Assimilation refers to the process by which plants and animals integrate the NO3- while forming ammonia through nitrogen fixation and nitrification. Plants absorb these types of nitrogen in form of nitrites, through their roots, and integrate them into plant proteins and nucleic acids. Animals obtain these forms of nitrogen by feeding on plant tissues.
Nitrogen is essential to life, as it constitutes a major component of amino and nucleic acids. In addition, it is a key part of ATP, which is the basic energy molecule for living things. Animals and plants cannot directly obtain nitrogen from the atmosphere. It is however, made available through nitrogen cycle processes. Nitrogen cycle is a complex process involving five steps: nitrogen fixation, nitrification, assimilation, ammonification and denitrification.
Nitrogen fixation involves conversion of gaseous nitrogen to ammonia (NH3 or NH4+), through biological fixation or nitrate through a physical process involving high energy. Nitrification is a two-stage process and involves conversion of NH3/NH4+ to NO3-. The third step in nitrogen cycle is assimilation. Another process involved in nitrogen cycle is ammonification. It involves breaking of the large quantities of organic nitrogen produced during assimilation, ammonia. The ammonia is excreted into the environment and is also available for assimilation or nitrification. Denitrification involves reducing of NO3- to gaseous N2 by anaerobic bacteria.