Effect of four organophosphorus insecticides on microbial activities in soil

Laboratory tests were conducted with four organophosphorus insecticides, Bay 37289 (O-ethyl O-2,4,5-trichlorophenyl ethylphosphonothioate), diazinon [O,O-diethyl O-(2-isopropyl-4-methyl-6-pyrimidinyl) phosphorothioate], Dursban (O,O-diethyl O-3,5,6-trichloro-2-pyridyl phosphorothioate), and Zinophos (O,O-diethyl O-2-pyrazinyl phosphorothioate), applied to a sandy loam at rates of 10 and 100 mug/g to determine whether these materials caused any serious effects on microbial activities related to soil fertility. All insecticides showed an effect on fungi and bacteria for the first and second week of incubation, but, subsequently, the populations returned to levels similar to those obtained in the controls. All insecticide applications increased ammonium production, but, in some instances, there appeared to be a slight depression of nitrification. Sulfur oxidation was equal to or better than that obtained with untreated soil in most cases. There was no significant effect on phosphorus mineralization. Oxygen consumption indicated that microbial respiration increased in proportion to the concentration of insecticides, suggesting the possibilities of microbial degradation of the insecticides or their degradation products and of uncoupling oxidative phosphorylation.