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
Denitrification is

reduction of nitrate (NO3-) to nitrogen gas
changing of atmospheric nitrogen (N2) to nitrogen compounds
both (a) and (b)
reduction of nitrate to organic nitrogen compounds

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