Soil diversity

The complexity of the soil system with many large gradients including nutrients and gases such as oxygen drive a massive diversity below ground. It has been estimated that there are over 40,000 different organisms contained within a gram of soil which make up a biomass, measured as the carbon content of organisms, of approximately one gram of biomass C in every kilogram of soil.

This diversity is possible because the population is composed of many different organisms each performing a different suite of functions. Indeed in soil there is a complete soil food web with a complexity that rivals any above ground system.

Our understanding is limited not only because of the high diversity but also because over 95% of the organisms residing below ground remain unculturable. However, we rely on the continued health of this community for the maintenance of functions such as food production, pollution attenuation and conservation of habitats without which life would be impossible.

Work at the James Hutton Institute is involved with a wide range of soil organisms many of which, since they are bacterial are difficult to visualise. For example, bacteria that cycle Nitrogen and others that can solubilise Phosphorus, an element that often limits plant growth; recent work isolated bacteria that could aid agriculture in sub-Saharan Africa by increasing P availability (see Figure 1 right).

Other work includes work on symbiotic fungi that aid plant nutrient uptake and can protect against pathogens and drought. These arbuscular mycorrhizal fungi are present throughout soil but enter plant roots and forming arbuscules (see Figure 2 below left). Other larger soil groups include amoebae (see Figure 3 below middle) that can alter shape to move through the complex soil structure and nematodes (see Figure 4 below right) that are in themselves highly diverse with plant, bacterial, fungal feeders as well as predators and omnivores. The majority of this group are beneficial and we are developing tools to use nematodes to gauge soil health.