Agriculture Reference
In-Depth Information
¤oxicant bioavailability is a leading aspect and should always be measured, preferably
working with field soils (instead of artificial soils). Soils should be characterized fully
by texture (percentages of sand, silt, and clay), pH, percentage organic carbon content,
water-holding capacity, and cation exchange capacity.
¤Chemical extraction techniques used to measure bioavailability, including the promising
new biomimetic techniques (diffuse gradient in thin films (DGT), semi-permeable mem-
brane devices (SPMD), solid phase micro extraction (SPME), should be verified properly
and validated.
¤oxicant bioavailability models should include temporal and spatial variations, as well
as internal distribution phenomena in earthworms.
¤Life cycle parameters as suitable end points have to be combined on one hand with
mechanistic biomarker responses and on the other hand with recorded population
changes.
¤Bioassays are important tools in both laboratory and field and should include the effects
of factors like drought, temperature, and acidity. Also, Terrestrial Model Ecosystems
(Knacker et al. 2004) could be useful in this perspective.
¤For regulatory purposes, there are a number of additional promising approaches; these
include behavioral aspects, critical body weight loss, and juvenile growth rates.
¤A proper definition of recovery from toxicants would be most helpful in assessing the
results of field tests, especially in the longer term. This also implicates an extended
duration for field tests, depending, for instance, on negative responses caused by exper-
imental treatments. Moreover, site-relevant species should always be included.
¤A proper assessment of the impacts of pesticides applied at a broader scale or the
implications of mixed contamination cannot be restricted to the field experiments but
should be extended to an adjacent set of field plots, thereby including landscape ecolog-
ical elements in the toxicity assessment.
TOXICOKINETIC BEHAVIOR BY EARTHWORMS (AVAILABILITY,
UPTAKE, ELIMINATION, BIOACCUMULATION)
Ecotoxicological work can be separated into a number of phases (Eijsackers, 1994). In the first
phase, the main question is how the organism is exposed to a contaminant. This comprises three
steps: how the contaminant is chemically available in the soil, by what routes and mechanisms
earthworms take up the contaminant, and how it is internally processed in the earthworm (whether
is it broken down, excreted, or stored). This can be summarized under headings of environmental
chemistry and toxicokinetics, which are discussed in this section.
The second phase deals with questions such as the following: What are the effects of a chemical
on individual earthworms? What are the implications for earthworm populations? What impacts
does this have on soil ecosystems and food chains?
With respect to the bioavailability of toxicants, the three major steps, each with a different
setting and characteristics, are (1) chemical availability; (2) biological uptake of the chemical into
earthworms (this also includes exoenzymes of microorganisms, which are active outside the organ-
isms and can ÑdigestÒ the contaminant there, which can be interpreted as an external uptake-and-
processing mechanism of the compound); and (3) internal transport and processing in the earthworm
body (
Figure 17.1
)
. Much progress has been made in the development and validation of the soil
pore-water approach, which suggests that most contaminants are taken up from the pore water
directly surrounding the earthworm through its skin.
For the effects of heavy metals on earthworms, much work has already been done on factors
influencing the uptake, internal distribution, and elimination of these toxicants (Beeby 1993). The
characteristics of the metal, the soil, and the earthworm are of prime importance and play significant
