Environmental Engineering Reference
In-Depth Information
microstructure of the soil (Cragg and Bardgett 2001). The modification of decomposi-
tion processes results in changes in nutrient mineralization and may ultimately affect
plant growth (Gange 2000). Bardgett and Chan (1999) and Scheu et al. (1999) showed
that the presence of Collembola resulted in changes in plant growth and plant shoot
N contents. The effects of Collembola on decomposition processes and nutrient miner-
alization may depend on species composition and dominance structure of collembolan
communities, since different species of Collembola have different feeding preferences
(Cole et al. 2004).
Rajagopal et al. (1990) reported that soil fauna that included mainly mites and collem-
bolans were affected by insecticide application in the field. The effect of pesticides on the
soil mesofauna is complex because of their action on both predacious and nonpredacious
groups. Therefore, several researchers have reported increased populations of collembo-
lan and mites after the use of pesticides, mainly because of mortality of predacious mites,
which exercise a great check on other mites and collembolans (Veeresh and Rajagopal
1989). Epstein et al. (2000) reported the effect of broad-spectrum insecticides on epigeal
arthropod biodiversity in Pacific Northwest apple orchards. They reported that the highly
mobile invertebrates were strongly susceptible to application of broad-spectrum neural
active insecticides.
The effect of dimethoate was found to be species specific. Folker-Hansen et al. (1996)
reported the effect of dimethoate on the body growth of representatives of the soil
mesofauna and it was proved to be sex and species specific. All the collembolan spe-
cies displayed a sexual dimorphism for body length, and females grew longer than the
males in the case of treatment with dimethoate. Diversity, a measure of community
structure, was higher in control farms, while dimethoate application reduced the soil
fauna diversity. In station one, Trips spp. disappeared in the treated farms, whereas they
were present in the control farms. In station two, Collembola spp. and seed corn mag-
gots disappeared in the treated farms, whereas they were present in the control farms.
On the other hand, Collembola spp., Symphyla spp., and beetle larvae were absent in the
treated farms, whereas at the same time they were present in the control farms in sta-
tion number three.
Everts et al. (1989) reported that fields, which were mechanically treated against weeds,
had higher species diversity than fields chemically treated by deltamethrin, fenitrothion,
and bromophos-ethyl. The present results are supported by the report of Stevenson
et al. (2002); they reported that pesticide-free cornfields had soil invertebrates' densi-
ties that were significantly higher than in the treated cornfields. Entry of pesticides into
the ecosystem may cause imbalances in the ecological equilibrium. Rogers and Potter
(2003) studied the effect of spring imidacloprid application for white grub control on
parasitism of Japanese beetle (Coleoptera: Scarabaeidae ) by Tiphia vernalis (Hymenoptera:
Tiphiidae ). They indicated that applying imidacloprid in early spring can interfere with
biological control by T. vernalis , whereas postponing preventive grub treatments until
June or July, after the wasps' flight period, will help to conserve T. vernalis population.
These deleterious effects greatly modify some biological functions, such as soil organic
matter decomposition and nutrient availability in the soil by reduction of the diversity
of soil biota (Ferraro and David 2000). From the outcomes of this study, we can conclude
that dimethoate residues in the soil of Zendan Valley are beyond the TTLC (total thresh-
old limit concentrations) for total dimethoate in soil. Soil microarthropod density and
diversity are low in the contaminated soil with dimethoate and omethoate; this will lead
to the reduction of agriculture production for such areas and thus lower the economy of
the country.
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