Terrestrial vertebrates use the urea cycle to convert the ammonium ion to urea such that the urea can be excreted. In the production of urea, only one of the nitrogens in the product comes from ammonium ions. What is the source of the other nitrogen?

Asparate is the source of other nitrogen

Explanation:

In the terrestrial vertebrate urea cycle is present which converts ammonium ions to urea and then this urea is excreted out of the by through urine.

First ammonia and bicarbonate combine to form Carbamoyl phosphate. The Carbamoyl phosphate and ornithine get condense to citrulline. Until here the reaction occurs in mitochondria.

Then the citrulline is transported in the cytosol where another nitrogen is added by an amino acid called aspartate and citrulline is condensed into argininosuccinate and aspartate converts and released as fumarate. So the correct answer is aspartate.