Effects of herbicides on nodulation of soybean inoculated with Bradyrhizobium sp. strains

Abstract

Soybean crop systems have been traditionaly based on the application of herbicides, frequently above the recommended dosages. The use of herbicides may be detrimental to the process of biological nitrogen fixation in legumes, dueto direct effects on rhizobia or indirect effects on the host plant. Two experiments were performed in greenhouse, in pots with soil, intending to evaluate the effects of dosages of herbicides and their residual effects over strains of Bradyrhizobium and over soybean nodulation. In both experiments, soybean seeds were inoculated with the strains SEM IA 587 (Bradyrhizobium elkanii), SEM IA 5074 and SEMIA 5079 (Bradyrhizobium japonicum). In the first experiment, the herbicides alachlor, imazaquin, metolaclhor and metribuzin were applied in the recommended dosage (1x) and in a dosage ten times greater (I 0x), besides a standard treatment without herbicide application. The number and dry matter of nodules and the dry matter and nitrogen content of soybean shoots were determined 30 and 55 days after the seeds germination. In the second experiment, the herbicides alachlor, imazaquin and metolaclhor were applied, in the recommended dosage, in the soil that had already received that dosage, in the previous experiment. In the soil where no herbicide had been applied, it was made an application of twice the recommended dosage. The soil that had received the dosage 10x received no further treatrnent, in order to evaluate the residual effect of the herbicides in soybean nodulation, 80 days after their application. As control treatments, soil was seeded with soybean inoculated with the Bradyrhizobium strains, with no herbicide. The experiment was harvested 40 days after soybean seeding and the number and mass of nodules and the dry matter of shoots were analyzed. The application of the recommended dosages of the herbicides had no effect, while the application of the dosage 10x was prejudicial to all variables analyzed. The Bradyrhizobium strain SEMIA 587 was more efficient and more tolerant to the applied herbicides. SEMIA 5079 was the most sensitive strain. The herbicide metribuzin was the most toxic, fallowed by metolachlor. The reapplication of the recommended dosage had no effect, showing that the herbicides were expressivelly dissipated 80 days after their application.