Field investigation was conducted in 1986 and 1987 at Ribeirão Preto, SP, Brazil, to determine the effect of linuron (1.00 and 2.00 kg/ha) and oryzalin (1.875 and 3.75 kg/ha) on plant growth, nodulation, nitrogen fixation and yield of soybean [Glycine max (L.) Merril.] cv IAC - 11. The soil type used was a Red Latosol. Beside herbicide treatments there were two controls: one not inoculated and another inoculated with Bradyrhizobium japonicum strain SMS-463 (=29W). All herbicide treatments were inoculated. The experiment was arranged in a split plot randomized block design with four replicates. Samples were taken at 28, 42, 56, 70, 84 and 98 days after sowing (DAS) and herbicide application in the first year and at 28, 42, 56, 70, 84 and 105 DAS in the second year. In both years, root, shoot and nodule dry weight were evaluated, as well as grain productivity. In the second year, nitrogenase activity was also evaluated. Generally, there wasn't a positive response of seed inoculation on plant growth and nodulation; but nitrogenase activity was improved by seed inocutalion. In the first year of application, the use of herbicides enhanced plant growth but nitrogen fixation decreased, mainly with the highest rates of both herbicides. Nitrogenase activity was lower with both herbicides, and was more affected by oryzalin. No effect of treatments were observed on soybean grain yield.
Herbicides; nodulation; nitrogenase activity; acetylene reduction