• Nitrogen cycle :
• Nitrogen is limiting nutrient for both natural and agricultural eco-systems as plants compete with microbes for limited nitrogen that is available in soil.
• Nitrogen fixation : it is the process of conversion of inert gaseous molecular dinitrogen (N₂) into salts of nitrogen that can assimilated by plants.
• Nitrogen oxides are formed in industrial combustions, automobile exhausts, power generating station and also by atmospheric reactions.
• Ammonification : the process of decomporition of organic nitrogen of dead plants and animals into ammonia.
• Most of ammonia retenters in at most phere but most of it is converted into nitrate by bacteria present in soil.

1NH₃ + 3O₂ => 2NO₂^- + 2H⁺ + 2H₂O

2NO₂^- + O₂ => 2NO₃^-

• Bacteria like nitrococcus and nitrosomonas oxidizes ammonia to nitrite.
• Bacterium nitrobacter further oxidizes nitrite into nitrate.

These toe steps involve in nitrification.

• Chemo autotrophic : nitrifying bacteria.
• This nitrate absorbed by plant and transported into leaves where it reduces to from ammonia.
• Denitrification is carried by bacteria pseudoinonas and thiobacillus.

Biological nitrogen fixation :-

• The process of reduction of nitrogen to ammonia by living organism is called biological nitrogen fixation.

N = N => NH₃

• Nitrogen fixes are of two types :

Free living and symbiotic :-

 

 

Free living nitrogen fixing bacteria	Aerobic : Azospirillum, Azotobacter Anaerobic : methanococues , clostridium.
Free living nitrogen fixing blue green algal.	Nortoc, Aulorira ,Anabaena
Symbiotic nitrogen fixing bacteria.	Aerorhizobium in stem of sesbania. Rhizobium in root nodules of legumes. Fronkia and Alnus.

 

Nodule formation :-

• Multiple interactions b/w roots of host plant and Rhizobium takes place during nodule formation.
• Rhizobia after multiplication, colonise around roots and attached to epidermal and root hairs.
•  Curling of root hair helps in invading bacteria into root hair.
• Bacteria intiate nodule formation in cortex of root after production of infection thread carrying bacteria in cortex of root.
• Then, differentiation of specialized nitrogen fixing cells has done after releasing of bacteria from thread to cell.
• Nodule consist of necessary biochemical components like enzyme leghaemaglobin and intro genase.
• Nitrogenase : catalyses conversion of atmospheric nitrogen into nitrogen

MO – Fe protein type.

Sensitive to molecular oxygen.

N₂ + 8e^- + 8H⁺ + 16A TP à  2NH₃ + H₂ + 16ADP + 16 Pi

• Enzyme is protected by oxygen by nodules having oxygen- scavenger called leghaemoglobin.
• Ammonia syntheses by nitrogenase requires very high input of energy ( 8 ATP for each NH₃ produced ) thus, energy required is obtained from respiration of host cell.

Fate of Ammonia :-

Reduction of nitrate is done in two steps to form nh₃.

1. Reductive amination :

 

• 
• 

In this process ammonia reacts with a  - retoglutaric acid to produce ghetamic acid.