Ecology
Earthworm species can be classed in one of three morpho-ecological groupings (Bouche, 1977 [summary in Lee, 1985]). Epigeic species live in organic horizons and ingest large amounts of undecomposed litter. These species produce ephemeral burrows into the mineral soil for diapause periods only. They are relatively exposed to climatic fluctuations and predator pressures, and tend to be small with rapid generation times. A common example is Eisenia foetida (redworm, manure worm) which is used in vermicomposting.
Endogeic species forage below the surface, ingest large quantities of soil with a preference towards organic rich soil, and build continuously ramifying burrows that are mostly horizontal. These species are apparently not of major importance in litter incorporation and decomposition since they feed on subsurface material. They are important in other soil formation processes including root decomposition, soil mixing, and aeration.
Species which build permanent, vertical burrows that penetrate the soil deeply were termed anecics by Bouche. These species are detritivores and come to the surface to feed on partially decomposed litter, manure, and other organic matter. The permanent burrows of anecics create a microclimatic gradient, and the earthworms can be found shallow or deep in their burrows depending on the prevailing conditions. Anecics have profound effects on organic matter decomposition, nutrient cycling, and soil formation. The most common examples are the nightcrawlers sold by fish-bait dealers consisting of Lumbricus terrestris and Aporrectodea longa.
Palatability of different types of litter to earthworms may depend on nitrogen and carbohydrate content, and the presence of polyphenolics such as tannins (Satchell, 1967). Earthworms prefer materials with a low C/N ratio, such as clovers, to grasses which have a higher C/N ratio (Ruz Jerez et al., 1988). Colonization of litter residues by microorganisms also increases palatability (Cortez et al., 1989), as does leaching of feeding inhibitors.
Benefits of Earthworms
Deep burrowing species such as L. terrestris can burrow through compacted soil and penetrate plough pans, creating channels for drainage, aeration, and root growth (Joschko et al., 1989). Recent work by Shipitalo and Protz (1989) elucidated some of the mechanisms by which earthworms enhance soil aggregation. Ingested aggregates are broken up in a liquid slurry that mixes soil with organic material and binding agents. The defecated casts become stable after drying. Stewart et al. (1988) also presented evidence that earthworms initiate the formation of stable soil aggregates in land degraded by mining.
In forest ecosystems earthworms, especially litter feeders such as L. terrestris, can consume all the litter deposited on the soil surface within a period of several weeks (Knollenberg et al., 1985) or months (Satchell, 1967). Incorporation of litter by earthworms in apple orchards can be an important mechanism for preventing outbreaks of scab fungus, spores of which are transmitted from litter to new foliage by spring rains. Raw (1962) found a high correlation between L. terrestris biomass and apple leaf litter incorporation, with over 90 percent of litter incorporated during the winter when this species was abundant. Incorporation of surface litter may be an important function of earthworms in no-tillage agroecosystems.
Earthworm casts are sources of nutrients for plants. Lumbricids in a pasture soil produced casts that contained 73 percent of the nitrogen found in the ingested litter; indicating both the importance of earthworms in incorporating litter nitrogen into the soil and the inefficiency of nitrogen digestion by earthworms (Syers et al., 1979). Earthworms increase the amount of nitrogen mineralized from organic matter in soil. Because nitrification is enhanced in earthworm casts, the ratio of nitrate-N to ammonium-N tends to increase when earthworms are present (Ruz Jerez et al., 1988). Nitrogen-fixing bacteria are found in the gut of earthworms and in earthworm casts, and higher nitrogenase activity, meaning greater rates of N-fixation, are found in casts when compared with soil (Simek and Pizl, 1989).
Earthworms may increase levels of metabolic activity in soils, as measured by the amount of CO2 evolved, yet nematode abundance and microbial biomass may decrease (Yeates, 1981; Ruz Jerez et al., 1988). This occurs because earthworms reduce the amount of substrate available to other decomposers, and because earthworms ingest other decomposer organisms as they feed. This process would tend to accelerate nutrient cycling rates.
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