Nitrogen Cycle Nitrification assimilation Ammonification

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The nitrogen cycle is the biochemical cycle by which gaseous N2 is converted into different chemical forms of N through biologic, physical, and geologic processes and it is a complex process. It include following points.
Nitrogen fixation
Nitrification
Nitrogen assimilation
Ammonification
Denitrification
1. Biological nitrogen fixation
 Major parts of nitrogen fixation occurs by natural method i.e. approx. 54x10⁶ metric ton /year and is carried and by certain prokaryotes like Rhizobium, Nostoc, Anabaena.
 Symbiotic nitrogen fixation : Some of bacteria (Rhizobium) live in symbiosis with certain legume plants and Frankia in the roots nodules of Alnus, Casuarina etc. Nostoc, in corallaoid roots of cycas, Anabaena in the leaf Azolla. The nitrogen fixation is catalysed by the enzyme nitrogenase. Nitrogenase is sensitive to molecular oxygen (O2) and thus legumes contains leghaemoglobin.
 Free living (asymbiotic) nitrogen fixation: BGA(Nostoc, Anabaena), Aerobic bacteria (Azotobacter, Beijerinkia, Bacillus, Klebsiella), Anaerobic bacteria (Clostridium).
 The nitrogen fixation or reduction of N2 to NH3 (ammonia) is a complicated, multistep process
(N2+ 8e- + 8H+ + 16ATP → 2NH3 + H2 + 16ADP + 16P).
NITRIFICATION
 Ammonia can be observed by some plants only (Coniferous forest) and most of the nitrogen taken up by plants is converted by bacteria from ammonia ion into nitrite ion and then into nitrate ion and the process is called nitrification.
 Thus nitrification is a two-step process.
1. At first Nitrosifyers, chemosynthetic bacteria Nitrosomonas, Nitrosococcus, convert ammonium ion into nitrite.
(NH4+ + O2 → NO- 2 + H2O +H+).
2. Later nitrifying bacteria, such as Nitrobacter, Nitrocystis oxidize nitrite into nitrate
(NO-2 + O2→NO-3 ).
NITROGEN ASSIMILATION
 Absorption of nitrogenous compounds by the plants and animals and utilizing it for making proteins is known as assimilation.
 Plants take up nitrate ions from soil through their root systems and 1st the nitrate in the cytoplasm is reduced to nitrite with the help of enzyme nitrate reductase and then nitrite into ammonium ions in the plastid by nitrite reductase.
 NH4+ then combine with organ acids to form amino acids essential for plant growth and development and are carried by enzyme GOGAT & GS.
AMMONIFICATION
 Ammonification is the process by which organic nitrogenous substance such as dead plant and animal matter containing Proteins, Amino acids, Manure, Urea, are decomposed and converted back into ammonia (NH3) or ammonium ions (NH4+) with the help of ammonifying bacteria.
 Bacillus mycoides, B. vulgaris and B. ramosus, etc.
DENITRIFICATION or NITRATE REDUCTION
 In this process the available from of N (nitrate) for plant are converted into free nitrogen by certain microbes.
 Anaerobic Bacteria such as Bacillus subtilis, Thiobacillus denitrificans, Micrococcus denitrificans, use nitrate as a substitute for oxygen during respiration and convert it to different nitrogenous compounds according to the following chain of reactions:
(NO3- → NO2 - → NO → N2O → N2)
 Denitrification helps to return nitrogen in atmosphere and thus eventually, nitrogen is released into the atmosphere as N2O and NO or as N2.
 Denitrification has a negative effect on agriculture, as it removes nitrogen from soils but are useful in wastewater treatment.
 Overall nitrogen cycle plays a crucial role in maintaining the balance of nitrogen in ecosystems and prevents environmental issues such as eutrophication in water bodies.

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