Editing Nitrogen cycle (section)

=== Assimilation ===
{{Main|Assimilation (biology)|Nitrogen assimilation}}
Plants can absorb nitrate or ammonium from the soil by their root hairs. If nitrate is absorbed, it is first reduced to nitrite ions and then ammonium ions for incorporation into amino acids, nucleic acids, and chlorophyll. In plants that have a symbiotic relationship with rhizobia, some nitrogen is assimilated in the form of ammonium ions directly from the nodules. It is now known that there is a more complex cycling of amino acids between ''Rhizobia'' bacteroids and plants. The plant provides amino acids to the bacteroids so ammonia assimilation is not required and the bacteroids pass amino acids (with the newly fixed nitrogen) back to the plant, thus forming an interdependent relationship.<ref name="Willey 2011" /> While many animals, fungi, and other [[heterotrophic]] organisms obtain nitrogen by ingestion of [[amino acid]]s, [[nucleotide]]s, and other small organic molecules, other heterotrophs (including many [[bacteria]]) are able to utilize inorganic compounds, such as ammonium as sole N sources.  Utilization of various N sources is carefully regulated in all organisms.