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Effects of Pesticides on Urease Activity in Soil

S. M. Imamul Huq
Department of Soil, Water and Environment, University of Dhaka, Dhaka-1000, Bangladesh

R. Sadia Afroze
BCSIR Laboratories, Dhaka, Bangladesh

Raquibul Amin
IUCN Country Office, Dhaka, Bangladesh.

Abstract/ Executive Summary:

A pot culture experiment was conducted to study the effect of two pesticides viz. heptachlor (I, 4, 5. 6, 7, 8. 8-heptachloro-3n. 4, 7, 8-tetrahydro-4,7-methanoindene) and diazinon [0,0-dicthyl-(2-isopropy 1-4-methyl pyrimidy 1-6) thiophosphate ] on the urease activity in a sandy loam (Melandaha series) and a silly clay loam (Bhatpara series) soil. Effects of these selected pesticides on urease activity were evaluated for up to 28 days of incubation. Three sets of each of the individual soil were treated with various doses of the pesticides; the recommended dose (0.8 and 0.5 mg active ingredient (a.i.) kg^-1 soil for heptachlor and diazinon, respectively], 100 and 400 mg a.i. kg^-1 soil. To the first set of treatments, no extraneous N was added to study the indigenous urease activity of both soils. Two and 100 ppm of N as urea were applied to the second and third set of experiments. respectively. Both the pesticides at any rate of application stimulated the enzyme activity signiftcantly in all soils. Degree of stimulation varied with the type of soils. More stimulation was observed in Bhatpara soil than in Melandaha soil. The effectiveness of two pesticides to stimulatee urease activity differed negligibly. The different concentrations of pesticides or incubation period did not show any effect on soil urease activity.

Keywords: Pesticides, Urease activity, Soil, Diazinon.

Location: Savar, Dhaka, Bangladesh

Start Date:

End Date:

Program Area: Crop-Soil-Water Management

Commodity: Becteriocides

Objectives:

To look deeply into the soil urease activity as affected by pesticide application.

Methodology:

Two representative soil s from Melandaha and Bhatpara series were used in the present experiment. Both the soils were collected from adjacent areas of Savar. A composite sample was collected to a depth of 15 cm in both the cases. Before use, each sample was screened for debris, roots and other dead plant materials. Each of the samples was air dried, crushed gently to pass a 2 mm screen and stored room temperature in a tightly sealed plastic container. The methods of extraction and subsequent analysis of different parameters have been described elsewhere. Two generic group of pesticides viz. the organophosphate and chlorinated hydrocarbon were used in the present experiment. These were diazinon (0,0-diethy 1-0-[2 isopropul-4-methylpyrimidy 1-6] thiophosphate) and heptachlor (I, 4, 5, 6, 7, 8, 8-heptachloro-3a, 4, 7, 7a-tetrahydro-4, 7-methanoindene). To the first set of experiments no additional N was applied. The second set received 1 ml of 2 mg N as urea (2 ppm in soil). To the third set, 1 ml of 100 mg N as urea (100 ppm in soil) was added. All the sets then received different doses of either diazinon or heptachlor. Diazinon was applied to soils in 1 ml of aqueous solution to achieve pesticide concentration of 0, 0.5, 100 and 400 mg a.i. (active ingredient) kg^-1 soil. 1 ml or homogenized suspension of heptachlor was applied to soils to achieve pesticide concentration of 0, 0.8, 100 and 400 mg a.i. kg^-1 soil. In each case, only urea solution was added in one control In another control, soil samples were incubated under identical conditions without addition of urea and pesticides. Sufficient water was added to bring the soil moisture content to 60% of its water holding capacity. The containers were capped with parafilm. No restoration of moisture was required during incubation, since parafilm was permeable to gases but not to water. All the treatments were replicated three times and incubated in an incubator at 30°C for 4 weeks. To determine NH/- N in the resulting soil suspension, 10 ml aliquot of the suspension was pipetted into a 100 ml distillation flask and NH₄+-N released was determined by steam distillation of the aliquot with 0.2 g of MgO for 4 minutes. Urease activity was expressed as mg of ammonium N released per kg soil in 5 hours.

Source of Information: J. Asiat. Soc. Bangladesh, Sci., 32( 1): 97- 109. June 2006

Article Link (Online Journal):

Funding Source:

Total Budget (Tk.) :

Achievement/Findings:

The results from the present experiment show that urease activity is enhanced in the presence of diazinon and heptachlor and this effect continued till the end of the observations. From these results. we opine in agreement with earlier report on baygon treated soil that both the processes i.e. the enhancement of the activity of ammonifiers anti the suppresion of nitrifiers result in increased ammonia form:uion. Another general observation is that ammonia formation or nitrogen mineralization is less sensitive to pesticides than is nitrification. Often the ammonifying activity of the heterotrophci bacteria, actinomycetes and resistant saprophytic fungi is little affected or actually even increases after pesticide treatment. Consequently, depending on the soil pH, rather large concentrations of NH₃ or NH₄ can accumulate following fumigation or fungicidal treatment. In Bhatpara soil, all doses of both the pesticides increased urease activity of N pool up to first 14 days of incubation, which decreased subsequently. The increase might be due to greater mineralization of organic mutter and the decrease at later stages, due to volatilization and immobilization. It needs to be mentioned here that the doses used in the experiment are not likely to occur under field condition. Moreover, heptachlor has been banned,so its use by the farmers is also not likely. But prolong use of pesticides might result in accumulation in centain amount to cause harm stated the possibility of zone accumulation of pesticides at 100 ppm or higher due to uneven distribution of pesticides through plow depth. This is particularly important in our country where farmers lack basic training and equipment for proper use of these toxic agrochemicals. Besides, insecticidal resistance by agricultural pests led farmers to use pesticides repeatedly and also to choose more persistent chemicals. All of these might lead to zonal or widespread ecological disaster.

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