Accumulations of inorganic nitrogen (NH4 (+), NO2 (-), and NO3 (-)) were analyzed to evaluate the nitrogen circulation activity in 76 agricultural soils. Accumulation of NH4 (+) was observed, and the reaction of NH4 (+) -> aEuro parts per thousand NO2 (-) appeared to be slower than that of NO2 (-) -> aEuro parts per thousand NO3 (-) in agricultural soil. Two autotrophic and five heterotrophic ammonia-oxidizing bacteria (AOB) were isolated and identified from the soils, and the ammonia-oxidizing activities of the autotrophic AOB were 1.0 x 10(3)-1.0 x 10(6) times higher than those of heterotrophic AOB. The relationship between AOB number, soil bacterial number, and ammonia-oxidizing activity was investigated with 30 agricultural soils. The ratio of autotrophic AOB number was 0.00032-0.26 % of the total soil bacterial number. The soil samples rich in autotrophic AOB (> 1.0 x 10(4) cells/g soil) had a high nitrogen circulation activity, and additionally, the nitrogen circulation in the agricultural soil was improved by controlling the autotrophic AOBs.