<P> Nitrification is the biological oxidation of ammonia or ammonium to nitrite followed by the oxidation of the nitrite to nitrate . The transformation of ammonia to nitrite is usually the rate limiting step of nitrification . Nitrification is an important step in the nitrogen cycle in soil . Nitrification is an aerobic process performed by small groups of autotrophic bacteria and archaea . This process was discovered by the Russian microbiologist Sergei Winogradsky . </P> <P> The oxidation of ammonia into nitrite is performed by two groups of organisms, ammonia - oxidizing bacteria (AOB) and ammonia - oxidizing archaea (AOA). AOB can be found among the β - proteobacteria and gammaproteobacteria . Currently, two AOA, Nitrosopumilus maritimus and Nitrososphaera viennensis, have been isolated and described . In soils the most studied AOB belong to the genera Nitrosomonas and Nitrosococcus . Although in soils ammonia oxidation occurs by both AOB and AOA, AOA dominate in both soils and marine environments, suggesting that Thaumarchaeota may be greater contributors to ammonia oxidation in these environments . </P>

The process of nitrification is carried out by what group of organisms