Analysis of Nitrification in Agricultural Soil and Improvement of Nitrogen Circulation with Autotrophic Ammonia-Oxidizing Bacteria

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 ⁺?→?NO₂ ^? appeared to be slower than that of NO₂ ^??→?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?×?10³–1.0?×?10⁶ 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?×?10⁴ 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.