Plants absorb nitrogen through their root systems. Nitrogen from belowground biomass in the soil is taken in for use.
Animals receive their nitrogen from food sources.
It can be found in plants by herbivores or in other animals by carnivores. However, most processes using nitrogen employed by living organisms require nitrogen to be altered to a different state. There are four processes which take place during the nitrogen cycle in which living organisms make use of this important element. Fixation is the conversion of N 2 into either ammonia by biological fixation, or to nitrates by lightning and other inorganic processes. Ninety percent of the earth's fixed nitrogen is produced by biological fixation each year.