EFFECT OF LONG TERM EXPOSURE TO GLYPHOSATE HERBICIDE ON SOIL MICROBIAL POPULATION DYNAMICS.

Glycophosate herbicide is one of the most widely used herbicides by farmers because of its efficacy. However, little is known about its possible long term effects on important soil microflora. To investigate this, mesocosm study involving four cell, Cell I, II, III, and IV contaminated with the herbicide at concentrations of 5, 20, 50 and 0 % v/v respectively were studied between October, 2016 to March, 2017. Soil samples analysed weekly from respective cells to determine the effect of herbicide on overall heterotrophic bacterial and fungal populations, and specifically on Nitosomonas, Nitrobacter and actinomycetes populations, qualitatively and quantitatively with time. The results showed that total heterotrophic bacteria counts (THBC) and actinomycetes counts decreased with time and concentrations for the first 28 day as compared to the control before attaining steady increases. However, beyond day 90, THBC and actinomycetes counts in control cell (Cell IV) were lower than all other cells with the exception of Cell III (P ? 0.5). The total fungal counts (TFC) were not significantly affected except for Cell I that had higher TFC than other cells at day 180 (P ? 0.5). Populations of Nitrosomonas and Nitrobacter witnessed a progressive decrease with time and concentrations as compared to uncontaminated soil (P ? 0.5). The following genera of bacteria, Pseudomonas, Bacillus, Acinectobacter, Micrococcus, Alcaligens, Achromobacter and fungi, Rhizopus, Trichoderma, Penicillium and Aspergillus were isolated. Actinomyces and Nocardia were the actinomycetes isolated. Though bacteria and fungi in soil generally recovered from the effect of glyphosate application in the long term following initial population decline except at the highest concentration of 50 % v/v, Nitrosomonas and Nitobacter (nitrogen fixers) specifically never recovered. This portend serious problem for nitrogen fixation in soil.