Causes and consequences of biological diversity in soil.

There is a vast diversity of organisms that live in the soil, and the activities of the total soil biota, together with the diverse forms and functions of plant roots, have critical roles in soil functioning. In this paper I discuss the likely determinants of soil diversity and also comment on recent studies that have explored whether or not there is a relationship between soil organism diversity and ecosystem function. There is little evidence to suggest that soil diversity is regulated in a predicable fashion by competition or disturbance; rather it is attributed to the nature of the soil environment, in that soil offers an extremely heterogeneous habitat, both spatially and temporally, proving unrivalled potential for niche partitioning, or resource or habitat specialisation, thereby enabling co-existence of species. Most evidence that is available suggests that there is no predictable relationship between diversity and function in soils, and that ecosystem properties are governed more by individual traits of dominant species, and by the extraordinary complexity of biotic interactions that occur between components of soil food webs. There is evidence of redundancy in soil communities with respect to soil functions, but the scale of effect of changes in soil diversity on process rates depends on which species are removed from the community and the degree to which remaining species can compensate. As in aboveground communities, therefore, it would appear that species traits and changes in species composition, and alterations in the nature of the many important species interactions that occur in soil, are likely to be the main biotic control of ecosystem function. In view of this, consideration of these important biotic interactions and their sensitivity to environmental change must be a key priority for future research.