Old urine is very alkaline--but what else is unique to it?

Understanding that old urine becomes very very alkaline makes it clear why it feels so refreshing to use on the skin or wounds, even though its ammonia-like smell suggests that it would burn.

What happens to the nutrients in urine when it ages? The minerals would remain... both the organic and inorganic? What about the vitamins? Are any reduced by the fermentation?

I suppose urine is left open and even put in the sun because the evaporation concentrates whatever is in it.