Question

Which bacteria fixes nitrogen in soil?

Answer 1

Microbes or bacteria that fix nitrogen in soil are called nitrogen fixing bacteria.There are many species of bacteria that can fix nitrogen in soil but popular are azotobacter and rhizobia, acetobacter and certain blue green algae.
These bacteria can fix atmospheric nitrogen and then convert it into usable nitrogenous compounds which can be used by plants for the photosynthesis and synthesis of plant proteins and other compounds.

Complete answer:
Nitrogen fixation is the process in which atmospheric nitrogen is converted to a usable nitrogen form like ammonia or nitrogen oxide. Nitrogen fixing bacteria are free living or symbiotic bacteria.
Free living soil bacteria like azotobacter also form associations with plants and can help in fixing nitrogen by converting atmospheric nitrogen to ammonia.

Rhizobia are the bacteria that live in symbiotic associations with the root nodules of leguminous plants and can do nitrogen fixation. Symbiotic bacteria need a plant host as they cannot fix nitrogen; rhizobia are vital sources of nitrogen.
The converted nitrogen in soil then can be easily used by plants for their plant protein synthesis and other compound formation.

Various strains of Rhizobium can fix the nitrogen in root nodules and few species of Rhizobium bacteria include for example Rhizobium leguminosarum and Rhizobium alamii. Examples of leguminous plants are beans.

Note:
Nitrogen fixing bacteria can transform atmospheric nitrogen into fixed nitrogen or ammonia or any usable form.
Nitrogen fixing bacteria are those which can fix atmospheric nitrogen in soil and these are such as Rhizobium , blue green algae ,acetobacter and azotobacter.
Fixing bacteria are of two kinds : free living nitrogen fixing bacteria like cyanobacteria and symbiotic nitrogen fixing bacteria like Rhizobium.