Toxicity of Agricultural Pesticides to Soil Bacteria

The annual application of pesticides to agricultural soil in a bid to increase crop yield is enormous. The consequence of this practice on bacterial population was assessed from soil collected from a control garden and two pesticide-contaminated vegetable farm soils. Eight pesticides; glyphosate, cypermethrin, 2,3-dichlorovinyl dimethyl phosphate (DDVP), paraquat, lambda-cyhalothrin, chlorpyrifos, emamectin benzoate and dichlorvos were applied to the soil at half the recommended field rate, recommended field rate (RFR) and double the recommended field rate. The control garden soil recorded the highest mean bacterial population compared to the pesticide-treated soil samples. Generally, glyphosate and lambda-cyhalothrin increased the bacteria population of the polluted farm soils with all the dose rates applied. Cypermethrin, DDVP and paraquat dramatically decreased the bacteria populations of the three soil samples. The 16S rRNA gene sequences showed that the bacteria belonged to the genera of Bacillus, Thalassobacillus, Stenotrophomonas, Luteimonas andPseudopropionibacterium. Genes responsible for pesticide tolerance were not plasmid-mediated. The antibiotic susceptibility test showed that all the Gram-negative isolates were resistant to augmentin (30 µg), amoxicillin (2 µg) and sensitive to ofloxacin (5 µg), tetracycline (10 µg), nitrofurantoin (200 µg) and gentamicin (10 µg) while Gram-positive was resistant to ceftriaxone (30 µg), ceftazidime (30 µg), cefuroxime (30 µg), cloxacillin (5 µg) and sensitive to ofloxacin (5 µg). This study showed that the application of agricultural pesticides affects the soil bacteria population and this can lead to a shift in soil health and fertility.