Sources of the nitrogen in urea cycle are the following:
The 2 nitrogen atoms of urea are derived from two sources, one from ammonia and other from alpha amino group of aspaic acid. Ref: Textbook of Biochemistry By DM Vasudevan, 3rd Edition, Pages 163-5
Aspaate and ammonia
Glutamate and ammonia
Arginine and ammonia
Uric acid