Source of nitrogen in urea cycle is:

The urea cycle/ornithine cycle is a cycle of biochemical reactions that produces urea from ammonia. In mammals, the urea cycle takes place only in the liver. Function: Organisms that cannot easily and quickly remove ammonia usually have to conve it to some other substance, like urea or uric acid, which are much less toxic. Reactions: The urea cycle consists of five reactions. The cycle conves two amino groups, one from NH4+ and one from Aspaate, and a carbon atom from HCO3-, to relatively nontoxic excretion product, urea, at the cost of four "high-energy" phosphate bonds. Nitrogen donors in the urea cycle are ammonia and aspaate.