Biodiversity is generally defined as the variety and variability of living organisms and the ecosystems in which this occurs. The variability of life in the soil encompasses not only plants and animals but also the invertebrates and microorganisms that are interdependent on one another and the higher plants they support. Biodiversity is composed of three interrelated elements: genetic, functional and taxonomic diversity as shown in Figure 1. Taxonomic diversity i.e. the number of species forms an important part of an ecosystem’s diversity and is controlled by the genetic diversity. Genetic diversity can be much more than the number of recognized species. Hence, several species may have the same functions, resulting in functional redundancy. Some species may also interact to perform functions not possible by any single species. Therefore, biodiversity is the interaction of all these elements.
Soil biodiversity is more extensive than any other environment on the globe when all living forms are considered. The soil biota contains representations of all groups of microorganisms, fungi, bacteria, algae and viruses, as well as the microfauna such as protozoa and nematodes. The total diversity is equal to greater than any coral reef or rain forest. Soil algae and protozoa, like higher plants and animals, can be identified by their morphology. Fungi and bacteria, however, require more extensive biochemical and genetic analysis to enable identification.
It has been estimated that only between 1 and 5% of all microorganisms on the earth have been named and classified. A large proportion of these unknown species is thought to reside in the soil. The possible numbers of existing species of different groups are 1.5 million species of fungi, 300,000 species of bacteria, 400,000 species of nematodes and 40,000 species of protozoa. New molecular techniques have been used to estimate that single gram of soil probably contains several thousand bacterial species.