A field investigation was conducted to determine the influence of pendimethalin [N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine] and trifluralin [2,6-dinitro-N,N-dipropyl-4-(trifluoromethyl)benzenamine] herbicides on soybean [Glycine max (L.) Merr. ‘Centennial’] — Bradyrhizobium japonicum symbiosis. Pendimethalin and trifluralin applied at rates of 1.1, 1.7, and 2.2 kg ai/ha delayed emergence and injured soybean seedlings grown on an Olivier silt loam. Nodule number, dry weight, and nitrogen (N2) fixation measured by acetylene (C2H2) reduction were decreased by all herbicide rates during the vegetative growth stages in 1984, with occasional decreases in nodulation noted during the late reproductive growth stage. Seedling injury was less severe in 1985 than in 1984, with most injury occurring at 1.7 and 2.2 kg/ha for both herbicides. Nodule number and dry weight in 1985 were decreased from vegetative through the early reproductive growth stages with little influence on N2(C2H2) fixation. Inhibitory effects of the herbicides on nodulation and N2(C2H2) fixation did not influence seed formation since seed yield was not affected either year.

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.