Impact on PhD application omitting a recommendation letter from research advisor in that field? If an applicant has research experience in a field that they are applying to and includes it in their application, would there be negative impact on the application if they chose not to include a recommendation letter from their research advisor for that research experience?

The reasons that they don't may be misconstrued as negative, but it could just be that there are others who could give stronger letters of recommendation, for instance from other research advisors in another field or a teacher with whom the applicant had a very good relationship with. Would not including the research advisor as a recommender be cause for speculation?
It would likely be cause for speculation if the applicant is promoting their research experience but does not include a letter from their advisor. There may be inert reasons and it does happen (an advisor that is too busy or too aloof; on a leave for medical or other reasons). There are also cases where the applicant and advisor do not get along well, or where their interests are not aligned. In these cases it may be worth a direct conversation to find out if they would offer strong support for the position and if not then ask why not. The applicant may be surprised to learn that the advisor with whom she/he argues holds them in high regard. At the same time, the advisor may have useful input to offer that the applicant may wish to weigh as they decide on an educational and career trajectory.

That said, if no advisor letter is provided then the applicant should succinctly provide an explanation for why the given recommenders were chosen, and leave it at that unless pressed. If the applicant has a good relationship with another recommender, she/he might ask that recommender to include a supportive explanatory note in their letter.