Microbiology of Soils
Nitrogen oxidation (nitrification)refers to the

conversion of ammonium ions into nitrates through the activities of certain bacteria.
changing of atmospheric nitrogen (N2) to nitrogen compounds
None of these
sulfur is oxidized to the sulfate form through Thiobacillus bacteria

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Microbiology of Soils
Nitrogen fixation

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

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Microbiology of Soils
A heterocyst is

a type of spore
a terminally differentiated cell that fixes nitrogen
a cell that carries out oxygenic photosynthesis
the progenitor of cyanobacterial vegetative cells

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