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

On storage of fresh urine  without any preservative , breakdown  of urea is hydrolysed  by the enzyme urease of bacteria,present in urine .With decomposition  of urea into  ammonium and bicarbonate ions ,pH is increased. There is a strong relationship between pH and the fraction of urea decomposed into ammonia levels of  urine on storage.With an increase in pH, ureolysis is found to increase and vice versa. 
Certain Food , food bye products and drugs  secreted in urine enhance accelerated breakdown of urea on storage into ammonia lead to increasing levels of urinary ammonia on storage.In Dehydration, urine becomes concentrated and urinary urea  levels are increased , subsequently ammonia levels will see upsurge by breakdown of this urea  concentrated stored urine.
In starvation, rate of urea secreted is is increased with breakdown of proteins.Urinary concentration of urea is increased with subsequent its breakdown leading increased ammonia formation in stored urine.
 Presence of increased naturally occurring  urease inhibitors including in cabbage extracts, onion or garlic,  are  inhibitors for urease, may  not see increased levels of ammonia on storage of urine. Metallic compounds like boron ,sulphur, thiols,  or fluoride compounds, and specific acids in urine are   urease enzyme inhibitors , may show decrease of ammonia levels on storage rather than its surge.Urine infection by   gram-negative bacilli,a GIT Commensals ,  are regarded as an  urea-splitting organisms, may further lead to increase of urinary levels on its storage  Urinary samples show   increased decomposition of urea under storage temperatures of 60 °C and 70 °C compared with storage under ambient temperature as activity of urease forming bacteria is affected under this temperature range

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

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

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.