Role in the Environment
Earthworms are thought to be the most ancient soil animals having started colonizing terrestrial environments about 600 million years ago (Spain and Lavelle 2001). They are the most predominant soil fauna except in dry and cold climates. Earthworms are semiaquatic animals which extract water continuously from the surrounding soil in order to maintain their cuticle in a moist state to facilitate gas exchange. Earthworms live mostly in horizontal burrows, selecting food from the soil, often feeding on organic materials that are on or just under the soil surface, deposit casts within their burrows or in other spaces within the soil. By living in burrows, earthworms are to some extent protected from diurnal and seasonal variability in the physico- chemical environment and from predatory pressure.
The burrows they create facilitate water and gas transport (Zhang and Schrader 1993), by mixing soil minerals with organic material (Hendriksen 1991). Moreover water, gas and solute transfer processes can be enhanced by the presence of earthworm burrows (Kretzschmar and Monestiez 1992). Burrow systems help understand (i) the actual role of earthworms in the soil ecosystem and (ii) the influence of these burrow systems on soil transfer properties and (iii) the difference in the microbial diversity compared to surrounding soil.
Earthworm burrows are important in maintaining soil aeration, drainage and porosity (Carter et al., 1982). Burrows and burrow walls are surrounded by soil rich in nutrients and polysaccharides and are lined with protein rich mucus that gives the burrow walls and castings their consistency. These maintain the stability of the channels by binding soil particles together. In addition to mucus secretions nitrogen excretion from the earthworm bodies (mostly urea and ammonia) is also added to the burrow walls and/or to castings. The diameter of the burrows varies with the dimensions of the earthworms but is generally in the range of 1-10mm, which places them among the largest of soil pores. Burrows enable earthworms to select conditions that suit them best from the range of microenvironments available in one or more soil horizons, while retaining access to forage for food at the surface at times when conditions are suitable.