Effect of carbon source supply and its location on competition between inoculated and established bacterial strains in sterile soil microcosm

The aim of this work was to study how the location of a carbon source and of bacterial cells in soil can enhance the growth of a bacterial inoculum. Studies were performed using irradiated soil and two pairs of strains of Bradyrhizobium japonicum and Pseudomonas fluorescens. For each species, an antibiotic-resistant mutant was used as inoculant and introduced into a sterile soil pre-colonised with its parent strain. The inocula and a glycerol amendment were introduced together or separately into soil using porous microgranules or by spraying a suspension onto the bulk soil. Competition was assessed by plate counts of both inoculant and established populations. Both B. japonicum and P. fluorescens parent strains established at a high density level (10(9) cells g(-1) of dry soil) in the sterile soil. The effects of the amendment on the inoculant strain in soil were strongly dependent on the way the cells and the substrate were introduced into the soil. Joint introduction of cells and substrate into soil improved the growth of the inoculant strain, but the effect of the amendment was also shown to be dependent on the inoculant carrier. Location of glycerol and the inoculant strain in microgranules gave a competitive advantage to the inoculated bacterium for a part of the amendment. We suggest that diffusion of glycerol from microgranules to the soil limits the competitive advantage of such location effects.