No one has answered, so here is my thinking…

If you look at the molecule of a common amino acid in a plant, glutamine, C5,H10,N2,O3

Then look at the forms of nitrogen:

Amino Sugar C6,H10,N2,O4 (no pH change…I think)

Urea C,O,N2,H4 (1x lowers pH thru nitrification)*

Ammonium N,H4 (2x lowers pH thru nitrification)*

Nitrite N,O2 (1x raises pH thru construction of amino acid)*

Nitrate N,O3 (2x raises pH thru construction of amino acid)*

You can very easily see which molecules are most proportional in composition to the amino acid. I listed them in order. Well, I believe that is the same order in which nitrogen is most efficiently taken up. It takes less energy to find all the parts need to assemble an amino acid.

The fact that Amino Sugar and Urea already carry some carbon in the molecule is a major benefit.

The other thing to balance in your choice of nitrogen is if you need to raise the pH, lower the pH or maintain the pH in the soil. You may choose a more efficient form of nitrogen, but if it changes your pH in the wrong direction, thereby making nutrients less available (by chemistry and/or by killing microbiology) then you will lose the efficiency you thought your were gaining.

*the 1x or 2x pH change does not represent a pH change of 1.0 or 2.0. This is just relative to each other in the list. Ammonium will lower the pH twice as much as Urea. Also, these are off the top of my head, and I could not quickly find an article that verifies these proportions. I know this is approximately correct but don’t quote me. Also, if someone does know the exact numbers, please do list them!