Effects of selected commercial glyphosate herbicides on bacterial population, activity and diversity in a cowpea farmland

Objective: This study investigated the impacts of three commonly available commercial glyphosate herbicides; Force up®, Sunphosate® and Sunsate® on bacterial load, activity and diversity in soil samples obtained from a cowpea farmland. Methods: Three concentrations of each herbicide, half of the recommended dose, the recommended dose, and twice the recommended dose, were applied to the soil samples in the laboratory for 24 hours. Distilled water served as control. Spread-plate technique and spectrophotometric measurement were used for bacterial enumeration and their growth activities respectively at 24, 48, 72 and 96 hours, while microbiological/biochemical properties were used to establish bacterial diversity. Results: Bacterial count was highest in soil sample treated with distilled water (control), followed by those treated with half the recommended dose of each herbicide and least in those treated with double the recommended dose of herbicide. Post-exposure, bacterial count increased in all the treated soil samples with increasing hours. Compared to the control, bacterial growth decreased significantly in samples treated with recommended doses of herbicides and those treated with double the recommended dose; while no significant difference was observed between the control and those treated with half the recommended doses. Aeromonas sp. and Bacillus sp. were the only bacterial genera recovered from the herbicide-treated soils. Conclusion: The three herbicides have significant adverse effects on soil bacterial population, activity and diversity, especially when they are applied at concentrations above the recommended doses.