Microbiology of Soils
Nitrogen fixation

both (a) and (b)
especially changes nitrogen compounds, mostly amines such as NH2
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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Microbiology of Soils
Nitrogen fixation by the microorganisms can be detected by adopting the approach of

measuring15N2 by mass spectrometer
cultivating the microorganisms in the presence of nitrogen labeled with isotropic nitrogen
All of these
demonstrating growth in a nitrogen free medium

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