Agriculture Reference
In-Depth Information
edge-to-edge contacts between clay domains with time but without water loss. These processes are
reversible, and organic matter reportedly slows or has no effect on thixotropic hardening (Blake
and Gilman 1970; Molope et al. 1985). As Marinissen et al. (1996) pointed out, however, thixotropic
process normally occur within hours, so they cannot be solely responsible for the stabilization that
occurs in casts over much longer time frames.
In addition to the physical processes, chemical processes can contribute to stabilization of
earthworm casts. Earthworms are known to secrete amorphous calcium carbonate (C.A. Edwards
and Bohlen 1996), a possible binding agent (Tisdall and Oades 1982). In addition, by using selective
chemical pretreatments, Shipitalo and Protz (1989) gathered indirect evidence that calcium and, to
a lesser extent, magnesium are involved in the clay-polyvalent cation-organic matter linkages that
stabilize soil microaggregates within casts. In fact, the change in CaCO
content in casts compared
with uningested soil (Zhang and Schrader 1993) might be useful as an index of the capacity of
various earthworm species to bond soil particles and reform new stable aggregates. When they
compared casts from various soils, Schrader and Zhang (1997) found positive correlations between
tensile strength and the clay and CaCO
3
contents of the soil. These parameters, however, correlated
negatively to water-stable aggregation. Hindell et al. (1997a) hypothesized that the greater dispers-
ibility of artificial casts compared with natural casts was because of a greater loss of calcium ions
from the artificial casts, which reduced coagulation of clay particles.
The production of microbial polysaccharides in casts (Chapman and Lynch 1985; Emerson et
al. 1986; Robertson et al. 1991) and polysaccharides that are added to casts in the mucus secreted
by earthworms and by mucilages produced by microorganisms living in their digestive tract (Barois
and Lavelle 1986; Kristufek et al. 1992) may also affect aggregate stability. However, the role these
polysaccharides play in cast aggregation is still uncertain.
Hindell et al. (1997a) suggested that the secretion of soluble carbohydrates in the earthworm
gut initially facilitates the dispersion of clay. On the other hand, Swaby (1950) showed that, as
populations of intestinal bacteria increased, the production of gums and glues increased, and cast
stability increased. Altemller and Joschko (1992) also showed that carbohydrates produced by
bacteria can serve as cementing agents, and Flegel et al. (1998) reported a significant correlation
between phosphomonoesterase activity and the water-stable aggregation of earthworm casts.
Other research, however, has demonstrated that microbial activity is not necessary for casts to
stabilize (Marinissen and Dexter 1990; Marinissen et al. 1996; Haynes and Fraser 1998). In fact,
microbial activity may be reduced in some casts because of limited gaseous exchange caused by
their high bulk density (Blanchart et al. 1993). In some instances, poor correlations of aggregate
stability in casts with the size of the microbial populations and polysaccharide content are probably
because the arrangement and location of these constituents within casts is more important than the
absolute quantities (Shipitalo and Protz 1989; Haynes and Fraser 1998).
Microbial activity can also physically stabilize earthworm casts. Fungal hyphae have been
reported to stabilize soil aggregates and casts (Tisdall and Oades 1982; Molope et al. 1987;
Marinissen and Dexter 1990; Lee and Foster 1991; Tisdall 1991; Tisdall et al. 1997; Kabir and
Koide 2002). Using scanning electron microscopy, Haynes and Fraser (1998) observed that fungal
hyphae emanating from within casts enmeshed aggregates. Tiunov and Scheu (2000) found that
most fungi can survive passage through
3
but the dominance structure of the fungal
community changes with time and remains cast specific for up to 100 days.
L. terrestris,
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In most studies, the amounts and source of organic matter incorporated into the soil by earthworms
has been shown to have a significant affect on aggregate stability within casts; thus, a positive
correlation between cast organic carbon content and cast stability is frequently reported (Shipitalo
and Protz 1988; Zhang and Schrader 1993; Schrader and Zhang 1997; Flegel et al. 1998). Earth-
worms play a large role in litter comminution and its repartitioning into the smaller aggregate size
