Microbiology of Soils
Some microorganisms have the ability to increase the nitrogen content of soils, are called as

all of these
nitrogen fixation
denitrification
nitrification

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Microbiology of Soils
Nitrogen fixation

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
especially changes nitrogen compounds, mostly amines such as NH2

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