BIODIVERSITY AND CHANGES IN SOIL MICROBES IN A MAIZE FIELD TREATED WITH SOLE AND HERBICIDES MIXTURES IN SOUTHWEST, NIGERIA

Abstract

The diversity and population of soil microbes play significant role in the health state of arable field. This research was designed to investigate the effect of diuron and S-metolachlor on soil microbial population and species diversity in a maize-based cropping system. Soil samples were taken at 0 – 15 cm depth using a soil auger five times at two weekly intervals, starting from the time before the herbicide was applied and after herbicide spraying and soil microbial population and species composition were determined. Variable mixtures of the two herbicides, Diuron (2.5 l/ha) and S-metolachlor (4 l/ha) constitute the 5 treatments and a control. Standard methods were used in isolation, enumeration and identification of microorganisms. Data on microbial population were analysed using Analysis of Variance with means separated by Duncan Multiple Range Test (DMRT) at 5% probability level while bacterial and fungal species composition were displayed as percentage occurrence on bar charts. The population growth of bacteria was enhanced by 96.8% and 107.4% respectively under the two herbicides when compared with the result obtained before herbicide was sprayed, Diuron and S-metolachlor at the recommended rate (100%) at 2 WAS with reference to the control. At the plots that received a quarter of Diuron concentration mixed with 75% S-metolachlor, bacterial population increased by 50% while the rate of increase reduced to 16.3% at 4 WAS. Further reduction in microbial population was observed at 6 WAS from 31% to 21% at 8 WAS. From 2 WAS to 6 WAS, there was no significant different in the fungi population among all the treatments and control. The most common species of bacteria that occurred and identified on the plots include, Proteus vulgaricus (38.92%), Pseudomonas telluria (50.04%) and Bacillus stearothermophilus (50.04%). Penicillium dangeardi while fungal species encountered were Aspergillus restrictus (44.48%), Penicillium roqueforti (38.92%). The two herbicides affected the bacteria and fungi in terms of species composition and population variation.