Bacteria convert ammonia into nitrates. Bacteria convert nitrates into nitrogen gas. Bacteria convert nitrogen gas into ammonia.

The atmospheric nitrogen cannot be utilized by the plants and this nitrogen should be converted into forms that can be utilized by the plants this is done by the nitrogen fixing bacteria.

Explanation:

Bacteria that converts the ammonia into nitrates is a nitrifying bacteria. It is a two way process. It involves the conversion of ammonia ions into nitrites and nitrites are further converted into nitrates. Examples are Nitrosomonas and Nitrosospira.

Bacteria that converts the nitrates into nitrogen gas are called denitrifying bacteria.  For example, Rhizobium.

Bacteria that converts nitrogen gas into ammonia are ammonium fixing bacteria. They convert atmospheric nitrogen into ammonia. For example,  Clostridium.