This is a response from Dr Robert Linderman to a question posed by Lon J Rombough (who also has a wonderful article on mycorrhiza) about whether mycorrhizae affect the pH of soils.

"...As it turns out, the real story about weathering mineral rock to release nutrients is that bacteria live in association with the mycorrhizal fungal hyphae, and they are the source of acid that can etch the rock, solubilising it for uptake by the mycorrhizal fungus and on to the plant. Pretty neat, huh? C.Y.Li, now retired from the Forest Service here in Corvallis, was the scientist who documented that fact. He was able to isolate the bacteria from the hyphae and show that they could dissolve even marble with the acids they produced. It is the bacterial association with the mycorrhizal fungal hyphae that perform many functions previously attributed to the mycorrhizae. I call this phenomenon the 'mycorrhizosphere effect'. We have shown that the fungi selectively increase populations of bacteria that can do many things as part of a taem, even help suppress diseases (the subject of my own research). So I have described a new paradigm for the mycorrhizosphere wherein the host plant associates with the mycorrhizal fungi and the soil bacteria (certain ones) associate with the mycorrhizal fungus, making a marvelous microbial team that has been a winner for some 460 million years!! By the way, we have shown that some of the bacteria can produce acids, some ammonia; both can change the pH around them..."