Microbiology of Soils
Denitrification is

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

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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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