The direct and interactive effects of elevated CO2 and additional nitrate on relative costs and benefits of legume-rhizobia symbiosis

Rising concentrations of carbon dioxide (CO 2 ) is likely to have important effects on growth and development of plants and on their relationship with symbiotic microbes. A rise in CO 2 could increase demand by plant hosts for nutrient resources, which may increase host investments in beneficial symbionts. In the legume-rhizobia mutualism, while elevated CO 2 is often associated with increased nodule growth and investment in N 2 -fixing rhizobia, it is yet unclear if this response depends on the mutualistic quality of the rhizobia. To test if host carbon allocation towards more-beneficial nodules are similar to less-beneficial (but still effective) nodules when plant N demand changes, we manipulated plant C and N status with elevated CO 2 and additional nitrate. We used two isogenic Rhizobium etli strains that differ in their ability to synthesize an energy reserve compound, poly-beta-hydroxybutyrate (PHB), as well as their efficiencies for nitrogen fixation and nodulation rates, resulting in two Phaseolus vulgaris host groups with either large number of small nodules or small number of large nodules. The addition of nitrate negatively affected carbon allocation towards nodules, and elevated CO 2 reversed this effect, as expected. However, this alleviation of nodule inhibition was greater on plants that started with greater numbers of smaller nodules. If smaller nodules indicate less-efficient or low-fixing rhizobia, this study suggests that increased demand for nitrogen in the face of elevated CO 2 has the potential to disproportionately favor less-beneficial strains and increase variation of nitrogen fixation quality among rhizobia.