1.1 Role in soil

Earthworms have a critical influence on soil structure, forming aggregates and improving the physical conditions for plant growth and nutrient uptake. They also improve soil fertility by accelerating decomposition of plant littre and soil organic matter. Earthworms are the most important invertebrates in this initial stage of the recycling of organic matter in various types of soils. Curry & Byrne (1992) demonstrated that the decomposition rate of straw which was accessible to the earthworms was increased by 26-47% compared with straw from which they were excluded. Organic matter that passes through the earthworm gut and is digested in their casts is broken down into much finer particles, so that a greater surface area of the organic matter is exposed to microbial decomposition. Martin (1991) reported that casts of the tropical earthworms had much less coarse organic matter than the surrounding soil, indicating that the larger particles of organic matter were fragmented during passage through the earthworm gut. Earthworm species, such as Lumbricus terrestris, are responsible for a large proportion of the overall fragmentation and incorporation of littre in many woodlands and farmland of the temperate zone, which resulted in the formation of mulls. As a result, the surface littre and organic layers are mixed thoroughly with the mineral soil (Scheu & Wolters, 1991).

The numbers of earthworm burrows have been counted between 50 and 200 burrows m-2 on horizontal surfaces (Edwards et al., 1990). Earthworms not only improve soil aeration by their burrowing activity, but they also influence the porosity of soils. Earthworm burrows was found to increase the soil-air volume from 8% to 30% of the total soil volume (Wollny, 1890). In one soil, earthworm burrows comprise a total volume of 5 litres m-3 of soil, making a small but significant contribution to soil aeration (Kretzschmar, 1978). Water infiltration was from 4 to 10 times faster in soils with earthworms than in soils without earthworms (Carter et al., 1982). They bring large amounts of soil from deeper layers to the surface and deposit as casts on the surface. The amounts which turned over in this way greatly differ with habitats and geographical regions, ranging from 2 to 268 tons ha-1 (Beauge, 1912; Roy, 1957). The importance of this turnover, which was discussed first by Darwin (2009), can be seen by comparing the profile of a stratified mor soil (with few earthworms) with that of a well-mixed mull soil. Blanchart (1992) reported in a formation of aggregates that under natural conditions with or without earthworms, large aggregates (>2 mm) comprised only 12.9% of soil with no earthworms, whereas in soil with worms, large aggregates comprised 60.6% of soil after 30 months in the field. Devliegher & Verstraete (1997) introduced the concepts of nutrient enrichment process and gut associated process. They noted that earthworms are performing these two different functions that may have contrasting their effects on soil microbiology, chemistry and plant growth. Earthworms, such as L. terrestris, incorporate and mix surface organic matter with soil and increase biological activity and nutrient availability. However, they also assimilate nutrients from soil and organic matter as these materials pass through their gut.