Environmental Engineering Reference
In-Depth Information
in soil bulk density) and some are decompacting (they
decrease soil bulk density and increase porosity) (Rossi,
2003). Therefore, 'big fat squishy' worms may have quite
different effects on a soil from 'little thin skinny' ones.
Although the role of earthworms might not be as sim-
ple as many hydrologists think (the basic model being:
more earthworms
soils. For example, Poier and Richter (1992) investigated
earthworm populations in a temperate loessic agricultural
soil near Hanover in Germany and found that all three
species of earthworm identified in their survey (
Lum-
bricus terrestris
L.,
Allolobophora caliginosa
Sav. and
A.
rosea
Sav.) showed distinct nonrandom variation in both
abundance (number of individuals per unit area, with a
range from 58-360 individuals m
−
2
) and biomass (mass
per unit area, with a range of 9.3-86.3 g m
−
2
), despite the
fact the soil appeared to be uniform at the surface and
had been ploughed for arable crops (see Figure 10.5). In a
somewhat similar study, Margerie
et al
. (2001) investi-
gated the spatial structure of populations of different
species of earthworm on a temperate chalk hillslope in
Normandy, France. In areas of the hillslope dominated
by apparently homogenous stands of vegetation, such
as tall grassland dominated by
Brachypodium pinnatum
(L.) P. Beauv., they found clear patterns in the species
composition of earthworm populations and noted that,
on the hillslope as a whole, it was not possible to
superimpose the spatial structures of the earthworm
populations on to those of the vegetation. Spatial
patterns have also been observed by Rossi (2003) in
tropical sandy soils in C ote d'Ivoire. Unlike Poier and
Richter (1992) and Margerie
et al
. (2001), Rossi (2003)
additionally considered changes in spatial pattern over
time and, in repeat surveys over a period of two years,
he found that patches of different earthworm species
tended to remain stable. Finally, Rossi (2003) measured
soil bulk density and found a very significant correlation
greater rates of
water flow through the soil), changes in bulk density
whether brought about by compacting or decompacting
species can be expected to affect water flow (hydraulic
conductivity -
K
), water storage, oxygen content and a
host of other biochemical and biophysical properties and
processes, and some of these may be self-reinforcing
so that patterns that develop may strengthen over time
or remain relatively stable (see below). This role may
also work across different scales. If a species of earth-
worm that creates conditions that enhance rates of water
flow through the soil has a patterned (nonrandom or
nonuniform) distribution within the soil, its effect on
whole-hillslope hydrological behaviour will depend on
the form of the pattern and on connections between
those patches where earthworm densities are highest.
Hydrologists seem to have missed the opportunity to
investigate such possibilities. Biologists and ecologists
have looked for subsurface patterns in soil faunal popu-
lations, although not necessarily from a hydrological or
ecohydrological perspective. Perhaps the time is right for
more crossover between these disciplines.
Striking spatial structures related to faunal activity
have been found even in otherwise
(apparently)
uniform
=
more macropores
=
Lumbricus terrestris
Biomass (g m
−
2
)
Abundance (individuals m
−
2
)
< 15.7
< 29.4
< 42.4
> = 42.4
< 72.0
< 97.0
< 120.0
> = 120.0
Figure 10.5
Patterns of biomass (g m
−
2
) and abundance (individuals (ind.) m
−
2
) of the earthworm
Lumbricus terrestris
L. in a
100
100 m area of loessic soil near Hanover, Germany (Reproduced with permission from Poier and Richter (1992). Redrawn from
the original. Poier, K.R., and Richter, J. (1992) Spatial distribution of earthworms and soil properties in an arable loess soil.
Soil
Biology and Biochemistry
, 24, 1601-8).
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