Mineral Nutrition

Biological nitrogen fixation

  • Nitrogen is abundant in the atmosphere (78%) but plants don’t absorb the same and hence is the most Critical element.
  • Nitrogen exists as dinitrogen atoms connected by three covalent bonds (N ≡ N)
  • Nitrogen fixation is converting inert atm. N2 into its usable compounds (Nitrate, ammonia, amino acids, amides etc.,) through biological(microbes) and Abiological methods (electric discharges, ozonisation and combustion).
  • Several bacterial species live in soil but cannot fix N2 by themselves.
  • Steps involved are (i) Bacterial collection over root hairs (ii) Degradation of cell wall (iii) formation of infection thread (iv) Bacterial multiplication. (v) Bacteroid formation covered by pinkish Leg haemoglobin (Lb). (vi) Protection of Nitrogenase (vii) Nitrogen fixation
  • Nitrogenase has 2 active centers – iron and Molybdenun to which dinitrogen attaches and reduces further to dimide (N2H2) , Hydrazine (N2H4) and then ammonia (2NH3) , which is the first stable product of N2 fixation
  • Free living N2 fixation happens through Annonification and Nitrification.
  • Bocillus ramosus, B. Vulgaris , B. mesentericus act upon proteins and convert them into ammonia (NH3), later converted into nitrites by Nitrosococcus and Nitrosomonas by Nitrification.
  • Nitrocystis and Nitrobacter convert nitrites and nitrates in the next step. Aspergillus flavus can perform both the reactions of Nitrification.
  • P. denitrificans , Thiobacillus denitrificans and Micrococcus denitrificans reduce nitrates into gaseous N2 under anaerobic conditions (water logging)
  • The obtained nitrates and nitrites are assimilated in plants by FAD/FMN/Nitrate reductase and Ferredoxin/ Nitrite reductase respectively to form 2NH3 (Ammonia), which is then converted to ammonium ions (NH4+).
  • Amino acids are the first Organic compounds of Nitrogen, which are synthesized by reductive amination, catalytic amidation and Transamination.
  • Amides are double aminated keto acids, which are derivatives of amino acid Eg : glutamine and Asparagine, Niacinamide
  • Vreides are mediated as a means of N2 transport from nodules of legumes (soyabean), which have a high N/C.
  • Finally, Proteins are synthesized from amino acids using ribosomes through the translation process.
  • Free living N2 fixing becteria : Azotobacter , Beijerinckia, Klebsiella , Desulfovibrio
  • Free living N2 fixing cyanobacteria : Anabaena , Nostoc , Calothrix, Aulosira
  • Symbiotic N2 fixing Bacteria : Frankia , Rhizobium , Xanthomonas , Mycobacterium
  • Symbiotic N2 fixing cyanobacteria : Anabaena, Nostoc.
  • Highly mobile elements : N, P, K, Mg
  • Immobile elements : Ca, S

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