Answer: Azolla with Anabaena.
- A Rhizobium in soybean
- B Azolla with Anabaena
- C Mycorrhiza on pine
- D Pseudomonas alone
Correct answer: B. Azolla with Anabaena
Explanation: The water fern Azolla harbours the cyanobacterium Anabaena and is used as a nitrogen biofertiliser in rice fields.
Atmospheric N₂ is fixed into ammonia by nitrogen-fixing bacteria (protected from oxygen by leghaemoglobin in root nodules), converted to nitrate via nitrification for plant uptake, and eventually returned to the atmosphere as N₂ via denitrification, completing the cycle.
Concept context
Essential mineral elements, their roles and deficiency symptoms. Nitrogen fixation and soil nutrition.