Agriculture Reference
In-Depth Information
walls with organic material by
pulls leaves and large fragments
of organic debris into its burrows, creating the black burrow lining observed by Graff, which resulted
from the decomposition of organic material.
In the burrows of many other earthworm species, a fine black layer is visible on the surface of
the burrow walls. Reused burrows are lined by a layer of mucus produced from the accumulation
of a mucus deposit about 10- to 15-´ thick each time an earthworm passes. Older burrows, which
can be considered abandoned, present cracks that connect the burrows to porosity in the soil matrix
(Kretzschmar 1987). Typical cracking patterns of the compacted layers around the burrows have
been observed for the burrows of
L. terrestris. Lumbricus terrestris
(Babel and Kretzschmar 1994). These
different patterns govern the contribution of burrow systems to overall soil transfer whether they
are connected to the soil surface or not. The presence of macropores and their connections with
the surrounding soil porosity create heterogeneous patterns of connectivity. It has been shown in
simulated earthworm burrow systems that the mean length of the burrows plays a role in transfer
properties at the scale of the soil profile; the average distance between burrows (governed by the
burrow number) controls the mass transfer at the local scale of a given horizon (Kretzschmar 1988).
The patterns of distribution of cracks around earthworm burrows have been described by Krebs
et al. (1994). It is nevertheless still difficult to quantify interactions between these two components
of the soil pore systems. Nevertheless, a similar interaction has been estimated for the distribution
of earthworm burrows that open at the surface and the surface cracks during the drying phase under
experimental conditions (Chadoeuf et al. 1994). The density of surface open burrows at a distance
of less than 2.5 cm from cracks is greater than would be expected if the distribution of burrows
and cracks were independent. The possibility of observing such Ñattraction-likeÒ interactions within
the soil profile between cracks and earthworm burrows depends on the soil structural context,
mainly the presence of roots. More interestingly, the intensity of this interaction could summarize
the history of the use of burrows both by earthworms and in water transfer.
Megascolides australis
I
E
B
M
T
P
NTERACTIONS
BETWEEN
ARTHWORM
URROWS
AND
ASS
RANSFER
ROCESSES
The presence of large numbers of earthworms has been correlated with high soil hydraulic conductivity
(Ehlers 1975); this observation has been confirmed many times with earthworms introduced into
pastures or into differently cultivated plots (Springett et al. 1992), into experimental compacted
columns (Joschko et al. 1992), and even when earthworms were expected to be present. Clothier and
Vogeler (1994) reported improvements in soil conductivity that ranged from 0.3
H
m
to 1.2
H
m
−
l
−
l
under disk permeameters with salt solution because the earthworms are attracted to or repelled from
the soil surface by salt solutions.
The contribution of macropores to hydraulic conductivity can also be shown by theoretical
considerations (Smettem 1992). Nevertheless, the effects of earthworm burrows depends on the
type of connectivity that they attain within the burrow system itself or between the burrow system
and pore spaces. Francis et al. (1994) reported differential effects of
A. caliginosa
and
O.
cyaneum
in opening top soil and subsoil burrows, respectively. Although the total porosity was the same in
both cases, the effects of the latter were much greater (
K
= 573 mm h
with O.
cyaneum
compared
−
l
sat
with 103 mm h
with no earthworms) than the effects of the former (
K
= 729 mm h
with
A.
−
l
−
l
sat
caligonosa
with no earthworms). The gaseous diffusivity was
improved in the presence of earthworm burrows only when air-filled porosity was below a threshold
(Kretzschmar and Monestiez 1992).
Thus, it is inappropriate to attempt to measure the effect of earthworm burrows on mass transfer
because such experiments tend to make this measurement only under conditions when they are
visible or obvious; in essence, the absence of visible effects of burrows could be governed by
opposite interactions because earthworm burrow volume is totally connected to the matrix pores,
and the burrow volume is totally isolated from the matrix pores because of the impermeable walls.
compared with
K
439 mm h
−
l
sat
