Effects Of Some Herbicides On Soil Nitrifiers and Nitrification

Abstract

The herbicides Ioxynil, Bromoxynil, NPH 1320. Totril, Dieamba, Tricamba, Trifluralin, Oxadiazon, Legurame, M and B 9057 and M and B 9555 were tested for their effects on the process of nitrification in soil as well as in pure cultures.

An improved version of the perfusion apparatus was developed and the perfusion technique was used as the principal experimental method in the soil studies in which the effects of different herbicides on the rates of nitrification in soils previously enriched with nltrifiers and in fresh soils continuously perfused with the herbicides were estimated. These estimates were made use of in the assessment of the degree of toxicities of the herbicides under consideration, on the metabolic rates per cell of nitrifiers and on their degree of proliferation in soil. An attempt was also made to study the possible differential effects of these herbicides on the two main groups of chemoautotrophic soil nitrifiers and the results obtained revealed that the metabolic activities of Nitrosomonas populations in soil were much less sensitive to the lower concentrations of many of the herbicides tested when compared with the sensitivities shown by the Nitrobacter populations, of the same soils, to the toxic effects of the same herbicides. But the rates of metabolic activities of Nitrosomonas populations were found to be the factor limiting the overall rates of the nitrification process in soils treated with the higher concentrations of most of these herbicides.

The nitrification experiments carried out with cell suspensions of pure cultures of Nitrosomonas Euronaea and Nitrobacter Winogradski indicated that the herbicides exerted differential effects on the metabolic activities of these two organisms even in artificial media.

The only other method used in studying the effects of herbicides on soil nitrification involved the measurement of the rates of oxygen uptake by samples of enriched soil treated with known concentrations of herbicides, making use of the conventional Warburg respirometrie technique.

The qualitative effects of most of the herbicides on the activities of nitrifiers, grown in artificial media and in the natural medium of soil, were found to be essentially similar although the concentrations effective in causing these toxicities in pure cultures were much less than the concentrations needed to cause similar levels of toxicities in soil media.