The foul smell of urine of a healthy man having healthy food, when spilled on floor, is mainly due to the bacterial decomposition of

The foul smell of urine when spilled on the floor is primarily due to the bacterial decomposition of urea into ammonia. Fresh human urine contains urea, which is an abundant organic compound. When urine is exposed to the environment, the urease enzyme—produced by various bacteria, fungi, and plants—catalyzes the hydrolysis of urea. This chemical reaction breaks down one mole of urea into two moles of ammonia and one mole of carbon dioxide (or carbonate). Ammonia is a volatile gas with a characteristic pungent and 'foul' odor, which also increases the pH of the urine, making it more alkaline [2]. While other organic processes like anaerobic digestion can produce methane from waste [1], the specific rapid odor associated with urine degradation in open environments is the direct result of this ureolytic activity and the subsequent release of ammonia gas [2].

Sources

1. [2] Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 7: Climate Change > 2. greenhouse gases > p. 11
2. [1] https://www.sciencedirect.com/topics/nursing-and-health-professions/urease