Agriculture Reference
In-Depth Information
and adaptations in correspondence with the soil environmental conditions (Lamparsky et al. 1987;
Kretzschmar 1987; Kretzschmar et al. 1992). Ingestion of soil by earthworms creates structures of
smaller size, mainly related to casts, which are deposited either on the soil surface or within the
soil profile, and to aggregation properties of casts. The microporosity of earthworm casts results
from the evolution and content of bound or free organic compounds after casts are deposited
(Blanchard 1992); cast pore properties spread into the soil matrix and contribute partially to the
soil matrix porosity characteristics that interfere with macropores. The two types of structures
described belong to the two main categories of soil pores: macropores, which are roughly defined
as larger than 0.5 mm diameter, and matrix porosity. Macropores are created either by physical
processes (clay swelling, freezing, etc.) or by biological processes (roots, meso invertebrates,
earthworms, etc.). Matrix pores are considered part of the basic soil fabric that defines the soil as
a porous medium.
T
R
E
-O
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S
HE
OLES
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ARTHWORM
RIGINATED
OIL
TRUCTURES
Macropores formed as earthworm burrows are expected to improve mass transfers in soil at the
profile scale. The extent of this improvement is based on connectivity and tortuosity of the earth-
worm burrows on the one hand (Kretzschmar 1987; Joschko et al. 1989; Edwards et al. 1992) and
connections between the burrow walls and the matrix porosity in the vicinity of these macropores
on the other hand (Kretzschmar 1987; Babel and Kretzschmar 1994). Earthworm burrows are not
the principal source of soil macropores; an earthworm burrow system could be responsible for only
1 to 2% of the total porosity. The importance of earthworm burrows in mass transfer relies on two
main factors. First, they are constructed in a cylindrical form with lightly compacted walls with at
least several coatings of mucopolysaccharides (Kretzschmar 1987). Earthworm burrows are con-
solidated structures that stay open even when soil moisture is at high levels (e.g., when the soil
matrix porosity is saturated with water, as are the macropores created by plant roots) and when
clay swelling has closed most of the larger cracks in soils. At this stage, air-filled porosity is at its
lowest level (Monnier 1992); the burrows might represent about 20% of the total air-filled porosity.
Second, in temperate climates, a noticeable proportion of the burrow systems created by earthworm
populations shows strong anisotropic orientations marked by a vertical direction. In this context,
earthworm burrows are regarded as preferential vertical pathways for gravitic mass transfer (water
and partial solutes) (Ehlers 1975; Shipita1o et al. 1990; Edwards et al. 1992; Smettem 1992).
Earthworm burrows are also expected to influence the spatial distributions of other living
organisms, such as roots and microorganisms. The distribution of plant roots can be affected by
the presence of earthworm burrows. The density of roots in the vicinity of pores (cracks and
burrows) is not the same as farther away from these pores (Krebs et al. 1994); nevertheless, no
experimental evidence has shown that roots grow preferentially toward earthworm burrows or
entered holes filled with earthworm casts (Hirth et al. 1998). The distribution of microorganisms
associated with earthworm burrows is due partially to the organic carbon sources transferred through
the earthworm burrow system, that is, mechanical transport of organic debris from the surface litter,
mucus deposition along the burrow walls, or infillings of old burrows with casts. The distribution
of microorganisms is also because of transfer of microorganisms through transit and the dispersion
of microorganisms with water flowing through the larger burrows. Evidence of dispersion of
microorganisms through earthworm burrows has been demonstrated in experimental conditions
(Reddell and Spain 1991; Stephens et al. 1994) (see
Chapter 10
, this volume).
D
E
B
A
S
S
F
S
?
O
ARTHWORM
URROWS
FFECT
PECIFIC
OIL
UNCTIONS
OR
TRUCTURES
The effects of earthworm burrows on soil structures and soil functions cannot be regarded as a
simple causal relationship. When transport properties are measured in soils where earthworm
burrows exist, it is not certain whether the burrow systems in these soils are so extensive because
