COS 265-2 - Synergistic multi-mutualistic effects occur in some prairie legume-rhizobia-AM fungi mutualisms, but are not stronger in later-successional plants

Abstract: The benefits of mutualisms can be unpredictable, and this contingency is compounded when considering multi-mutualist effects. For some pairs of mutualisms, such as those providing complementary benefits, multi-mutualist effects could even be synergistic (more than the sum of their parts)—but understanding when and where synergistic multi-mutualist effects occur, and how to manage them, is a major challenge. We tested the hypothesis that successional status should affect the frequency and strength of synergistic multi-mutualist effects, with late-successional species more likely than early-successional species to experience super-additive benefits of multiple mutualisms. Prairie legumes associate with two complementary guilds of mutualists (arbuscular mycorrhizal (AM) fungi and rhizobia) and could be establishment-limited in restored prairies because of a lack of suitable mutualists. We tested the effects of legume life-history on multi-mutualistic benefits to plants and rhizobia by inoculating 12 native prairie legumes with a mixture of locally-collected prairie rhizobia or AM fungal strains, both, or neither, and growing them for 10 weeks in a growth chamber. We used log response ratios of plant biomass to indicate plant benefit from single or multiple mutualists, nodule number to indicate rhizobia benefit, and coefficient of conservatism to estimate legume successional status.

We found that although mid- and late-successional legumes were more likely to respond synergistically to dual inoculation with AM fungi and rhizobia (Dalea purpurea, Lespedeza virginica, and Tephrosia virginiana; 3 of 9 species) than early-successional legumes (0 of 3), the magnitude of synergistic effects did not increase with legume successional status. Rhizobia did not experience synergistic multi-mutualist effects. One early-successional legume (Desmodium canadense) exhibited significantly sub-additive multi-mutualistic effects, where dual inoculation resulted in less plant biomass than expected from the sum of individual mutualist effects. Only one species (Desmodium canadense, an early-successional legume) responded significantly positively to AM fungi alone while two late-successional species responded significantly negatively (Lespedeza virginica and Tephrosia virginiana). Legume responses to rhizobia alone were generally positive (significant in 6 of 9 mid- to late-successional legumes and one early-successional legume). The benefit of the rhizobial mutualism (for both plants and rhizobia) tended to be stronger for late successional legumes. Our findings suggest a high degree of context-dependency in multi-mutualist effects that is only partially explained by legume life history.