Our current understanding of plant-microbial interactions is limited to pairwise investigations of the dynamics between a host and symbiont.   While such studies are necessary and interesting, they lack biological reality by assuming that plants engage in only a single interaction at a time. In nature, plants often interact with multiple symbionts concurrently, and must evaluate allocation to multiple partners.  We conducted a meta-analysis of published studies that manipulated two microbial symbionts on a common host plant in a full factorial design.  For each study, we recorded the plant species, experimental design, and types of symbionts.  We also recorded the mean values of control treatments, independent effects of each symbiont, and the interactive effects of both symbionts on plants.  The response ratios of each symbiont individually and the interaction ratio were calculated.  We used one-sample t-tests to determine if the interaction ratios of various combinations of the symbionts were significantly different from their expected additive effects.

Results/Conclusions

Although often considered as complementary nutritional symbionts, the preliminary results of this study indicate antagonistic interactive effects between arbuscular mycorrhizal fungi (AMF) and microbial nitrogen-fixers (including Rhizobia spp., Bradyrhizobium spp., Frankia spp., and others).  While the independent effects of AMF and nitrogen-fixers on plant performance were positive overall, plant performance in presence of both AMF and nitrogen-fixers was significantly less than expected given their individual effects (t = -3.32; p=0.003).  We found no significant non-additive effects when examining overall interactive effects of simultaneous mutualists, or of simultaneous mutualists and pathogens.  These results indicate that the presence of additional microbial plant symbionts, especially nutritional symbionts, can influence the nature of an interaction by altering the cost: benefit relationship between the host and symbionts through competition for, or enhancement of, a common resource.