Plants often harbor a wide variety of microbial organisms within their bodies. Interactions between plants and these microbial symbionts influence the diversity, structure, and function of terrestrial plant communities. In many such interactions the microbe provides host plants with nutrients in exchange for carbon from the plant. Arbuscular mycorrhizal fungi, a soil fungus that specializes in the acquisition of phosphorus, and rhizobia, a nitrogen-fixing soil bacterium are two common nutritional plant symbionts of legume plants. The manner in which each of these symbionts benefits or antagonizes their plant partner varies and is partially determined by soil nutrient levels. Moreover, plants can allocate resources to multiple partners, allowing for the presence of simultaneous interactions to influence the costs and benefits of the interaction. This project evaluates the relative importance of biotic (i.e. co-occurring symbionts) and abiotic (i.e. soil nutrient levels) factors on the consequences of plant-microbial interactions.
The majority of investigations on plant-microbial interactions have focused a single host and a single symbiont. Yet in nature plants simultaneously interact with many symbionts. This project experimentally examines the importance of this feature of natural plant-microbe interactions. Results of this project are applicable to many goals of ecological restoration. Because of their ability to fix nitrogen, legumes are important members of plant communities, and are often priority species in restoration management. Many restoration projects begin in post-agricultural fields. While the soils of natural areas are often nutrient limited, disturbed environments are often subjected to high levels of anthropological nutrient deposition. Thus this project aids the success of restoration efforts by shedding light onto the consequences of both unnatural nutrient levels and the soil community the legume-rhizobia interactions. We will incorporate work from this project into a special program at a local children's science museum that elucidates below-ground biological process and organisms.
Plants often harbor a wide variety of microbial organisms within their bodies. Interactions between plants and these microbial symbionts influence the diversity, structure, and function of terrestrial plant communities. In many such interactions the microbe provides host plants with nutrients in exchange for carbon from the plant. Arbuscular mycorrhizal fungi (AMF), a soil fungus that specializes in the acquisition of phosphorus, and rhizobia, a nitrogen-fixing soil bacterium are two common nutritional plant symbionts of legume plants. The manner in which each of these symbionts benefits or antagonizes their plant partner varies and is partially determined by soil nutrient levels. Moreover, plants can allocate resources to multiple partners, allowing for the presence of simultaneous interactions to influence the costs and benefits of the interaction. This project evaluates the relative importance of biotic (i.e. co-occurring symbionts) and abiotic (i.e. soil nutrient levels) factors on the consequences of plant-microbial interactions. We inoculated the legume Amorpha canescens (leadplant) with one of two AMF inocula or planted them free of AMF. We also inoculated plants with one of two rhizobia strains, or a non-inoculated control. We grew all possible combinations of these AMF and/or rhizobia treatments. In addition to these biotic treatments, we applied one of four types of fertilization: nitrogen (N) addition, phosphorus (P) addition, N and P addition, and unfertilized. After four months, we harvested plants for above and belowground biomass. Additionally, we measured the N and carbon content of plant material, and the N, P, and carbon content of the soil. We also evaluated fitness measures of the microbial symbionts. We measured the percentage of plant roots colonized by AMF, AMF spore production, and the number and weight of rhizobia nodules on plant roots. AMF and rhizobia independently promoted biomass production of A. canescens. We found no difference between the AMF inocula in their ability to promote plant growth, but did find that one rhizobia strain increased plant biomass more than the other. Overall, plants had greater biomass with nitrogen addition, but without phosphorus addition. We found a strong synergistic interactive effect of AMF and rhizobia on plant biomass. When AMF and rhizobia are inoculated in combination plant biomass was much greater than expected given the independent effect of AMF and rhizobia independently. This synergism was consistent between the different AMF inocula and rhizobia strains. We found that the effect of specific AMF inocula and rhizobia strain combinations depended upon the fertilization treatments. One combination of AMF inocula and rhizobia strain is more beneficial than the other combination with phosphorus fertilization, but not without phosphorus fertilization. This result demonstrates that both the biotic and abiotic environments are important in shaping the outcome of interspecific interactions. The presence of AMF benefited rhizobia. The number and weight of rhizobia nodules on plant roots increases in the presence of AMF. Additionally, we found that fertilization with nitrogen decreased nodule number in non-AMF plants. However, plants that were coinfected with both AMF and rhizobia did not have decreased number of nodules with the addition of nitrogen. Our work has strong implications for restoration. A. canescens and other long-lived legume species are important members of the tallgrass prairie community. Unfortunately, these species are often lost from prairie remnant areas and difficult to establish in restoration attempts. A. canescens depends upon microbial symbionts for growth and nutrition. The addition of nitrogen did not influence the nitrogen content of plants. However, plants with either AMF or rhizobia had higher nitrogen contents than non-inoculated plants. Therefore, this plant may be competitively disadvantaged compared to plants that may be able to utilize available nutrients independently. This study strongly suggests that a diverse and healthy soil community is essential for the success of A. canescens, regardless of the abiotic soil nutrient levels in an environment. We have incorporated this research into an ongoing prairie restoration project conducted by Environmental Biology students on the campus of Indiana University. As part of this project, students inoculate legume plants with AMF, rhizobia, both, or neither symbiont. Students then take part in planting this experiment in the field, and the following semester, students measure and analyze data from plants established the spring beforehand. In this way, we test the results of our greenhouse experiment in a restoration context, as well involve undergraduate students in a scientific experiment. We have also worked with the City of Bloomington, Indiana Parks and Recreation Department to create an exhibit relating to this project. We presented this exhibit at the community farmers market.
