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pigmentation

Modified from Barois et al., 1999.

Modified from Barois et al., 1999.

FIGURE 4.64. Pictorial representation of some of the characteristics of earthworm ecological strategies (categories) as proposed by Bouché (1977), Lavelle (1981), and Lavelle et al. (1989) (from Brown et al, 1995).

have become a popular means for segregating earthworm communities into functional groups of species.

Within a particular soil, less than a half-dozen earthworm species are typically found, and the species within such an association often effectively partition the soil volume according to their functional categories. Further, the activities of earthworms influence soil processes in various ways according to these functional categories. For example, epigeic species promote the breakdown and mineralization of surface litter, whereas anecic species incorporate organic matter deeper into the soil profile and facilitate aeration and water infiltration through their formation of burrows.

Influence on Soil Processes

Earthworms, as ecosystem engineers (Lavelle et al., 1998), have pronounced effects on soil structure as a consequence of their burrowing activities as well as their ingestion of soil and production of castings (Lavelle and Spain, 2001; van Vliet and Hendrix, 2003). Casts are produced after earthworms ingest mineral soil and/or particulate organic matter, mix them together and enrich them with organic secretions in the gut, and then egest the material as a slurry or as discrete fecal pellets within or upon the soil, depending on earthworm species. Darwin (1881) observed that surface casting by earthworms buried chalk to considerable depths in soil over a 20-year period. Turnover rates of soil through earthworm casting range from 40-70 t • ha-1 •y-1 per hectare per year in temperate grasslands (Bouché, 1983) to 500-1000 t • ha-1 •y-1 per hectare per year in tropical savannas (Lavelle, 1978).

During formation in the earthworm gut, casts are colonized by microbes that begin to break down soil organic matter. As casts are deposited into the soil, microbial colonization and activity continue until readily decomposable compounds are depleted. Eventually, casts may harden into stable soil aggregates. Mechanisms of cast stabilization include organic bonding of particles by polymers secreted by earthworms and microbes, mechanical stabilization by plant fibers and fungal hyphae, and stabilization due to wetting and drying cycles and age-hardening effects (Tomlin et al., 1995). Earthworm casts are usually enriched with plant-available nutrients and thus may enhance soil fertility; plant-growth-promoting substances have also been suggested as constituents of earthworm casts. Castings from vermicomposting operations are sold commercially as soil amendments that purportedly enhance plant growth (Edwards, 1998).

Earthworm burrowing in soil creates macropores of various sizes, depths, and orientations, depending on species and soil type. Burrows tend to be similar in diameter to that of the earthworms that produced them, ranging from about 1mm to larger than 10 mm in diameter and constituting among the largest of soil pores (Edwards and Shipitalo, 1998). Burrows of epigeic earthworms (e.g., Dendrobaena octaedra) are often small and limited to upper layers of soil; they may be horizontal to vertical in orientation. Endogeic species (e.g., Diplocardia mississippi-ensis or Pontoscolex corethrurus) may form networks of variously oriented burrows, as the earthworms ingest soil and cast behind them as they burrow. These networks may form continuous pores over some depth, but castings within the burrows may impede free water movement. Anecic earthworms (e.g., Lumbricus terrestris) may create deep vertical burrows that form continuous macropores to depths of 1 m or more (van Vliet and Hendrix, 2003). These burrows tend to be very stable because their walls are lined with organic matter drawn in or secreted by earthworms, and they often have higher bulk density than that of surrounding soil (Lee, 1985). Continuous macropores resulting from earthworm burrowing may enhance water infiltration by functioning as bypass flow pathways through saturated soils. These pores may or may not be important in solute transport, depending on soil water content, nature of the solute, and chemical exchange properties of the burrow linings (Edwards and Shipitalo, 1998).

