Studies on the effects of some herbicides on soil nitrification

An investigation was carried out into the effects of Bromoxynil, Chloroflurazole, Chlorothamid, Dichlobenil, Diquat, Endothal, Ioxynil, Paraquat, Picloram, Propanil and 2,3,6-TBA on soil nitrification. The perfusion technique was employed for studies on herbicide effects on the nitrification process in soil previously enriched with nitrifiers, and also on their effects on the enrichment growth of nitrifiers in a soil continuously treated with the herbicides.The results obtained with these methods were compared with those from nitrite production and utilization by pure cultures of Nitrosomonas sp and Nitrobacter sp respectively, and with measurement of oxygen uptake by these organisms and enriched soil, by conventional Warburg techniques for these herbicides. The results indicate that there were differential effects of the herbicides on the growth and metabolism of the nitrifying organisms; the nitrification process was affected at a lower herbicide concentration than the growth rate. The evidence suggests that there was an adaptation of the nitrifying organisms during growth, i.e. a selective proliferation of a tolerant strain. Ioxynil, propanil, chloroflurazole and bromoxynil were the most toxic while dichlobenil, paraquat and endothal were the least toxic of the herbicides. Endothal in thesoil perfusion experiments, and also in the pure cultures, stimulated nitrification, while paraquat was more inhibiting in the pure cultures than in the perfuser experiments.