Question

If wheat field is inoculated with $Rhizobium$

• A. Soil will become rich in calcium
  
• B. Soil will become nitrogen rich
  
• C. Soil will be depleted of nitrogen
  
• D. No effect on soil nitrogen
  

Answer 1

The correct option is D No effect on soil nitrogen


Role of $Rhizobium$ and leguminous plants in nitrogen fixation
Rhizobium forms a symbiotic association with roots of leguminous plants only. They have a special enzyme called nitrogenase which helps in nitrogen fixing. However, the enzyme gets inactivated in the presence of oxygen.
To protect it, the roots of the leguminous plants contain a pigment called leghaemoglobin. This pigment acts as an oxygen-scavenger and maintains an anaerobic environment helping in nitrogenase enzyme activity.

Plants like wheat lack the pigment leghaemoglobin, therefore nitrogen fixation cannot take place.

Final answer
(B) No effect on soil nitrogen