Vesicular-arbuscular mycorrhizal (VAM) fungi are involved in the formation or preservation of soil structure and in the uptake of bound soil water by plants. Together with their well-known function in mediating and enhancing mineral nutrition of their host plant, they are thus agents of resource conservation as well as of crop production. Under severe photosynthate stress, such as heavy grazing, VAM colonization is impaired. When this occurs, an interrelated chain of events may be initiated with adverse effects on agricultural plant-soil systems.