Persistence of Bradyrhizobium japonicum in arable soils of Argentina

Argentinian soils do not contain indigenous Bradyrhizobium japonicum., therefore when soybeans were first cultivated in Argentina, they were inoculated with imported rhizobia. Since the inoculants were not adapted to the climatic conditions, their beneficial effects decreased rapidly. With the objective of evaluating the possibilities of improving inoculation, the nodulation capacity of a presumptive inoculant and its persistence under different clopping regimes with or without the host were studied in field experiments in the Argentine soybean production area. Crop rotations included soybean inoculated with Bradyrhizobium japonicum strain E110 (presumptive inoculant), in the first year followed by maize, or a soybean-soybean rotation using soybean with or without inoculation with E110. Initially, the soil lacked Bradyrhizobium japonicum. A naturalized strain isolated originally from the soybean area, E112, was therefore introduced into the soil to represent the indigenous population to facilitate inter-strain competition studies. Movement of rhizobia in soils was restricted by polyethene barriers placed to a depth of 10 cm, Bacterial strains from nodules at flowering time of soybean plants were identified by ELISA. Strain E110 required at least three seasons as an inoculant before it was able to dominate nodule occupancy. However, it proved capable for persisting in soil for up to 5 years in the absence of the host and could therefore be recommended for use as an inoculant from the standpoint of its competitive abilities and persistence. (C) 1998 Elsevier Science B.V.