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 adap ted to the climatic conditions, their beneficial effects decreased rap idly. With the objective of evaluating the possibilities of improving inoculation, the nodulation capacity of a presumptive inoculant and it s persistence under different clopping regimes with or without the hos t were studied in field experiments in the Argentine soybean productio n area. Crop rotations included soybean inoculated with Bradyrhizobium japonicum strain E110 (presumptive inoculant), in the first year foll owed by maize, or a soybean-soybean rotation using soybean with or wit hout inoculation with E110. Initially, the soil lacked Bradyrhizobium japonicum. A naturalized strain isolated originally from the soybean a rea, E112, was therefore introduced into the soil to represent the ind igenous population to facilitate inter-strain competition studies. Mov ement of rhizobia in soils was restricted by polyethene barriers place d to a depth of 10 cm, Bacterial strains from nodules at flowering tim e of soybean plants were identified by ELISA. Strain E110 required at least three seasons as an inoculant before it was able to dominate nod ule occupancy. However, it proved capable for persisting in soil for u p to 5 years in the absence of the host and could therefore be recomme nded for use as an inoculant from the standpoint of its competitive ab ilities and persistence. (C) 1998 Elsevier Science B.V.