Agriculture Reference
In-Depth Information
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
0-2.5
2.5-5.0
10-20
Soil Depth (cm)
Parent soil
Casts (no dung)
Casts (plus dung)
FIGURE 12.3
Organic carbon levels in parent soil from three sampling depths and in the casts of
A. trape-
zoides
maintained in those soils in the presence and absence of added sheep dung. The sandy soil was taken
from three depths (0 to 5 cm, 5 to 10 cm, and 20 to 30 cm) in a field near Monato, South Australia. (Adapted
from Doube et al. 1994b.)
(1992) confirmed this by direct observation of earthworm gut contents. Using the same technique,
Wolter and Scheu (1999) reported that the anecic species
L. terrestris
fed on soil microsites that
were enriched in bacteria and fungi.
Earthworms of different species and ecological categories differ greatly in their ability to digest
various organic residues and assimilate nutrients from ingested OM (Lattaud et al. 1998, 1999), so
selective feeding on OM- and microbe-rich microsites such as the rhizosphere (Spain et al. 1990;
Brown et al. 2000) or decomposing plant litter (Wolter and Scheu 1999) may provide earthworms
with additional soluble C sources, such as carbohydrates, root exudates, and those derived from
microbial metabolism.
In studies on the feeding behavior of several earthworm species (
A. trapezoides, Aporrectodea
rosea, Microscolex dubius, Eisenia fetida
), use of an organic dye mixed with sheep dung (Doube
and Davoren, unpublished) showed that, when offered buried or surface-deposited sheep dung,
these species fed in an alternate manner on both dung and soil. Their castings thus consisted of a
series of bands of dyed and undyed material, deposited sequentially. Organic carbon levels in the
dyed portions (from the sheep dung) were 5 to 10 times higher than in the nondyed soil sections
of the cast. This indicates that organic residue heterogeneity within casts is much higher than
previously thought, and that differences in microbial activity may be correspondingly variable.
The role of microorganisms as a source of earthworm nutrition is still a matter of debate.
Various enzymes isolated from earthworm guts allow them to digest some bacteria and fungi,
microinvertebrates (e.g., protozoa, nematodes), and partly decomposed plant debris (Brown et al.
2000). However, the amounts of microorganisms consumed and the ability of the earthworms to
digest and assimilate microbial biomass varies greatly with the earthworm species, its ecological
category, character of the food, and the environmental conditions in which the earthworms are
living. For instance, Wolter and Scheu (1999) showed that levels of digestion of fungi and bacteria
in the gut of
were generally low and varied with the type of food substrate.
Nevertheless, other authors (Dash et al. 1979a, 1986; Edwards and Fletcher 1988) have shown
that fungi may be an important source of food for many earthworm species, but the level of dietary
L. terrestris
