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 o f bacterial cells in soil can enhance the growth of a bacterial inoculum. S tudies were performed using irradiated soil and two pairs of strains of Bra dyrhizobium japonicum and Pseudomonas fluorescens. For each species, an ant ibiotic-resistant mutant was used as inoculant and introduced into a steril e soil pre-colonised with its parent strain. The inocula and a glycerol ame ndment were introduced together or separately into soil using porous microg ranules or by spraying a suspension onto the bulk soil. Competition was ass essed 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 th e way the cells and the substrate were introduced into the soil. Joint intr oduction of cells and substrate into soil improved the growth of the inocul ant 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 microgr anules to the soil limits the competitive advantage of such location effect s. (C) 1999 Federation of European Microbiological Societies. Published by Elsevier Science B.V. Ali rights reserved.