Question

What is the role of nitrifying, nitrogen-fixing, denitrifying, and ammonifying bacteria? (What is converted to what by each of them?)

Answer 1

Nitrifying bacteria - oxidize ammonia and/or nitrites to nitrates
Nitrogen-fixing bacteria - use atmospheric nitrogen gas to produce ammonia, which is incorporated into composition of organic molecules.
Denitrifying bacteria - nitrogen oxides are converted into nitrogen gas
Ammonifying bacteria - decompose organic molecules with the release of ammonia