The influence of earthworms on organic matter and nutrient cycling in soils is closely related to the density and feeding ecology of resident populations, as described previously (Lee, 1985; Barois et al., 1999). Epigeic species typically inhabit the surface litter and the O and upper Ahorizons of soil, where they mix mineral soil and plant residues, fragment organic particles, inoculate them with microbes, and thus increase organic matter decomposition rates. Anecic earthworms pull surface litter into their burrows, thus transporting organic material deeper into the soil profile. They cast on the soil surface, mixing organic and mineral particles in the litter layer. The activities of both epigeic and anecic earthworms produce "mull" soil horizons, in which organic matter is intimately incorporated into the upper mineral soil of a well-developed A horizon overlain with a recently deposited litter layer. The extreme case is termed "vermimull," in which the Ah horizon is granular and characterized by organo-mineral complexes consisting of earthworm casts (Green et al., 1994). Endogeic earthworms feed within the soil on organic matter and microbes associated with plant roots or mineral soil. As mentioned previously, they are termed oligo-, meso-, or polyhumic, depending on the level of organic enrichment of their substrate. Casts and burrows of endogeic earthworms are also sites of increased microbial activity and organic matter decomposition (Brown, 1995). Mineralization of organic matter in earthworm casts and burrow linings produces zones of nutrient enrichment that are different from those in bulk soil. These zones are referred to as the "drilosphere" and are often sites of enhanced activity of plant roots and other soil biota (Lavelle et al, 1998).

Despite the many beneficial effects of earthworms on soil processes, some aspects of earthworm activities may be undesirable (Edwards and Bohlen, 1996; Lavelle, 1998; Parmelee et al., 1998). Detrimental effects include removing and burying of surface residues that would otherwise protect soil surfaces from erosion; producing fresh casts that increase erosion and surface sealing; increasing compaction of surface soils; depositing castings on the surface of lawns and golf greens where they are a nuisance; dispersing weed seeds in gardens and agricultural fields; transmitting plant or animal pathogens; riddling irrigation ditches, making them less able to carry water; increasing losses of soil nitrogen through leaching and denitrification; and increasing soil carbon loss through enhanced microbial respiration. Furthermore, there have been reports of earthworms transmitting pathogens, either as passive carriers or as intermediate hosts, raising concerns that some earthworm species could provide a mechanism for the spread of certain plant and animal diseases.

Thus it is the net result of positive and negative effects of earthworms, or any other soil biota, that determines whether they have detrimental impacts on ecosystems (Lavelle et al., 1998). An effect, such as mixing of O- and A-horizons, may be considered beneficial in one setting (e.g., urban gardens) and detrimental in another (e.g., native forests).

Earthworm Management

There is interest in managing earthworms to utilize their beneficial effects in organic waste reduction, in land reclamation, and in reduced-intensity agriculture (Lee, 1995; Edwards and Bohlen, 1996). Because of their effects on organic matter decay, earthworms are increasingly being used to accelerate decomposition of organic waste materials. Vermicomposting involves culturing of earthworms outdoors in beds or in confined chambers in the presence of waste materials, which are reduced in volume and carbon-nitrogen ratio as they are processed by earthworms and decomposed by enhanced microbial activity within the earthworms and their castings (Edwards, 1998). A variety of approaches and designs has been developed for vermicomposting systems, but the basic principle is the feeding of acceptable organic materials to earthworms in continuous or batch culture, and the collecting of processed wastes that ultimately consist of stabilized castings. Earthworm biomass is also harvested from vermicomposting systems for a variety of uses, including further composting operations, animal protein, and fishing bait. Organic wastes that have been used successfully in vermicomposting include animal manures, sewage sludge, food production wastes, and horticultural residues. Small-scale vermicompost-ing is becoming popular for reduction of household wastes such as kitchen scraps and yard trimmings. Earthworm species typically used for vermicomposting included the European lumbricids, Eisenia fetida and Lumbricus rubellus, often called "red worms"; the African "night-crawler," Eudrilus eugeniae; and the Asian "blue worm," Perionyx exca-vatus. The latter two species are tropical and best suited to composting under warm conditions (Edwards, 1998). A number of vermicomposting publications have appeared in the popular literature in the last decade, including the periodical Worm Digest and the ever-popular Worms Eat Our Garbage (Appelhoff et al., 1993).

The potential for earthworms to ameliorate soils during land reclamation or in degraded agricultural sites is also of increasing interest (Lee, 1995; Baker, 1998). In many situations, it may be desirable to introduce earthworms. Techniques have been developed for large-scale inoculation of areas devoid of earthworms (e.g., reclaimed polders) and for introduction of species that may perform desired functions (e.g., epi-endogeic species for thatch removal from pastures). It is usually necessary that favorable soil conditions (e.g., adequate water and organic matter, appropriate temperatures) exist at the time of inoculation and/or that refugia (e.g., blocks of native sod or containers of native soil) are provided from which earthworms may disperse. Introductions of earthworms into unfavorable environments often fail.

