Studies on the effects of herbicides on soil nitrification,II

The effects of selected herbicides have been studied on the following parameters of soil nitrification processes: the rates of nitrate and nitrite formation from ammonia in freshly perfused soils and in soils previously saturated with nitrifying organisms in an improved perfusion apparatus; the rates of oxygen consumption and of oxidation of ammonia and nitrite in washed cell suspensions of Nitrosomonas europaea and Nitrobacter winogradskii, respectively; the rates of growth of those two organisms in newly established cultures; the rates of oxygen uptake by soil enriched in nitrifying organisms; the rates of proliferation of nitrifying populations in freshly perfused soil. ID50 values were computed for all parameters and herbicides studied. On average, the most sensitive parameters were the metabolism and growth of the two organisms in pure culture, while the least sensitive were the corresponding measures in the soil environment. Similarly, herbicides fell into four distinct groups. The most toxic were the formulated octanoates of bromoxynil and ioxynil (NPH1320 and Totril, respectively); next in order of toxicity were chlorbufam, phenmedipham, formulated oxadiazon, formulated legurame, ioxynil, formulated trifluralin, and bromoxynil; low toxicity was shown by terbacil, dicamba, and tricamba, whereas asulam and the related experimental herbicide MB9555 showed activity on some parameters at the very highest concentrations only. Comparisons of soil with pure-culture parameters showed that the relative toxicities of herbicides to Nitrosomonas in culture bore little relationship to those in soil. The inhibitions of Nitrobacter proliferations in soil on the other hand were correlated with the inhibitions of growth and metabolism in pure culture. Within these overall effects, individual herbicides showed marked differential actions on various parameters. Thus, the formulated octanoates of bromoxynil and ioxynil were extremely toxic to the growth of nitrifying organisms in culture, an action probably due to an unknown formulation component. Relative to other herbicides, bromoxynil and ioxynil were more effective on nitrification processes in the soil environment. Both in culture and in the soil, Nitrobacter is more sensitive than Nitrosomonas to these four herbicides. Legurame and oxadiazon are relatively more toxic to Nitrosomonas in culture, but this differential action is not demonstrable in the soil. Dicamba, tricamba, trifluralin, and chlorbufam are more toxic to Nitrobacter than to Nitrosomonas in the soil environment. Formulated trifluralin seems to exert a stimulating action on the growth of nitrifying organisms, but only in the soil; suppression of antagonistic organisms is suggested as a possible cause. Extrapolation of these results to the field situation suggests that the only herbicide which might cause small inhibitions of nitrification at field rates is terbacil, which is disproportionately toxic at low concentrations. At rates somewhat in excess of normal, the formulated octanoates of bromoxynil and ioxynil and possibly dicamba and oxadiazon could also cause small inhibitions.