Why does day old urine smell rank while fresh unine does not?

In scenarios where the toilet is clogged or simply remains unflushed, urine that sits in the bowl the better part of the day smells pretty rank after awhile, but fresh urine often has relatively little smell. What is going on that causes it to stink if it sits over time?

Bacteria converting the urea to ammonia and other nitrogenous breakdown products. (IIRC)

Beat me to it. Fresh urine is usualy sterile. Old urine is not.