Nitrogen Cycle:
Regular circulation of nitrogen amongst living organisms, reservoir pool in the atmosphere (78% of atmosphere is N.) and cycling pool in lithosphere. Plants obtain nitrogen from soil as nitrate, ammonium nitrite and urea.
The atmospheric nitrogen is fixed into oganic combinations such as amino acids, protenis, nucleic acids etc. in living organisms via inorganic forms such as NH4 +.
Ammonification:
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The dead remains of animals and plants are decomposed through microbial activities to produce ammonia.
Nitrification:
Ammonia is rapidly converted first to nitrites and then to nitrates by the process of nitrification.
Denitritication:
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Nitrate is converted to nitrogen gas by the micro-oiganisms e.g., Pseudomonas.
Biological nitrogen fixation: Nitrogen gas fixed in the form of NH/ by both free living and symbiotic bacteria.
Freeliving nitrogen fixing bacteria includes cyanobacteria, Azotobacter and Clostridium.
Symbiotic N2 Fixation:
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Symbiotic bacteria are found in the root nodules of the members of family Leguminosae, they have the ability to take up atmospheric N2 through a process called Symbiotic N2 fixation. Rhizobium (e.g., R. leguminosarum, R. meliotii) present root nodules of fabaceae e.g., Cajanus, Cicer, Glycine, Lens, Phaseolus, Vigna, Pisum etc.
Root hair of legume secrete flavonoids and betaines which attract bacteria of Rhizobium species (e.g., R. leguminosarum, R. melilotii).
Bacteria release nod factors that cause curling of root hair, degeneration of cell wall, passage into root hair where infection thread carrying bacteria is formed.
Finer branehes of infection thread pass into cells where bacteria stop dividing and form irregular, polyhedral bodies called bacteroids. These bacteroids present in groups or vesicles surrounded by host membrane.
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A pinkish pigment i.e., leg haemoglobin (Lb) develops around each vesicle which is related to blood pigment haemoglobin. In root nodules, it found as oxygen scavenger, protecting enzyme nitrogenase from oxygen.
Symbiotic nitrogen fixation requires Nod genes of legume, nod, fix and nif gene clusters of bacteria.
Nitrogen fixation requires an enzyme nitrogenase, ATP, a source of hydrogen (NADPH, FMNH) and organic acids to trap ammonia produced through nitrogen fixation. From this other amino acids and nitrogenous substances of different types are formed.
Amino acids are translocated to other parts of the plant, which act as building blocks for the synthesis of various types of proteins.