Pesticides are an integral part of modern farming practice. Pesticides can enter the soil by a variety of routes, e.g., intentional application, spillage, overspraying, runoff, aerial transport with soil, or leaching. Organic matter plays a major role in the binding of pesticides in soil. Fulvic and humic acids are most commonly involved in binding interactions. Pesticides or their degradation intermediates can also be polymerized or incorporated into humus by the action of soil microbial enzymes (Bollag et al., 1992).

Soil fumigation with general biocides such as methyl bromide decreases micro-bial populations and nearly eliminates nematodes (Yeates et al., 1991). Although recovery occurs, population densities may not return to prefumigation levels even after 5 months (Yeates et al., 1991). Fumigation with general biocides return the successional status of soil to that of a depauperate soil matrix that can only be inhabited by primary colonists. However, within 60 weeks after soil fumigation and manuring, a progression of colonization by early successional species followed by more-specialized, later successional taxa can be observed (Ettema and Bongers, 1993).

Broad-spectrum insecticides that are applied for the control of insect pests can be toxic to predaceous and parasitic arthropods. A single surface application of chlorpyrifos reduced populations of predatory mites in plots of Kentucky bluegrass for 6 weeks and similar applications of isofenphos reduced populations of non-oribatid mites, Collembola, millipedes, and Diplura for as long as 43 weeks (Potter, 1993). Densities of Collembola were lower in aldicarb-treated soil than in untreated soil, but only the collembolans in the suborder Arthropleona were influenced negatively, whereas Symphypleona were not affected or occurred in higher numbers in soil treated with aldicarb (Koehler, 1992). Mesostigmatid mites did not occur at the site for the first 2 months after treatment, and their abundance was reduced for 6 months. After 3 and 4 years, abundance was similar in treated and untreated soil. Koehler (1992) noted a change in species composition associated with aldicarb treatment and categorized three groups of reaction. The most sensitive organisms were absent from 9 months to 1 year after application; other groups showed no reaction to treatment, or a positive reaction. Surface-dwelling microarthropods appeared to be affected less negatively than were soil-dwelling microarthropods.

Badejo and Van Straalen (1992) tested the effects of atrazine on the growth and reproduction of the collembolan Orchesella cincta. The lethal concentration (LC50) for atrazine was estimated at 224 |g/g atrazine in food. Mortality and molting frequency increased with increasing concentrations of atrazine. The no observed effect concentration (NOEC) on egg production of O. cincta was 40 |g/g. Based on data for five collembolan species, 2.7 | g/g was estimated to be the hazardous concentration for 5% of soil invertebrates, which corresponds to the recommended field rate of 2.5 |g/g. House et al. (1987) investigated the impact of seven herbicides on miroarthropods and decomposition. No effect of any herbicide was observed on numbers of microarthropods, but decomposition of wheat straw was more rapid in soils without than with herbicide.

Generally, phenoxy acetic acid herbicides (e.g., 2,4-D, 2,4,5-T, 2-methyl-4-chlorophenoxyacetic acid) do not depress soil fauna directly with toxic effects, but indirectly through reduced vegetation and smaller additions of organic matter to soil (Andren and Lagerlof, 1983). Simazine, a triazine herbicide, is deleterious to most soil fauna (Edwards and Stafford, 1979).

Certain compounds such as the fungicide benomyl and its conversion product carbendazim have negative effects on soil biota even in low concentrations (Andren and Lagerlof, 1983). Applications of the fungicide captan to field soil reduce the abundance of saprophytic fungi and fungal-feeding mites compared with untreated field soil (Mueller et al., 1990).

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