Agriculture Reference
In-Depth Information
indirect effects in the test. He concluded that these last effects seemed to be more important than
the impacts of the applied chemicals.
In addition to improvements in physical soil conditions, earthworm activities can contribute to
better chemical soil conditions. In a study of a peat polder soil contaminated with polycyclic
aromatic hydrocarbons (PAHs), Eijsackers et al. (2001) showed that the introduction of earthworms
could result in an accelerated breakdown of the PAHs present. Kersant et al. (2002) observed
another impact: earthworm activity promoted the adsorption of the pesticide atrazine to the ingested
soil, thereby causing a slower microbial breakdown. Singer et al. (2001) observed an enhanced
microbial breakdown of polychlorinated biphenyls by microorganisms introduced by the introduc-
tion of earthworms, which stimulated mixing of the microorganisms through the soil. Eijsackers
and Doelman (2000) further specified this for removal of a number of contaminant groups (orga-
nochlorines (OCls), PAHs, and heavy metals) from a riverbank soil. Here, the soil biological
processes are combined with physical forces caused by soil erosion from the river as well as the
implications of inundation.
For heavy metals, bioremediation is related mainly to differences in their availability under
aerobic or anaerobic conditions. However, next to that, the consumption of soil by the earthworms
may lead to changed availability of the heavy metals. Salisbury (1925, cited by Lee 1985) observed
that earthworm casts had a higher pH than the surrounding soil. Research aimed specifically at
assessing the role of earthworms in changing the bioavailability of contaminants resulted in mixed
observations: Ablain et al. (2002) observed increased contaminant availability, whereas Zorn (per-
sonal communication) suggested decreased bioavailability.
Another type of indirect relationship that deserves further study is between earthworms and
soil structure. Tomlin et al. (1993) observed the impacts of three types of treated sewage sludge
on soils and reported that earthworm burrows were lined with fecal material that had higher heavy
metal concentrations than the surrounding soil. Moreover, soluble forms of heavy metals could be
leached more easily through these burrows.
E
FFECTS
OF
T
OXICANTS
AT
F
OOD
C
HAIN
AND
E
COSYSTEM
L
EVELS
The best-recognized use of earthworms for the general public is as fish bait. Many other animal
species prefer earthworms as food, including songbirds, raptors (buzzards and tawny owl), various
mammals (hedgehog, badgers, and boars), and amphibians and reptiles (Edwards and Bohlen 1996).
Hence, earthworms act as an important route for transfer of contaminants from the soil to food
chains in terrestrial ecosystems. They have a considerable ability to accumulate contaminants from
the soil matrix into their body parts, greater than most other soil invertebrates, and therefore play
an important role in food chain transfer of contaminants.
With respect to contaminant food chain research, a number of models of the food chain transfer
of toxicants have been developed. These start with rather simple models, such as those suggested
by Romijn et al. (1991) and Luttik et al. (1993). In these models the maximal permissible concen-
tration (MPC) of a toxicant is calculated by dividing the NOEC for these bird or mammal predators
(NOEC
bird/mammal
) by the bioconcentration factor (BCF) of the contaminant from the soil into the
earthworm (BCF
earthworm
). MPCs were calculated for standard soil situations and compared with
MPCs for terrestrial organisms.
As a next step, the biological characteristics of the predator were introduced. Noppert et al.
(1994) combined BCFs for earthworms with the uptake rate by their predators, including feeding
rates, uptake efficiency, elimination rates, and life expectancy. Gorree and Tamis (1992) extended
this concept by introducing the bioavailable proportion of a compound (the soil pore-water parti-
tioning approach). Everts et al. (1993) introduced the basal existence and field metabolic rates of
contaminants for predators, both in laboratory feeding experiments and (based on general assump-
tions) in the field, and developed a formula:
