Hello AskHistorians.
https://en.wikipedia.org/wiki/Battles_of_Khalkhin_Gol
According to the wikipedia article "Furthermore, up to 30% of the total casualties were due the dysentery which Japanese believed was delivered by Soviet biological-warfare aerial bombs, as confirmed by laboratory testing. To reduce susceptibility to this form of biological warfare and to wage the biological warfare on their own, future Japanese divisions will commonly include a specialized Epidemic Prevention and Water Purification Departments."
However there seems to be no sources attributed to this, and I have not been able to find any other sources that talk about Soviet biological warfare at this battle.
I'm also suspicious of the assertion. Wikipedia's talk page is also a little bit leery. This is most likely vandalism, in my opinion.
In English, the major source would be Coox's two volume history, which is sited by the Wiki article. However I haven't read Coox and definitely don't have access to a copy to check the citation. A google books search (mods forgive me) suggests that [this assertion is NOT in Coox] (https://books.google.com/books?id=SIAwKiNRmrAC&pg=PA1167&lpg=PA1167&dq=nomonhan+biological+warfare&source=bl&ots=uNiBWwGziA&sig=8xJXivtoMROSiNWq_NQ848DbPf0&hl=en&sa=X&ved=0ahUKEwiRyKO8s7_QAhWBwiYKHV2mCSkQ6AEISTAJ#v=onepage&q=nomonhan%20biological%20warfare&f=false)
In Edward Drea's more recent (and much smaller and more accessible) work on Nomonhan/Khalkhin_Gol has no mention of any biological warfare.
[There are some works on Japanese biological warfare unit 731 which document some ineffectual activity at Nomonhan] (http://apjjf.org/-Tsuneishi-Keiichi/2194/article.html) but as far as I know, no Soviet ones.
And lastly, dysentery was quite common in the China theatre and the Pacific War as a whole among troops of all nationalities because water and sanitation in far flung areas was a difficult logistical problem that even the most logistically sophisticated armies struggled with. So it makes sense that Japanese troops, who often lacked adequate logistics would have experienced it, but of course it was not due to biological warfare.