Globally accelerating rates of species loss make it imperative that relationships between biodiversity and ecosystem function are analysed, yet resolution of these interactions has presented one of the most intractable challenges in ecological research. Because biodiversity in soil is considerably greater than that aboveground, and the identities and functions of many soil microorganisms are uncharacterized, the difficulties involved in establishing diversity-function relationships in the below-ground environment are compounded. In this paper, the mycorrhizal fungi are selected as a representative keystone group of soil microorganisms, and the relationship between their biodiversity and function in soil ecosystems are explored. For some keystone groups of soil microorganisms, prominent among which are the mycorrhizal fungi, diversity-function relationships are starting to be elucidated. Mycorrhizal symbionts are present in virtually all terrestrial ecosystems where they are major components of the soil microbial biomass. There is increasing evidence that mycorrhizal diversity is of central importance in agro-ecosystem functioning, and that intensification of agriculture and forestry, combined with air and soil pollution, is reducing their diversity and compromising their functioning. Two lines of evidence support the case that mycorrhizal diversity is of major functional significance, namely: (1) that mycorrhizal associations are multi-functional and exhibit complementarity, assisting plants in nutrient acquisition, mediating carbon transfer between plants and protecting their roots from pathogens; and (2) that based on emerging evidence of a combination of high specificity and dependency in many mycorrhizal associations, especially those involving myco-heterotrophic plants, it is hypothesized that the extent of functional 'redundancy' is low.