name the group of bacteria which converts atmospheric nitrogen into nitrites and nitrates

The group of bacteria that convert atmospheric nitrogen into nitrites and nitrates are called nitrifying bacteria. For example, Nitrosomonas and Nitrobacter. The complete reactions can be illustrated as below:

Nitrogen  ----------- > Rhizobium ----------- > Ammonia

Ammonia  ----------- > Nitrosomonas ---------- > Nitrite 

Nitrite  ---------- >  Nitrobacter  ----------- >  Nitrate