Agriculture Reference
In-Depth Information
particles of plant material (Graff, 1971; Aldag and Graff, 1975) and a larger proportion
of small
mineral components
than
in
the surrounding soil
(see
e.g
., Bolton
and
Phillipson, 1976).
Anecics also produce subterranean casts which may partly fill their burrows, or are
deposited into other sorts of soil macropores. The abundance, structure and composition
of such casts has not yet been documented. Some results suggest that they are different
from casts produced by endogeics: in a microcosm study, Shaw and Pawluk (1986)
showed that the endogeics
Octolasium tyrtaeum
and
Allolobophora turgida
created
granular structures in sandy loam and clay loam soils whereas the anecic
Lumbricus
terrestris
created a fused structure. According to these authors, the combination of both
functional groups allows the formation of the most favourable structure.
The annual production of surface casts has been frequently estimated with average
values of
ca.
in temperate areas where anecic earthworms are dominant
(reviewed in Lee, 1985). However, these represent only a fraction of the overall materi-
al actually passed through the earthworm gut, and the casts produced by anecics and
endogeics are rarely distinguished.
4.2.3.2
Galleries
Abundance and structure
Anecic earthworms may create relatively sophisticated burrow systems (Lamparski
et al.,
1987). In French temperate climate pastures, the burrow system comprises a large
number of galleries with total lengths of and overall surface areas of
depending on the season (Kretzschmar, 1982). Galleries are concen-
trated in the upper 60 cm of soil while an average of 16.3 % occur below this; they may
also be found at depths of 2-3 m. The majority (56.9 %) of galleries are subvertical
whereas subhorizontal galleries only account for 18.9 % of the total. Most galleries are
rather narrow and their average diameter is in the range 2-4 mm (Table IV. 17). Similar
observations were made in a Dutch polder recently colonised by anecic earthworms
(especially
Allolobophora
): density of galleries rapidly increased up to a maximum of
ca.
at 5 cm depth after four years and then diminished slightly following
a decrease in earthworm biomass. Most burrows were found in the upper 40 cm of soil
(Ligthart, 1996). Estimates of 'active galleries', opening at the soil surface, at seven
French sites gave much lower figures, ranging from with average
distances between active burrows ranging from 10 to 30 cm in the upper 40 cm of
soil (Lopes-Assad, 1987). This indicates that a large part of the functional galleries may
not open at the soil surface, either because they have never done so or because they have
been partly refilled, especially when earthworms have become inactive or have died.
Burrow systems are dynamic entities which are periodically renewed. In temperate-
climate regions of the northern hemisphere, burrows are actively dug and maintained
during winter, from October to February whereas in late Spring and Summer, surface
activity diminishes and burrows are partly filled in as anecic worms enter diapause
(Kretzschmar, 1982). Ligthart (1996) found that cattle trampling and earthworm casting
in burrows were the main reasons for burrow destruction. Estimates of annual rate of
destruction were respectively 1172 ± 401 burrows
at 2.5 cm depth and
