Microbiology of Soils
Nitrogen fixation

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

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Microbiology of Soils
Syntrophism involves

exchange of nutrients between two species
exchange of nutrients among species
no exchange of nutrients between two species
no exchange of nutrients among species

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