodospirillum (Purple bacteria)
It is estimated that when Azotobactor is grown along with other crops lead to higher yield and reduce the nitrogen requirement by 10-25 kg/hectare.
(c) Loose Association of N2-Fixing Bacteria:
E.g. Azospirillum lipoferum with the roots of Digitaria, Sorghum, Zea mays, etc.
3. Industrial Nitrogen Fixation:
Nitrogen and hydrogen combines to form ammonia industrially under extremely high temperature of 400°C and a high pressure of about 200 atmospheres.
It has been estimated that electrochemical and photochemical fixation result in an average amount of nitrate of 7.6 x 10” metric tonnes/year while biological fixation is estimated at 54 × 10″ metric tonnes/year.
Biological nitrogen fixation involves following steps:
So net energy input for nitrogen fixation is 147 kcal/ mole.
B. Ammortification:
It involves the decomposition of proteins of dead plants and animals, and nitrogenous wastes like urea, uric acid, etc. of animals to ammonia in the presence of ammonifying bacteria- also called bacteria of decay or putrefying bacteria.
The Common ammonifying bacteria are Bacillus ramvsus, B. vulgaris and B. mycoides. Some of ammonifying micro-organisms are substrate specific e.g. using urea but not uric acid. But some species use a variety of organic nitrogen sources. In the process, energy is also produced, so is an exothermic process.
C. Nitrification:
It involves the oxidation of ammonia to nitrates thought nitrites in the presence of nitrifying bacteria in the following manner:
D. Denitrification:
It is a biological process by which ammonium compounds, nitrates and nitrites are reduced to molecular nitrogen in the presence of denitrifying bacteria like. Thiobacillus denitrificans, Bacillus subtilis, Micrococcus denitrificans, Pseudomonas stutzeri etc. So it reduces the soil fertility and is stimulated by waterlogging, poor drainage, lack of aeration and accumulation of organic matter.
It involves the following steps:
NO3 → NO2-(Nitrite)
NO2 → NO (Nitrous oxide)
NO → N2O (Nitric oxide)
N2O → N2 (Nitrogen)
Denitrification of nitrate to nitrous oxide occurs in anaerobic conditions and in the presence of glucose. It is an exothermic reaction yielding 545 kcal/mole, while denitrification of nitrate to molecular nitrogen yields 570 kcal/mole.
Nitrogen Budget:
Delwiche has estimated that the total annual N2 fixation of 92 x 106 metric tonnes (54 biologically, 30 industrially, 7.6 photo chemically and 0.2 volcanic) is replaced through denitrification by only 83 × 106 metric tonnes (43 terrestrially, 40 marine and 0.2 in sediments).
A disbalance of 9 × 106 metric tonnes is being built up annually (due to increased industrial fixation) in ground water, rivers, lakes and the ocean which is a significant factor in water pollution. It is compensated by volcanic eruptions and erosion of sedimentary rocks.