Effect of common pesticides used in the Niger Delta basin of Southern Nigeria on soil microbial populations

The effects of eleven pesticides on the populations of bacteria, actinomycetes, fungi and protozoa was investigated by treating a garden soil with their recommended rates. The microbial populations were estimated using the standard plate-count technique. Of the 11 pesticides investigated, phenylmercuric acetate (agrosan) at 50 mug g(-1) inhibited bacterial density the most, i.e. from 4,600,000 to 220 cells g(-1). The pesticides were Pentachloronitrobenzene ( PCNB), tetramethyl-methylthiuram disulphide ( thiram), 1-naphthylmethylcarbamate (Vetox 85), 1,2,3,4,5,6-hexachloro-cyclohexane (Gammalin 20), phenylmercuric acetate (Agrosan), tetrachloroterephthalic acid (Dacthal), 4-nitrophenyl-2-nitro-4-trifluoromethylphenyl ether (Preforan), 2-ethyl-6-methyl-N-2-methoxy -1-methyl ethyl-chloroacetanide ( Dual), Benlate, Brestan and Gramoxone. Pentachloronitrobenzene (PCNB) at 240,000 mug g(-1) reduced bacterial population from 4,600,000 to 2,100 cells g(-1), whereas tetramethylthiuram disulphide (thiram) at 100 mug g(-1) suppressed it by 2 log orders of magnitude. Soil application of 1-naphthylmethylcarbamate (Vetox 85) at 100 mug g(-1) and 1,2,3,4,5,6,-hexachlorocyclohexane (Gamalin 20) at 1,300 mug g(-1) repressed the bacterial numbers by 2 log orders of magnitude each. Pentachloronitrobenzene reduced the actinomycetes density from 340,000 to 320 cells g(-1) and completely eliminated all fungal and protozoan propagules from the soil. The Gammalin 20 completely wiped out all the fungi, whereas phenylmercuric acetate totally eliminated all the protozoa and reduced the fungal population from 34,000 to 60 cells g(-1). In general, protozoa and fungi were more susceptible to fungicides than bacteria and actinomycetes. Pentachloronitrobenzene, 1,2,3,4,5,6,- hexachlorocyclohexane and phenylmercuric acetate were toxic particularly to soil microorganisms, whereas the herbicides dacthal, Preforan and Dual were quite harmless in soil at application rates of 0.1, 0.06 and 0.02 mug g(-1) respectively.