Why does urine smell like ammonia?
All animals (including humans) consume significant amount of proteins. Proteins consist of aminoacids, which include amine nitrogen. This nitrogen traces back to mineral fertilizers humans use in agriculture. As proteins in our bodies are destroyed, the waste is utilized, formine urea (carbamide), so ammonia didn't cause too much trouble. However, urea is slowly hydrolized in water back into ammonia, so old urine stinks of (among other things) ammonia.