• Nitrogen fixation is conversion of atmospheric N2 to NH3 (ammonia). It is carried out by N2 fixers such as Rhizobium. • NH3 is converted to NO2−(nitrite) by nitrifying bacteria such as Nitrococcus. • Then NO2−is converted to NO3−(nitrate) by nitrfying bacteria called Nitrobacter. • Thiobacillus carries out denitrification, a process where NO2−∕NO3− is converted to N2.