ABSTRACT
Nitrification is the aerobic conversion of ammonium (NH4 þ) into nitrite (NO2) and nitrate
(NO3 ) by nitrifying bacteria. There are several chemoautotrophic bacteria such as Nitro-
somonas, Nitrosospira, Nitrosococcus, and Nitrosovibrio involved in the first step of the process whereby ammonium is oxidized to nitrite (Schmidt 1982; Paul and Clark
1996). In soil systems, Nitrobacter and Nitrosospira-like bacteria are involved in the second step whereby nitrite is converted to nitrate (Schmidt 1982; Bartosch et al. 2002). The
ammonia-oxidizing bacteria and the nitrite-oxidizing bacteria are usually found together,
and as a result, nitrite rarely accumulates in soil (Paul and Clark 1996).
