Nitrogen is a key nutrient for plants but it exists primarily as inert nitrogen gas in the atmosphere. The nitrogen cycle describes the process by which nitrogen is converted between various chemical forms as it moves between the atmosphere, soil, organisms, and various environmental reservoirs. Key steps in the nitrogen cycle include nitrogen fixation by bacteria, nitrification of ammonia to nitrites then nitrates, assimilation of nitrogen by plants and animals, ammonification of organic nitrogen waste by decomposers, and denitrification of nitrates back to nitrogen gas. The nitrogen cycle is essential for providing plants with nitrogen needed for growth while enriching soils with nutrients.

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