pH drop might be due to the formation of organic acids (see Yang et al., Journal of Industrial Microbiology & Biotechnology 42:1533-1545) rather than the consumption of ammonia.

Dear  Uriel Maor,

 Aspergillus is types of fungus. The fungus is commonly found growing on dead leaves, stored grain, compost piles, or in other decaying vegetation. It can also be found on marijuana leaves. Foods utilized by humans and our domestic animals are good nutritional sources for Aspergillus and it can convert ammonia in acidic form causing low PH.

Regards

Prem Baboo

Aspergillus usually convert ammonia to ammonium hydroxide and change pH to alkaline form then the compound interact with other compounds results in the formation of acid which lead to change pH to acidic format 

Uriel Maor,

Fate and role of ammonium Ions during fermentation by Aspergillus niger  can you see in attachment