PS 34-226 - Some Prairie Legumes Grow Better with Mycorrhizal Fungi and Rhizobia in Restored Prairie Field Sites

Abstract: Many native prairie legumes form symbiotic relationships with arbuscular mycorrhizal fungi (AMF) and/or nitrogen-fixing rhizobia bacteria to better extract nutrients from their environment. Though the benefit of the individual relationships between plant and AMF or plant and rhizobia have been studied, the effect and consistency of the tripartite multi-mutualism is not well understood. Resource availability affects the magnitude of the interaction with individual mutualists since the mutualisms can be less beneficial when plants can directly access enough of the traded resource (nitrogen for rhizobia, phosphorus for AMF). We expected changes in resource availability might also affect the multi-mutualism, as AMF and rhizobia might compete for plant carbon if one or more soil nutrients are abundant.

To test these hypotheses, we germinated two native prairie legumes (Amorpha canescens and Lespedeza capitata) in a greenhouse and inoculated the seedlings with local strains of AMF and/or rhizobia. After several weeks of growth, we transplanted the seedlings into 20 restored prairies in Southwest Michigan. We tracked seedling survival, health, and height throughout the growing season and measured soil nutrient availability and aboveground biomass production at each site.

We found that across all sites, both species grew taller and were more likely to survive when inoculated with AMF. Dual-inoculation with both AMF and rhizobia further increased A. canescens survival. We also saw consistently synergistic multi-mutualistic effects on A. canescens height. Mutualisms with L. capitata were more sensitive to environmental conditions. As soil phosphorus increased, L. capitata plants inoculated with rhizobia were less likely to survive – the opposite of those without rhizobia.

And as soil phosphorus increased, the effect of AMF with L. capitata became more positive, but synergistic multi-mutualist effects eroded. As site aboveground biomass increased, L. capitata benefits from rhizobia declined. Our results show that these single- and multi-mutualisms can vary depending on which mutualists are present, and some mutualisms can vary with resource availability or biomass of the surrounding plant community. Averaged across soil conditions, however, inoculation with one or both mutualists can increase prairie legume performance.