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Effect of Pesticides on Nitrification in Soils

S. M. lmamul Huq
Departmet of Soil, Water and Environment, Dhaka University, Dhaka - 1000, Bangladesh.

Raquibul Amini
IUCN Country Office, Dhaka, Bangladesh

R. Sadia Afroz
BCSIR Laboratories, Dhaka, Bangladesh..

Abstract/ Executive Summary:

A laboratory experiment was conducted to study the effects of t.wo pesticides viz. diazinon (0,0-diethy l-0- [2 isopropyl-4-methylpyrimidyl-6) thiophosphate) and heptachlor (1, 4, 5, 6, 7, 8, 8-heptachloro- Sa, 4, 7, 7a tetrahydro-4, 7-methanoindene) on nitrification in soil. The two pesticides were applied at the recommended dose (0.5 and 0.8 mg active ingredient/ kg soil for diazinon and heptachlor respectively) and 200 and 400 mg active ingredient (a.i.) 1kg soil in presence or absence of 200 mg N/kg as (NH₄ hS0₄ and their effect on nitrification was studied over 28 days of incubation period. Two general effects of addition of pesticides on nitrification were apparent . At the recommended dose both the pesticides stimulated or did not affect the process. But at the higher doses both the pesticides inhibited the process. Degree of inhibition varied with the type of soils and type of pesticides. First order equations were used to calculate the maximum nitrification rate (K_^max), duration of lag period (t'), period of the maximum nitrification, (t) and duration of lag plus maximum nitrification period (ts). Nitrification rates were significantly affected by the application of pesticides at higher doses.

Keywords: Pesticides, Nitrification, Soil

Location: Melandaha and Bhatpara soil series, Savar, Bangladesh

Start Date:

End Date:

Program Area: Crop-Soil-Water Management

Commodity: Land management

Objectives:

To know the effect of pesticides on an important microbial process, viz., nitrification.

Methodology:

Two representative soils from Melandah a 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 and sieved in a field moist condition to pass through a 2 mm sieve. Sub samples were removed and stored at subzero temperature in sealed polyethylene bags for studies requiring field moist and freshly air-dried soil samples. The remainder of each of the samples was air-dried, crushed gently to pass a 2 mm screen, and stored at room temperature in a tightly sealed plastic container. The two soils were found to be sandy loam and a silty clay loam after particle size analysis performed by hydrometer method and subsequent textural class was determined by following Marshall's triangular co-ordinates as designed by USDA. Two generic group of pesticides viz. the organophosphate and chlorinated hydrocarbon, were used in the present experiment. These were diazinon (0,0- diethyl-0-[2 isopropyl-4-methylpyrimidyl-6) thiophosphate) and heptachlor or (1, 4, 5, 6, 7, 8, 8-heptachloro- 3a, 4, 7, 7a-tetrahydro-4, 7-methanoindene). The method used was identical to that described by Schmidt and Belser with some modification. Soil samples amounting to 10 g (air-dry basis ) was poured into plastic pots. All the treatments were replicated two times and incubated in an incubator at 28°C for four weeks. At 1, 2, 3 and 4 weeks 10 g of duplicate samples were extracted with 100 ml of 2 M KCl and the levels of NO₃ and NO₂ were determined calorimetrically by salicylic-sulfuric acid method and calorimetrically by Griess-Llosvay reagent, respectively. N0₃- plus NO₂ accumulation in excess of that in the control samples was used as the primary criterion for assessing nitrification of the applied ammonium. NO₃ = a/{1+ [a/( NO₃-)_o - 1]exp (- ak[t- t_o])}, t' = (1/ak)ln[a/ (N0₃-)_o -1] + [(N0₃-)o - a/2]/k_max, The SPSS/PC + statistical software was used to calculate k_max. t', ?t and t_s.

Source of Information: Dhaka Univ. J. Bioi. Sci. 13(1) : 77-89, 2004 (January)

Article Link (Online Journal):

Funding Source:

Total Budget (Tk.) :

Achievement/Findings:

The stimulation might have resulted from the killing of a portion of the soil microflora. However, these low concentrations of pesticides did not affect the nitrifying bacteria, which were able to utilize the newly available substrate. Such partial sterilization would bring about changes in the many complex interactions that are likely to upset the dynamic equilibrium of the soil microflora and thus affect soil fertility . From the nitrification model used in the current study, it was seen that diazinon lengthened the maximum nitrification period to about 11 days in Melandaha soil. Question arises as to what effect the resulting ammonium nitrogen accumulation would have on plant growth. On relatively infertile well drained soils delayed oxidation of ammonium fertilizers can prevent losses by leaching and denitrification when irrigation and rainfall is excessive, and this could increase nitrogen availability for such crops as corn and small grains during their early stages of growth. Under soil conditions where significant losses of fertilizer nitrogen are not likely to occur, delayed nitrification should not adversely influence nitrogen uptake unless there is a specific response to ammonium versus nitrate nutrition. It needs to be mentioned here that the doses used in the experiment are not likely to occur under field condition. But prolong use of pesticides might result in accumulation in certain amount to cause inhibition. Kearney et al.(44) stated the possibility of zone accumulation of pesticides at 100 ppm or higher due to un even distribution of pesticide through the 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 ff.rmers 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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