Mixed-species assemblages of earthworms may influence a wider array of soil processes, such as organic matter turnover as well as soil structural properties, than a single species can (Lee, 1995). Introductions of such assemblages might include one or more anecic species that make deep vertical burrows and that cast on the surface and bury residues, and one or more endogeic species that feed belowground on dead roots and organic matter and that make horizontal burrows. Inclusion of epigeic species might accelerate decomposition of plant residues on the soil surface.

Earthworm Sampling and Identification

A variety of sampling methods have been used for collecting earthworms, both quantitatively and qualitatively (see Table 9.2). Hand digging and sorting of soil is the most commonly used method for quantitative sampling of earthworms. Pits of known dimensions (e.g., 25 by 25 by 25 cm) are dug with a shovel, often in layers of defined thickness, and the soil broken by hand. More elaborate modifications of this method, which may improve collection ofjuveniles and cocoons, include dry or wet sieving of soil through screens of known mesh size, and flotation of sieved material in high-density solutions to separate earthworms and other soil fauna. Earthworms may be collected alive or immediately preserved in 70% ethanol or 5% formalin for later counting and identification.

Earthworms are also collected by applying solutions of chemical irritants to the soil, which bring earthworms to the surface where they can be hand collected. A number of chemicals have been used, including HgCl2, KMnO4, formalin, and mustard powder slurry (Lee, 1985; Zaborski, 2003). The latter has received much attention recently because of its safety and availability. Chemical extraction techniques may be effective on anecic earthworms such as Lumbricus terrestris but may be less so on other species. Effectiveness varies with earthworm species and activity, soil water content, porosity, and temperature. Comparisons with hand sorting should be done before adopting extraction techniques for quantitative sampling.

Mechanical or electrical stimulation may also bring earthworms to the soil surface. A technique known as "grunting" employs a wooden stake driven into the soil, vibration of the stake with a bow or flat piece of metal, and collection of earthworms that emerge. Some megascolecid species have been sampled with this technique (Hendrix et al., 1994) but it may not be effective on other earthworm groups.

Electrical extraction of earthworms uses metal rods connected to a source of electrical current and inserted into the soil; the current brings earthworms to the surface. Different voltages and amperages have been used with varying degrees of success; effectiveness of the technique is highly dependent on soil water content, electrolyte concentration, and temperature. As with mechanical vibration, the soil volume sampled with electrical current is not known and therefore these methods may be best suited for qualitative or comparative sampling (Lee, 1985). However, Schmidt (2001) used a commercially available "Octet" device to quantitatively sample earthworm communities in arable soils in Ireland; the electrical method gave estimates of species composition and population size comparable to those from hand digging, with the exception of one very small species. The electrical technique is potentially very dangerous and should only be used with extreme caution.

Earthworm populations also may be sampled with trapping techniques. Pitfall traps (described above) may give some idea of surface-active earthworm species present in an area. Baited traps consist of porous containers (e.g., clay flower pots) filled with bait such as animal manure and buried in the soil for appropriate periods of time. Trapping techniques are highly selective of certain earthworm species and thus are best suited for qualitative or comparative sampling (Lee, 1985).

For earthworm species that cast on the soil surface (e.g., Lumbricus terrestris), numbers and types of castings may give an indication of population activity. Because casting is dependent on soil water content and temperature, this technique is highly variable and not suitable for quantitative estimates of population density.

Many earthworm species in the family Lumbricidae can be identified from external body characteristics if the specimens are sexually mature. Several taxonomic keys are useful for the common lumbricids found worldwide (Reynolds, 1977; Sims and Gerard, 1985; Schwert, 1990). Most other earthworms require dissection for accurate taxonomic identification; position and characteristics of sexual organs, the gut and associated glands, and other structures are required. The procedures must be done carefully and require a degree of skill and practice. Additional general taxonomic references include Jamieson (1988), Fender and McKee-Fender (1990), James (1990), Edwards and Bohlen (1996), and Fragoso et al. (1999).

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