Studies on bacterial populations in soil treated with herbicides

An investigation was carried out into the effect of 2-4 dichlorophenoxyacetic acid (24D) and other herbicides on soil populations. Experiments were devised to test the hypothesis that the initial lag period in detoxication is a combination of a period of adaptation followed by proliferation. Methods employed were dilution plate counts, variations in conditions of treatment, respiration studies, and the use of a bacteriostat and pre-enrichment of the soil with autotrophic organisms. An active organism, which would decompose 24D when inoculated into an aerated suspension of sterile soil, was isolated from soil enriched to this herbicide. There is a significant correlation between 24D concentration and bacterial numbers in the soil. Applications of this herbicide stimulated the rate of respiration of an enriched soil and of a culture of the isolated active organism. Results indicate complete oxidation of the molecule. The evidence suggests that the organisms responsible occupy sites on the surfaces of the soil crumbs where the anion is adsorbed. These sites are quite distinct from those occupied by the nitrifying and thiosulphate-oxidising autotrophs. Proliferation of organisms begins on the fifth or sixth day of treatment, at which time it is calculated that a slow breakdown of herbicide begins. It was concluded that the organism responsible for the detoxication of 24D in soils undergoes a period of adaptation to the new substrate, lasting five or six days. During the following time, all the sites on the soil crumbs where the 24D anion can be adsorbed are occupied to give a saturated soil. When herbicide is reapplied, rapid breakdown ensues accompanied by a high rate of carbon dioxide evolution and sufficient proliferation to replace those bacteria which have died. It is probable that a similar process occurs when soil is treated with other phenoxyacetic acids.