Soil is one of the most diverse habitats on Earth. Nowhere in nature are species so densely packed as in soil communities. For example, a single gramme of soil may contain millions of individuals and several thousand species of bacteria. The complex physical and chemical nature of the soil, with a porous structure, immense surface area and extremely variable supply of organic materials, food, water and chemicals, provides a range of habitats for a multitude of organisms. These range from macro- to micro- levels depending on climate, vegetation and physical and chemical characteristics of a given soil. The species numbers, composition and diversity in a particular ecosystem depend on many factors including temperature, moisture, acidity, nutrient content and the nature of the organic substrates.
Soil biota includes archaea, bacteria, protists, tardigrades, rotifers, nematodes, acari (mites), collembolans (springtails), worms (enchytraeids and earthworms), macroarthropods (e.g. ants, termites, centipedes, millipedes, woodlice, etc.) and burrowing mammals.
It also includes plant roots, fungi and lichens. Root exudates attract a variety of organisms that either feed directly on these secretions or graze on the microorganisms concentrated near the roots, giving this busy environment the name ‘rhizosphere’. There are also animals, such as beetle larvae, flies and butterflies that use the soil as a temporary habitat to reproduce or to spend their early life stages feeding on different live and dead plant materials until they reach their maturity.
Soil communities are so diverse in both size and numbers of species, yet they are still extremely poorly understood and in dire need of further assessment. Research has been limited by their immense diversity, their small size and the technical challenge of identifying them.
Since ancient times, scientists have been trying to classify living organisms. By the end of the 20th century, the English biologist Thomas Cavalier-Smith created a new model with six kingdoms. During the 21st century, a phylogenetic approach, based on DNA comparisons to classify organisms has gained strength. However future changes in the classification might be needed.
Overall climate plays an overriding role in soil formation. Hence, soils generally differ from one major climatic zone to another.
Equatorial regions, with high temperature and rainfall levels, have deep, strongly weathered and very leached soils with low nutrient levels.
More arid conditions, with low precipitation and high evaporation, produce soils containing easily soluble components such as calcium carbonate or gypsum.
Soils in temperate climates tend to have more organic matter while the effects of parent material and precipitation levels are more evident.
In cold climates, soil formation is restricted and strongly influenced by freeze-thaw processes and the presence of ice in the subsoil (‘permafrost’).
Past climates also play an important role in determining current soil distribution, especially in the subarctic and northern temperate regions where glaciers have removed all soil material and new soils were formed after the retreat of the ice. Consequently, soils of these regions are relatively young or ‘immature’.
Soil classification schemes generally reflect different concepts of soil formation. For more information on the simplified descriptions of the world’s major soil types according to the World Reference Base for Soil Resources (WRB), an internationally used soil classification system please visit www.fao.org/soils-portal/soil-survey/ or http://www.isric.org/explore/wrb
Ref: Global Soil Biodiversity Atlas p21. p27