Symbiotic rhizobial fixation provides an important source of nitrogen for legumes and soil partially replacing mineral nitrogen fertilizer inputs and adding more organic nitrogen into the cropping and pastures systems which stimulate beneficial soil microbial cycles such as decomposition. There is a capacity to further exploit rhizobial associations in Australian soils but this requires more knowledge of limitations for plant-rhizobial symbiosis due to various biotic and abiotic factors such as high temperature, low moisture content, soil acidity and having ineffective native rhizobial populations in soil. For instance, a single legume host can accommodate multiple rhizobial strains but the consequences of interactions among these strains for symbiotic outcomes are poorly understood. We tested the effects of single and multi-strain inoculations of Rhizobium leguminosarum on nodulation and on nitrogen fixation in field pea. Multi-strain inoculants differed in the degree of genetic similarity between paired strains and we hypothesized that genetically similar pairs would exhibit stronger competition and negatively affect nodulation and N fixation compared with genetically diverse pairs.
We observed that co-inoculation with rhizobial pairs with low genetic similarity resulted in significantly higher nodule numbers per plant compared to the responses of each isolate inoculated on its own. In contrast, highly genetically similar rhizobial pairs showed less nodulation in co-inoculated treatments compared to single inoculations. Furthermore, we observed a significant increase (~20%) in fixed nitrogen for co-inoculated treatments compared to the treatments inoculated with single rhizobial strains, however the degree of genetic similarity was not observed to affect the strength of this increase. Thus, interactions among strains during nodulation appear to result in synergistic effects on nitrogen fixation regardless of whether the interactions are competitive. Our work suggests that careful selection of multi-strain inocula may improve the outcomes for managing fertility of field pea and other legumes. Future work will address the consequences of these interactions for rhizobial fitness, particularly when coping with environmental stress.