Nitrogen cycle

NITROGEN CYCLE

• Nitrogen is a constituent of amino acids, proteins, hormones, chlorophylls and many of the vitamins.
• Plants compete with microbes for the limited nitrogen that is available in soil; thus, nitrogen is a limiting nutrient for both natural and agricultural eco-systems.
• Nitrogen exists as two nitrogen atoms joined by a very strong triple covalent bond (N ≡ N).
• The process of conversion of nitrogen (N2) to ammonia is termed as nitrogen fixation.
• Decomposition of organic nitrogen of dead plants and animals into ammonia is called ammonification.
• Some of this ammonia volatilises and re-enters the atmosphere but most of it is converted into nitrate by soil bacteria in the following steps:

2NH₃+ 3O₂ à 2NO₂^- + 2H⁺ + 2H₂0

2N0₂^- + O₂ à 2N0₃^-

• Ammonia is first oxidised to nitrite by the bacteria Nitrosomonas and/or Nitrococcus.
• The nitrite is further oxidised to nitrate with the help of the bacterium Nitrobacter and the step is known as nitrification.
• In the leaves, nitrates reduced to form ammonia that finally forms the amine group of amino acids, and the step is known as assimilation.
• Denitrification is the process in which nitrate in the soil is reduced to molecular nitrogen by Pseudomonas and Thiobacillus.

Fig. Nitrogen fixation