Question

why are some bacteria called biological nitrogen fixers?​

Answer 1

The symbiotic nitrogen-fixing bacteria invade the root hairs of host plants, where they multiply and stimulate formation of root nodules, enlargements of plant cells and bacteria in intimate association. Within the nodules the bacteria convert free nitrogen to ammonia, which the host plant utilizes for its development.

Answer 2

Some bacteria are called nitrogen fixers because of they help to fix the amount of nitrogen in plants.

For ex:- Rhizobium