Insecticidal effects on soil microorganisms and their biochemical processes related to soil fertility A.C. Das* and D. Mukherjee Among the four insecticides under study, hexachlorocyclohexane (BHC) followed by phorate signi®cantly stimulated the populations of (total) bacteria, actinomycetes, fungi, aerobic non-symbiotic N 2 -®xing bacteria and P-solubilizing microorganisms in soil. Carbofuran signi®cantly stimulated total as well as N 2 -®xing bacteria. Fenvalerate had no effect on P-solubilizers. All the insecticides stimulated the proportion of Penicillium in soil. Similarly, Pseudomonas with BHC, Sarcina with phorate, Corynebacterium, Azotobacter and Streptomyces with fenvalerate were also stimulated. On the other hand, Erysipelothrix with BHC, Staphylococcus with phorate, Staphylococcus, Nocardia and Fusarium with fenvalerate were inhibited. Almost all the insecticides reduced the proportions of Micrococcus and Rhizopus in soil. Insecticides also augmented the non-symbiotic N 2 -®xing and P-solubilizing capacities of the soil and the augmentation was more pronounced with BHC followed by phorate. Key words: Insecticides, microorganisms, N 2 -®xation, P-solubilization, population, predominant genera, soil. In modern agriculture, insecticides are frequently ap- plied to the crop ®elds to increase the crop production. Besides combatting insects, a signi®cant amount of the insecticides eventually reaches the soil in the form of ``insecticidal fall-out'' and is accumulated in the top soil (0±10 cm) where the maximum microbiological activi- ties occur (Alexander 1978). As microorganisms are scavengers in soil and possess physiological variability, they degrade a great variety of chemical substances including the insecticides to derive energy and other nutrients for their metabolism (Stadnyk et al. 1971; Bhuyan et al. 1993) with the resulting increase in the biomass of the insecticide-utilizers which may favour- ably in¯uence nutrient transformations in soil (Agni- hotri et al. 1981; Das & Mukherjee 1994). On the other hand, there are some insecticides which exert adverse effect on the proliferation of microorganisms and their associated transformations in soil (Moorman 1989; Martinez-Toledo et al. 1992). Moreover, the insecticidal effects on soil microorganisms and their associated transformation of nutrients are very speci®c since indi- vidual members within a group vary in toxicity (Simon- Sylvestre & Fournier 1979). Therefore, to sustain the fertility status of soil, it becomes necessary to study the effect of different insecticides on the microbial trans- formation of different nutrients, especially nitrogen and phosphorus in soil. An incubation study has been made under laboratory conditions to investigate the effect of four insecticides, viz. BHC (1,2,3,4,5,6-hexachlorocyclohexane), phorate (O,O- diethyl-S-ethylthiomethyl-dithiophosphate), carbofuran (2,3-dihydro-2,2-dimethylbenzofuran-7-yl-methylcarba- mate) and fenvalerate [(RS)-cyano(3-phenoxyphenyl)- (RS)-methyl-4-chloro-a(1-methyl)benzeneacetate], one each from chlorinated hydrocarbon, organophosphate, carbamate and synthetic pyrethroid insecticides respec- tively, at their recommended ®eld doses (7.5, 1.5, 1.0 and 0.35 kg a.i.ha )1 , respectively) on the changes of popula- tion and distribution of bacteria, actinomycetes and fungi in general, and the growth and activities of non-symbiotic N 2 -®xing bacteria and phosphate-solubilizing microor- ganisms in a Typic Fluvaquent soil. World Journal of Microbiology & Biotechnology, Vol 14, 1998 903 World Journal of Microbiology & Biotechnology 14, 903±909 ã 1998 Rapid Science Publishers The authors are with the Department of Agricultural Chemistry and Soil Science, Bidhan Chandra Krishi Viswavidyalaya, Mohanpur-741252, India. *Corre- sponding author.