Environmental Engineering Reference
In-Depth Information
all cases, the structures and functions of soil communities are modified by pesticide use
(Dindal 1980).
Soil invertebrates are often directly exposed to pesticides, as they live in a number of habi-
tats that are deliberately sprayed to control insect pests, fungi, or weeds or to protect human
beings from disease vectors. Others are directly exposed by the deposition of insecticide
sprays that miss their target, for example, soil-dwelling invertebrates in sprayed forests. In
either case, exposure may be through contact or ingestion. Other invertebrates may be indi-
rectly affected through the removal or reduction of food sources, be they vegetable, fungal,
or animal. Insecticides are designed specifically to kill insects, and thus most invertebrates
are sensitive to these chemicals. Sensitivity to other pesticides varies, but some herbicides and
fungicides are also directly and highly toxic to this group of organisms.
Thompson and Edwards 1974, discussed the effects of pesticides on nontarget animals.
Brown 1978, reviewed the effects of insecticides, herbicides, and fungicides on soil inverte-
brates. The pesticide-dependent population reduction has been documented (House et al.
1987; Reddy 1989; Reddy et al. 1996).
The impact of agrochemicals on soil fauna diversity and soil functions has become an
issue of great concern. Agrochemicals are applied in the environment to fulfill a specific
purpose but, at the same time, may cause damage to the soil biota, decreasing its diversity,
growth, or reproduction, and consequently decreasing organic matter decomposition and
soil fertility. Therefore, there is an increasing need for appropriate methods to assess the
side effects of these chemicals on soil ecosystem.
12.3.2 Pesticide Residues in Earthworms
Earthworms are very useful and important members of the soil invertebrate fauna. They
help in soil formation by feeding on the decaying organic matter. Their burrowing and
feeding activities help in the continued maintenance of soil structure, aeration, drainage,
and fertility. They erect pebble-free top layers by moving large amounts of soil from deeper
strata up to the surface. They enhance the soil microbial fauna and hence the microbial
degradation of various substances. Aristotle called them “the intestines of the earth” and
Charles Darwin defined the extreme importance of earthworms in the breaking down of
dead plants and animal matter that reach the soil and in the continued maintenance of soil
structure (Darwin 1881). Their contribution to complex processes such as litter decomposi-
tion, nutrient cycling, and soil formation is very important, hence the presence of earth-
worms is beneficial to agroecosystems (Edwards and Fletcher 1988; Lavelle 1988; Lavelle
et al. 1997; Edwards 1998; Eriksen-Hamel and Whalen 2007). As a major component of the
soil ecosystem, they are susceptible to xenobiotics such as pesticides, and effects could
be seen at the species, population, and even the community level (Edwards and Bohlen
1992). In fact, it was Kennel (1972, 1990) who first reported that reduced litter decomposition
in orchards could indicate pesticide effects on earthworms. Since then, the use of earth-
worms in ecotoxicological studies is common, and a large database on pesticide effects on
earthworms exists (Spurgeon et al. 2003; Frampton et al. 2006), varying from biomarkers
(VanGestel and Weeks 2004; Rodriguez-Castellanos and Sanchez-Hernandez 2007) to field
effects (Förster et al. 2006; Reinecke and Reinecke 2007; Casabé et al. 2007). Earthworm
casts and burrow walls exhibit higher concentrations of total and plant-available elements
than surrounding soil, and it has been recognized that surface-feeding species horizon-
tally and vertically disseminate microorganisms, spores, pollen, and seeds and can reduce
plant pathogens through the digestion of fungal spores. Therefore, practices that reduce
earthworm populations in the soil can lead to a reduction in soil health.
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