Urea fermenta soluble ferment formed by certain bacteria, which, however, yield the ferment from the body of their cells only after they have been killed by alcohol. It causes urea to take up water and decompose into carbonic acid and ammonia. Many different bacteria possess this property, especially Bacterium ureæ and Micrococcus ureæ, which are found abundantly in urines undergoing alkaline fermentation.
The common source of ammonia in such cases is from decomposition of the urea in fermenting urine.
"Special Report on Diseases of Cattle" by U.S. Department of Agriculture