Microbiology of Soils
The nonsymbiotic bacteria which fix nitrogen live in the soil independently are

Azotobacter
All of these
considerably less important in comparison to the symbiotic bacteria
Clostridium

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Microbiology of Soils
Nitrogen fixation

especially changes nitrogen compounds, mostly amines such as NH2
both (a) and (b)
fix the free nitrogen (N2) by which it should not be usable by plants
changes the free nitrogen (N2) to a form usable by plants

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