Effect of pesticides on microbial diversity on agricultural soil in the locality of Akonolinga in the Centre Region - Cameroon

Bacteria and fungi constitute microorganisms which provide essential services to the soil ecosystem, not only for the maintenance of its fertility, but also for the degradation of certain pollutants. The present study was carried out to evaluate the effects of a synthetic fungicide and seed extracts of two fungicidal plants (Anonidium mannii and Ricinus communis) on microorganisms in an agricultural soil sample. With the solvents acetone, water and methanol, seed extracts of A. mannii and R. communis were used for phytochemical screening and for the preparation of solutions with concentrations of 50 µl/ml corresponding to fungistatic doses. For microbial colony enumeration, 10-1 dilutions for fungi and 10-2 dilutions for bacteria were spread on the surface of SCA medium supplemented with recommended doses of the chemical fungicide and 50 µl/ml concentrations of A. mannii and R. communis seed extracts. Petri dishes containing these mixtures are incubated with bacteria at 30°C for 24 hours and with the fungi for 7 days. At the end of each period, the types of microorganisms are differentiated by the colours of the colonies formed. For doses recommended for farmers, the chemical fungicide Monchamp 720WP destroys all bacterial and fungal life on solid SCA agar. In extracts containing abundant alkaloids, bacterial colony counts are low while in extracts containing metabolites, bacterial colonies are high. Different groups of fungi were differentiated by the coloration of the colonies formed. A significant number of fungal colonies of different colours were observed in the presence of extracts containing alkaloids and sterols/triterpenes. Microbial diversity will reduce the bactericidal and fungal activity of the plant extracts used and will favour their multiplication in the media. The inability of these plant extracts to destroy microorganisms continues to provide beneficial ecologically services to the ecosystem which ensures soil fertility and environmental preservation. Thus, the use of these plant extracts ensures good environmentally agriculture.