Why is the ammonia/ammonium concentration highly increased during storage of human urine?

Ammonia/ammonium concentrations increase during urine storage primarily because of bacterial urease activity. Urease hydrolyzes urea into ammonia and carbon dioxide. 

During the storage process, urease produced by bacteria will hydrolyze urea into ammonia and carbon dioxide, resulting in a significant increase in ammonia/ammonium concentration

Urease, produced by bacteria, converts urea in the urine to ammonia. 

During storage, urea is transformed to ammonium increasing ammonium concentration and pH avoiding pathogens' survival.