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other recalcitrant substances (which earthworms cannot digest by themselves) (Barois and Lavelle
1986; Lavelle and Gilot 1994). Most metabolites were reabsorbed in the earthworm hindgut.
The highest relative mucus production (ratio of the soluble C in the foregut to that in the
ingested food substrate) was observed in temperate earthworm species, probably because these
have greater need to stimulate the microflora (because of lower annual mean temperatures) than
those in tropical climates (Trigo et al. 1999). Higher rates of mucus production were also observed
when earthworms fed on poorer-quality substrates. Thus, litter-feeding epigeic earthworm species
feeding on higher-quality, less-recalcitrant materials (litter, fresh organic materials) may not need
to stimulate their intestinal microflora with mucus secretions to the same extent as endogeic species
that feed on poorer-quality (soil) material (Trigo et al. 1999). In addition, epigeic species have a
more complete enzymatic system (including cellulase) than do endogeic species (Urbsek and Pizl
1991). However, a comprehensive description of the digestive system and the origin of different
gut enzymes has so far been made for only seven species (five endogeic, one anecic, and one
epigeic), and further research is needed, particularly for epigeic and anecic species (Brown et al.
2000).
Ingested actinomycetes may inhibit the growth of other organisms (particularly fungi and Gram-
positive bacteria) in the earthworm gut by releasing antibiotics, leading to a predominance of
antibiotic-resistant Gram-negative bacteria and other actinomycetes in earthworm guts (Kristufek
et al. 1993). However, the activity of antibiotics has been tested only in vitro with specific test
microorganisms (Ravasz et al. 1986; Kristufek et al. 1993) and has not yet been tested in vivo .
Hence, the extent to which such an inhibition actually occurs in the guts of living earthworm is
still unknown. Finally, in the microaerophilic guts of some earthworms (e.g., P. corethrurus and
A. caliginosa ), free-living Clostridia spp. may fix N 2 (Barois et al. 1987; Striganova et al. 1989a),
although this contribution to the overall N budget in casts is relatively small.
Transit time of microorganisms through the earthworm gut may also be an important deter-
mining factor of the fate of ingested organisms in earthworm intestines (Scheu 1992; Brown 1995).
This varies greatly among different species and furthermore is dependent on the quality of ingested
materials (Hendriksen 1991) and on temperature (Barois and Lavelle 1986). Gut transit times range
between 1 and 3 hours for Millsonia anomala (Martin et al. 1987), A. rosea (Bolton and Phillipson
1976), E. fetida (F. Hartenstein et al. 1981), A. caliginosa, and O. lacteum (Scheu 1992); 3 to 5
hours for Hormogaster elisae (Diaz Cosn et al. 2002); and maybe 8 hours or longer for L. terrestris
(Parle 1963b; R. Hartenstein and Amico 1983), Lumbricus festivus (Hendriksen 1991), and L.
rubellus (Daniel and Anderson 1992). In such a short transit, there is little potential for microbial
multiplication (although there may be a large increase in activity and Ă‘awakeningĂ’ of dormant
bacteria), but with longer gut transit times, there may be sufficient time for microbial multiplication,
particularly because many Lumbricus spp. tend to feed on litter or organic-rich materials, which
already contain substantial microbial populations.
Microbiological Composition and Activity in Earthworm Burrows and Casts
Earthworms have high consumption rates, ranging from less than 1 time up to as much as 30 times
their body (fresh) weight of soil per day in endogeic species (Lavelle 1988). Litter consumption
rates may also be very high, representing from a small percentage up to more than 85% of the
annual litter fall in sites containing large populations of anecic and epigeic earthworms (e.g.,
Knollenberg et al. 1985). However, assimilation efficiencies are generally low in endogeic species,
ranging from only about 1% of the ingested C for A. rosea (Bolton and Phillipson 1976) and 3 to
19% for M. anomala and P. corethrurus (Martin and Lavelle 1992; Lavelle and Spain 2001). On
the other hand, assimilation efficiencies may be higher (30 to 75%) in litter-feeding species such
as L. rubellus and L. terrestris (Dikschen and Topp 1987; Daniel 1991). Therefore, earthworm casts
may have large amounts of OM that is not assimilated but may have been modified greatly by gut
 
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