Nitrogen assimilation is the general process by which plants and microbes take up ammonium for further incorporation into amino acids, nucleic acids and chlorophyll (Fig. 1). When nitrate is absorbed, it must first be reduced to ammonium before incorporation. In plants associated with rhizobia the plant receives ammonium directly from the root nodule in exchange for simple sugars.
Figure 1. Nitrogen can be taken up as ammonium or as nitrate and used to build up organic component.