I remember your post about aragonite. It is mainly CaCO3, right?
Sorry I didn't notice the link was limited to my institution (I emailed PDF to you just in case it is something interesting). And you are right, it is relatively short term study (30 weeks). From my half-ass understanding, the authors are suggesting that the pH drop is mainly due to the excretion from root. According to the other papers, concentration/composition of fertilizer could influence the acidification, but this paper was trying to show the existence of root is more relevant. The rhizosphere chemistry seems to be pretty complex (and the cited studies are not from epiphytes), but the discussion section talks about other studies which showed that pH decline also happens with deficiency in K, P, or Fe. So, if you are not seeing pH decline in your low-K, this may be a good thing (i.e., not deficient in K?).