Pesticide Residues in Soil Invertebrates - Pesticides: Evaluation of Environmental Pollution - page 338

ecologically important processes such as nutrient cycling, soil formation, and decomposi-
tion as well as composing the vital second, third, and fourth trophic levels of food webs.
Species of invertebrates inhabiting soils have been estimated to constitute 23% of the total
diversity of all living organisms (Decaëns et al. 2006). They include such animals as earth-
worms, slugs, land snails, ants, and other insects. These animals carry out the early stages
of the physical and chemical decomposition of all types of organic debris in or on the soil.
Most soil invertebrates also act as carriers of microbial propagules (e.g., seeds, spores), and
so they inoculate the organic matter as it is passed through their bodies. The final stages of
biochemical decomposition are accomplished by microbes, thus recycling nutrients, form-
ing humus, and fostering soil particle aggregation (Dindal 1980).
These invertebrates provide essential regulatory services (decreasing greenhouse gas
emissions, carbon sequestration, flood control, and more). The size of soil invertebrates is
diverse, ranging from the smallest microfauna such as nematodes and protozoa (<200 μm)
to the macrofauna such as mollusks and annelids. The invertebrates inhabiting soils are
very sensitive to disturbance as the soil is their habitat and the place from which they
can extract all the resources they need. Microarthropods, Enchytraeidae , and the many
groups of the mesofauna (0.2-2 mm) live in the air-filled soil pores. The largest arthropods,
Mollusca, Annelida, and Crustacea constitute the macrofauna that live in the surface lit-
ter or in nests and burrows that they create in the soil (Lavelle et al. 1997). In some places,
vertebrates of the megafauna may become conspicuous elements of the soil fauna. Soil
invertebrates and the many important functions they provide have resulted in the use of
their diversity and abundance as a measure of soil quality.
A greater proportion (>80%) of biomass of terrestrial invertebrates is represented by
earthworms, which play an important role in structuring and increasing the nutrient con-
tent of the soil. Therefore, they can be suitable bioindicators of chemical contamination
of the soil in terrestrial ecosystems, providing an early warning of deterioration in soil
quality (Culy and Berry 1995; Sorour and Larink 2001; Bustos-Obregón and Goicochea
2002). This is important for protecting the health of natural environments and is of increas-
ing interest in the context of protecting human health (Beeby 2001) as well as other ter-
restrial vertebrates that prey upon earthworms (Dell'Omo et al. 1999). The suitability of
earthworms as bioindicators of soil toxicity is largely due to the fact that they ingest large
quantities of the decomposed litter, manure, and other organic matter deposited on soil,
helping to convert them into rich topsoil (Reinecke and Reinecke 1999; Sandoval et al.
2001). Moreover, studies have shown that earthworm skin is a significant route of contami-
nant uptake (Lord et al. 1980), and thus the investigation of earthworm biomarkers in the
ecological risk assessment (ERA) can be helpful (Sanchez-Hernandez 2006).
The technological advances in agriculture have led to an increased production and
emission of chemical substances, which end up in the soil. The soil constituents, such as
clay and organic matter, have a great capacity to retain chemicals. Therefore, the soil is a
net sink for all kinds of chemicals, and their concentrations are often considerably higher
than in any other environmental compartment (Verhoef and Van Gestel 1995).
12.3 Pesticides in Soil
Pesticides reach soil as a result of direct application to it or by drift during dusting and
spraying to foliage or by being washed down by rains after application. Soil is the main