The nitrogen-fixing microbes could be free-living or symbiotic. Examples
of free-living nitrogen-fixing aerobic microbes are Azotobacter ,Beijernickia.
Since amides contain more nitrogen than the amino acids, they are transported to other parts of the plant via xylem vessels. In addition, along with the transpiration stream the nodules of
some plants (e.g., soyabean) export the fixed nitrogen as ureides. These compounds also have a particularly high nitrogen to carbon ratio.