Effects of Herbicides on the Survival ofRhizobium japonicumStrains

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Abstract
The survival of two Rhizobium japonicum strains was determined in soils treated with 13 herbicides used in soybeans [Glycine max (L.) Merr.]. Survival in soils treated with concentrations equivalent to or 10-fold greater than those from field applications at recommended rates was compared to survival in untreated soil under controlled temperature and moisture conditions. Nine of the herbicides had transitory or no effects, regardless of rate. Acifluorfen {5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoic acid}, bentazon [3-(1-methylethyl)-(1H)-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide], fluchloralin [N-(2-chloroethyl)-2,6-dinitro-N-propyl-4-(trifluoromethyl)benzenamine], and dinoseb [2-(1-methylpropyl)-4,6-dinitrophenol] consistently reduced survival of one or both strains at rates 10-fold greater than recommended. At rates of application equivalent to suggested field use rates, effects on survival were reduced or eliminated. Reductions in survival attributable to herbicides were in some cases comparable to those obtained by a weekly wetting and drying of the soil, indicating that herbicide effects on R. japonicum may be equivalent in magnitude to effects due to environmental stress. The herbicides were not sufficiently toxic to reduce R. japonicum populations in soil to levels that would be likely to affect nodulation.