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The Soil Habitat

Where are the organisms located in soil?

Most organisms are found in the top layers of soil, usually the top 2-3 centimetres, since this is typically where most of the organic matter is. Organisms do occur to depths of several kilometres below the soil surface, but the types of organisms that occur this far down are not the same as those close to the surface. The organisms in soil are often commonly found close to root surfaces in the rhizosphere, within living and dead roots, on soil particles, or amongst aggregates of soil particles.

Earthworms and other soil animals are able to move through most of the top layers of soil. Fungi can form a mat of hyphae, which can extend centimetres or even metres through the soil. They can also form a network of hyphae inside soil aggregates. Bacteria tend to accumulate inside soil aggregates because they are less likely to be eaten by soil animals such as protozoa and mites in this environment. Bacteria can be carried down further into the soil in water that is percolating downwards, but generally they do not move far.

Soils that are clayey often have more bacteria than sandy soils because the clay creates lots of small pores (spaces) which offer protection for bacteria. Sandy soils with fewer aggregates and small pores are less suitable habitats for bacteria and fungi unless a large amount of organic matter is added to the soil.

What are the main characteristics of the rhizosphere?

The rhizosphere is the region of soil that is immediately adjacent to and affected by plant roots. The rhizosphere is a very dynamic environment where plants, soil, microorganisms, nutrients and water meet and interact. The rhizosphere differs from the bulk soil because of the activities of plant roots and their effect on soil organisms.

A major characteristic of the rhizosphere is the release of organic compounds into the soil by plant roots. These compounds, called exudates, make the environment of the rhizosphere very different from the environment in the bulk soil. The exudates increase the availability of nutrients in the rhizosphere, and also provide a carbon source for heterotrophic microorganisms. The exudates cause the number of microorganisms to be far greater in the rhizosphere than in the bulk soil. Their presence attracts larger soil organisms that feed on microorganisms and the concentration of organisms in the rhizosphere can be up to 500 times higher than in the bulk soil. However, their growth and reproduction is even higher when grazing by predators is taken into account. This grazing helps release nutrients in microbial pool back into the soil. Thus the turnover of nutrients can be more important than the mount of microbes present at any point in time.

Another characteristic of the rhizosphere is the uptake of water and nutrients by plants. Water is drawn from the surrounding soil towards the roots. The balance between the movement of water and nutrients towards the roots and their removal from the soil by roots means that their concentration in the rhizosphere is usually very different from what it is in the bulk soil. This can affect microbial growth and activity.

What is the rhizoplane?

The rhizoplane is the surface of plant roots in the soil. The rhizoplane is the site of water and nutrient uptake and the release of exudates into the soil. Like the rhizosphere, the rhizoplane is a constantly changing environment. As roots grow they cast off dead cells and navigate around soil particles, making the rhizoplane highly irregular, blurring the dividing line between the root surface and the soil.

How do organisms in the rhizosphere influence plant roots?

  1. Organisms in the rhizosphere can affect the plant roots by altering the movement of carbon compounds from roots to shoots. This alteration occurs when organisms compete with root cells for carbon that is fixed by photosynthesis.
  2. The burrows of earthworms in soil provide an easy route for plants roots as they grow through the soil.
  3. Various root microorganism associations can increase nutrient uptake by plants in nutrient poor environments, such as symbiotic (e.g. mycorrhizal) and other specific (e.g. nitrogen fixing) associations and rhizobia.
  4. Some soil organisms are pathogenic and attack living plant roots.
  5. Azospirillum is a bacterium that commonly lives in the rhizosphere of grasses. Some strains of this organism produce hormones that stimulate plant growth. Other strains of Azospirillum fix atmospheric nitrogen and may or may not make it available to the grass.

Are soil organisms dependent on each other in any way?

The soil food web is a way of illustrating the way in which soil organisms relate to each other based on what they eat. The soil food web begins with organic matter, such as crop residues, pasture or any animal or plant material in the soil. Bacteria and fungi consume organic matter and are, in turn, consumed by nematodes, protozoa, earthworms, collembola and some mite species. Nematodes and protozoa are consumed by some mites. Mites and collembola are eaten by beetles and ants.

It is important to remember that the soil is a very complex habitat, in which the diversity of organisms is greater than that of the most diverse plant or animal community. This also means that it s not easy or accurate to generalize about all soil food web processes because the extent to which they occur will be different in different soils, at different times. The same processes (eg. mineralization) can be carried out by different groups of microorganisms.

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