Pesticide Effect on Acetylene Reduction and Modulation by Soybean and Lima Bean

Abstract

The influence of 12 pesticides on acetylene reduction (N2 fixation) and modulation of soybean (Glycine max L. Merrill cv. Williams 82) and lima bean (Phaseolus lunatus L. cv. Geneva) was evaluated. All pesticides except diazinon were found to be harmless to nitrogen fixation at 3× the manufacturer's recommended rate, Diazinon significantly decreased C2H2 reduction of soybean 2 days after application, but not after 7 days or at normal label rates, Acetylene reduction of excised nodules imbibed with diazinon indicated that the chemical may have affected nitrogenase function directly. Soybean nodule counts were significantly decreased by application of 3× rates of methomyl and trifluralin, whereas lima bean nodule counts were decreased only by trifluralin. Tritluralin also depressed soybean modulation at label rates, but had no effect on lima bean modulation. Methomyl was innocuous to soybean modulation at the recommended label rate. Both chemicals were nontoxic to Bradyrhizobium/Rhizobium sp. based on a disc inhibition study. Chemical names used: O,O -diethyl O -(2isopropyl-4-methyl-6-pyrimidinyl phosphorothiote (diazinon); S-Methul- N -((methylcarbamoyl)oxy)-thioacetimidate (methomyl); a,a,a -Trifluoro-2-6dinitro-N-N -dipropyl-p-toluidine (triflnralin).