Nitrogen cycle

1. Nitrogen Cycle
Dr. S. Sivasankara Narayani., M.Sc.,M.Phil.,Ph.D., MRSB (UK)

2. Intro
• Nitrogen is key nutrient for plants.
• The abundant nitrogen in the atmosphere cannot be used directly by plants or animals.
• Nitrogen cycle is a biogeochemical process through which nitrogen is converted into many
forms, consecutively passing from the atmosphere to the soil to organism and back into the
atmosphere.
• Definition: “Nitrogen cycle is a biogeochemical process which transfoms the inert nitrogen
present in the atmosphere to a more usable forrm for living organism

3. Nitrogen Gas types
• Organic
• It exist in living organism.
• They get passed through food
chain by the consumption of other
living organisms
• Inorganic
• Abundant in atmosphere
• Nitrogen is made available to plants
by symbiotic bacteria which convert
the inert nitrogen into a usable
form such as nitrites and nitrates

4. Stages
• Nitrogen fixation
• Nitrification
• Assimilation
• Ammonification
• Denitrification

6. Nitrogen Fixation
• Initial step
• N2 ------- NH3
• Inert form is deposited into soils from the atmosphere and surface waters, through
precipitation.
• Two nitrogen atoms get separated and combines with hydrogen to form ammonia (NH4+).

7. Cont…
• Entire process of nitrogen fixation is completely by symbiotic bacteria which
are known as Diazotrophs.
• Ex: Azotobacter , Rhizobium
• Nitrogenase enzyme – N + H = Ammonia

8. Types
• Atmospheric Fixation
• Lightning breaks nitrogen into nitrogen oxides – used by plants
• Industrial mode
• Direct combination of nitrogen and hydrogen to produce ammonia
• Converted into various fertilisers such as Urea.
• Biological
• Rhizobium and Blue green algae

9. Nitrification
• Ammonia is converted into nitrate
• Nitrites are formed by oxidation of ammonia by Nitrosomonas
• Nitrites are converted into nitrates by Nitrobacter
2NH4
+ + 3NO2 2NO2
- + 4H+ + 2H2O
2NO2
- + O2 2NO3
-

10. Assimilation
• Plants - ammonia, nitrite ions, nitrate ions or ammonium ions
• Used in the formation of plant and animal proteins
• Enters the food web

11. Ammonification
• Plant or animal die – released into soil
• decomposers – bacteria and fungi - convert organic matter back into
ammonium.
• Used for the biological process

12. Denitrification
• Conversion of nitrate into gaseous nitrogen.
• Occurs in the absence of oxygen
• Done by Denitrifying bacterial species – Clostridium and Pseudomonas
• Organism will process nitrate to gain oxygen and gives out free nitrogen gas as
byproduct.

13. Importance
• Synthesis Chlorophyll
• Decomposition
• Enriching the soil – with esseential nutrients required for cultivation.

14. 12-07-2023
Dr.SS