Effect of Some Commonly Used Herbicides on Soil Microbial Population

Abstract

Herbicide application has become an integral part of vibrant agricultural productivity in the whole world since its benefit has been overwhelming over the years. However, its toxic impact on the non-target soil microorganisms which play roles in degrading organic matter, nitrogen and nutrient recycling and decomposition needs to be considered.  In the present study, the effect of four (4) most commonly used herbicides in Ghana; Atrazine, 2, 4-D amine, Glyphosate and Paraquat on soil microorganisms was assessed over a period of fifteen continuous days (exposure period). The herbicide treatments were the normal recommended field rate, (6.67 mg active ingredient per gram of soil for Atrazine, 6.17 mg for 2, 4-D amine, 5.56 mg for Glyphosate, and 2.46 mg for Paraquat), half and double of the recommended field rate. Bacterial and Fungal populations were then determined at a five-day interval up to the 15^th day after treatment. The data gathered from bacterial enumeration was logarithmically transformed before graphs of mean bacterial were plotted against the exposure period for each selected herbicide. Bacterial population and percentage organic matter did not show any significant differences relative to the exposure period in this study (p < 0.05). However, the deleterious impact of the herbicides was seen as Paraquat treatment resulted in reduction in the bacterial population for five, ten and fifteen Days after treatment (DAT) in the treatment with half the recommended field rate. Glyphosate followed with 69.3%, 12.7%, and 18.0%; 2,4-D amine had 44.8%, 33.5%, and 21.6%; and lastly Atrazine had 41.8%, 44.5% and 13.6% bacterial population 5DAT, 10DAT and 15DAT respectively. The inhibition effect on the fungal population was very specific as some fungi (such as Aspergillus niger, Trichoderma viride, Collectotrichum gloeosporioides, Aspergillus flavus, Mucor, Penicillium, Curvularia lunata) which were present in the baseline determination (control) did not appear in the treatment. Percentage organic matter for the treatment did not vary much with the baseline determination (control) but the impact was observed in the various levels of treatments for all the herbicides. A similar study should be conducted on a normal field condition where herbicide treatments would be carried out on a normal field condition since most of the previous studies had the herbicide treatment carried out under laboratory condition. It will also be very appropriate if further research work is carried out to identify the specific components of these herbicides which favour the growth and development of certain beneficial microorganisms such as fungi and bacterial.

Keywords: herbicides, microbial population, biomass, baseline, organic matter, treatments.