第一节 The nitrogen cycle Nitrogen exists predominantly in an oxidized state in the environment, occurring principally as n2 in the atmosphere or as nitrate ion (NO3) in the soils and oceans. Its acquisition by biological systems is accompanied by its reduction to ammonium ion (NH4) and the incorporation of nh4 into organic lin kage as amino group The reduction of NO3to NH4 occurs in green plants, various fungi, and certain bacteria in a two-step meta bolic pathway known as nitrate assimilation第一节 The nitrogen cycle Nitrogen exists predominantly in an oxidized state in the environment, occurring principally as N2 in the atmosphere or as nitrate ion (NO3 - ) in the soils and oceans. Its acquisition by biological systems is accompanied by its reduction to ammonium ion (NH4 + ) and the incorporation of NH4 + into organic linkage as amino group. The reduction of NO3 - to NH4 + occurs in green plants, various fungi, and certain bacteria in a two-step metabolic pathway known as nitrate assimilation