Agriculture Reference
In-Depth Information
to monitor. Nevertheless, estimated soil ingestion rates for earthworms in temperate regions are
usually less than 100 Mg ha
(Tomlin et al. 1995). In tropical areas, such as the Ivory
Coast, where climatic conditions are less likely to seasonally inhibit activity, Lavelle et al. (1989)
reported a cast production rate of 1200 Mg ha
year
−
1
−
1
. High cast-production rates such as this
are attributable to the fact that geophageous, endogeic earthworms can ingest 5 to 30 times their
body weight per day (Lavelle 1988). According to Lee (1985), earthworms can process up to
25% of the Ah horizon (organo-mineral soil layer enriched in humified organic matter) in 1 year
and thus can be important aggregate-forming agents through the production of casts in the soil
and on the surface. In a laboratory study, Ziegler and Zech (1992) showed that
year
−
1
−
1
could
bind up to two thirds of the beech litter and unstructured artificial soil into 200- to 2000-
E. fetida
H
m
diameter aggregates in 446 days.
Earthworm casts, deposited on the burrow walls, within the burrow, or on the soil surface
(Brown et al. 2000), usually contain more clay and less sand than the surrounding soil because of
selective ingestion, with this effect more prominent with endogeic species, which tend to be smaller
than anecic species of earthworms. This concentration of fine particles in earthworm casts may
need to be taken into account when using methods such as dispersible clay or turbidity to compare
the stability of casts with uningested soil. Moreover, the relative differences in texture between
casts and uningested soil are probably dependent on the coarseness of the parent soil. For example,
Shipitalo and Protz (1988) noted that casts of
Lumbricus rubellus
, an epigeic/endogeic species,
contained less sand than those of
(anecic species), and both had less sand than the
uningested soil (18% sand). Schrader and Zhang (1997) reported only small differences in the
texture between the casts of
L. terrestris
L. terrestris
and
Aporrectodea caliginosa
(endogeic) and the parent
soils with initial sand contents less than 4%.
Likewise, the amount of organic matter incorporated into casts is dependent on whether the
earthworms are actively burrowing or feeding and the food source. Shipitalo et al. (1988) reported
that food ingestion rates and organic carbon contents of casts were higher for more palatable food
sources, as reflected in earthworm weight gains, and that casts of
L. rubellus
were generally higher
in organic carbon than those of
L. terrestris.
In a study by Schrader and Zhang (1997), however,
L.
terrestris
casts. Specific
organic compounds such as reducing sugars, amino sugars, phenolic materials (Mora et al. 2003),
and carbohydrates (Scullion and Malik 2000) can also be concentrated by earthworms in their casts.
Earthworm casts also usually have higher bulk density than the uningested soil (Edwards and Bohlen
1996; Grres et al. 2001), unless the soil is already compacted (Joschko et al. 1989), and are higher
in pH, contain more available nutrients, and have higher levels of microbial activity.
casts were enriched in organic carbon to a greater extent than
A. caliginosa
R
S
A
E
EMOLDING
OF
OIL
GGREGATES
BY
ARTHWORMS
The muscular contractions of the earthworm crop and gizzard, the peristalsis of the gut wall, and
contractions of the body wall create a great range of pressures that mechanically disrupt soil microag-
gregates during passage through the digestive tract. The mean pressure applied to soil by
Aporrectodea
rosea
, Newell (1950) reported
that the average coelom pressure was 1.6 kPa in segment 28 and 0.8 kPa near the tail region. Such
pressures, concomitant to the addition of large amounts of watery mucus (Barois et al. 1993), can lead
to the mobilization of clay (Marinissen et al. 1996) and the disruption of existing interparticle water
and cation bridges in the aggregates (Shipitalo and Protz 1988, 1989). Conversely, soil remolding also
brings clay minerals into close association with newly formed or released bonding agents originating
from the ingested organic matter (Shipitalo and Protz 1989). Consequently, the soil fabric is reorganized
in the posterior intestine of earthworms (Barois et al. 1993), with resistant organic fragments becoming
the foci for new microaggregates (Shipitalo and Protz 1989). Earthworm gut transit time probably also
affects the degree of microaggregate disruption. Reportedly it takes 2 to 24 hours for soil to pass through
the digestive tract of lumbricid earthworms (Barley 1959; Piearce 1972; Bolton and Phillipson 1976).
was estimated as 259 Pa (McKenzie and Dexter 1987). For
L. terrestris
