Competition and co-association, but not phosphorous availability, shape the benefits of phosphate solubilising root bacteria for maize (Zea mays).

Predicting the conditions under which rhizobacteria benefit plant growth remains challenging. Here we tested the hypothesis that benefits from inoculation with phosphate-solubilising rhizobacteria will depend upon two environmental conditions: phosphate availability and competition between bacteria. Maize-associated rhizobacteria with varying phosphate solubilisation ability were used in experiments in soil, sterilised soil and gnotobiotic microcosms under conditions of varying orthophosphate availability, while  we manipulated intensity of competition by varying the number of isolates in plant inocula. Growth promotion by microbes did not depend on phosphate availability but was affected by interactions between inoculants: beneficial effects of one Serratia isolate were detectable only when plants were inoculated with a single strain and beneficial effects of a competition sensitive Rhizobium was only detectable in sterilised soil or in microcosms inoculated with single strains. Moreover, microcosm experiments suggested that facilitation of a parasitic isolate, not competitive interactions between bacteria, prevented plants from gaining benefits from a potential mutualist. Competition and facilitation affected colonization of plants in microcosms but growth promotion by Serratia was more affected by inoculation treatment than culturable densities on roots. Experimental manipulation of seed inocula can reveal whether plant growth stimulation is robust with respect to competition, as well as the ecological strategies of different rhizobacteria. From an applied perspective, phosphate solubilisation may not provide the mechanism for bacterial growth promotion but may indicate mutualistic potential due to phylogenetic associations. Importantly, benefits to plants are vulnerable to interactions between rhizobacteria and may not persist in mixed inoculations.