Agriculture Reference
In-Depth Information
proceeded beyond the laboratory experimentation phase, and there seems to be much scope for
incorporating them in routine regulatory testing when applicable. The soil microcosm test has
various advantages over the standardized artificial soil test. It accounts for interactions with other
organisms, can be used with different soil types, has high reproducibility, and can provide repro-
duction data. The more complex TME closely approximates field exposure conditions but is usually
more expensive and labor intensive.
The use of earthworms as tools in the detection of chemical contamination includes bioassays
at contaminated sites in the field or in the laboratory using soil taken from the contaminated site
in microcosms. Contaminated sites are often characterized by combining the effects of complex
chemical mixtures rather than single chemicals. Adverse biological effects associated with a con-
taminated site can therefore seldom be ascribed to a single chemical. Determination of causal
relationships between specific chemicals and observed effects is not possible because of the
complexity and variability of biological responses. To determine the environmental impact associ-
ated with complex mixtures in a site, a toxicity-based approach rather than a chemically based
approach should be adopted (Callahan and Linder 1992), thus incorporating biological information
into the assessment process.
FIELD TOXICITY TESTS USING EARTHWORMS
Field tests can use replicated experimental field plots or use continuous monitoring in the field,
but they are expensive, labor intensive, and time consuming because the most accurate method for
quantitative sampling of earthworms is still hand sorting. Field tests have obvious advantages, such
as realism, because they are done under natural climatic conditions. Moreover, they can take several
earthworm species into account because different species are exposed, and a broad database is
obtained. The logical sequence of events would be to undertake these longer-term, labor-intensive
tests only after the potential toxic effects of a chemical have been established reliably from both
acute and chronic toxicity testing (first and second tier).
The aim is to determine whether an ecologically significant earthworm population change will
occur as a result of exposure to the chemical, taking full cognizance that earthworm populations
can fluctuate enormously because of seasonal changes or cropping. The importance of a sensitive
and carefully designed test is obvious. Test designs were proposed by Edwards (1992, 1998, 2002),
H. Kula (1992), and Lofs (1992). The literature showed that the results of most field experiments
are extremely variable and do not allow for meaningful comparisons because of poor design and
inadequate replications and controls. The distribution of chemicals in soils is also extremely variable
(Beyer 1992). Natural, seasonal changes in environmental factors could also cause large fluctuations
in earthworm population densities, thereby masking possible effects of toxicants during certain
periods.
Field tests can also provide valuable data on the indirect effects of chemicals on earthworms
through effects on food supply and soil cover. The limitations of field tests for assessing toxicity
of chemicals to earthworms were discussed fully by Edwards (1992, 1998), who proposed a
standardized field test design along the following lines:
Site variables:
At least six species of common earthworms with at least 100 earthworms per square meter
Preferably a loam-based soil type
No history of chemical use for at least 5 years
Treatment variables:
Clearly defined chemical with known physicochemical properties such as solubility, wa-
ter/lipid partition coefficient, and volatility
The highest recommended dose
 
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