Prolonged Usage of Herbicides Reduces Heterotrophic Aerobic Bacteria and Fungi Population and Alters Soil Physicochemical Parameters

The study aimed to examine the effect of commonly used herbicides on soil microbiota (bacteria and fungi) as well as its physiochemical properties. Topsoil (0 to 5cm depth) samples were collected from different plots at Ayepe, Iwaro Oka Akoko, Ondo State, Nigeria with known prior herbicide treatments. The plots had different history of glyphosate, paraquat and both combination application. The soil samples were serially diluted and cultured on nutrient agar and potato dextrose agar for bacteria and fungi, respectively. The physicochemical properties of the soil samples showed that all the soil samples had pH values which ranged from 7.4 to 9.4. The moisture contents ranged from 2.6 to 15 with soil sample without prior usage of herbicide having the highest moisture content. The average total bacterial counts ranged from 4.0 x 108 to 2.5 x 109 cfu/g while the average total fungal counts ranged from 3.0 x 103 to 3.1 104 cfu/g with the untreated soil containing the highest loads. Soil samples with prolonged use of herbicides generally contained lower soil moisture contents, organic matter contents, as well as lower microbial loads. It could be concluded from the study that the pattern of change in the bacteria and fungi population vary as a result of differences in the composition of herbicides and year of usage.