Legumes are plants in the bean and pea family (Fabaceae) that associate with mutualistic rhizobial bacteria. This mutualism can be highly beneficial for legumes, allowing them to differentiate their nitrogen niche from non-legumes. This symbiotic association may have implications for plant species coexistence and ecosystem function, including legume invasions around the world, soybean production in a changing climate, and the critically endangered longleaf pine ecosystem.
CoexistenceThe partnership between legume plants and rhizobial bacteria to symbiotically fix atmospheric N comprises the largest natural source of N into the terrestrial biosphere, making up approximately a quarter of total N inputs. Thus, improving our understanding of symbiotic N fixation (SNF) presents a major opportunity to advance our understanding of terrestrial N cycling.
Legume species exist along a spectrum from specialists to generalists in their preferences for rhizobial partners. Theory and limited data indicate that these partnership preferences can have dramatic impacts on the coexistence of legume species and individual SNF rates, but these patterns remain poorly characterized in nature and have never been linked to ecosystem-scale SNF. Coexistence and plant-soil feedback theory suggest that mutualist specificity may lead to negative interactions among different specialist species growing in mixture, reducing coexistence of specialist legumes. Generalist legumes should exhibit the opposite pattern, growing well in multi-species mixtures and therefore having higher abundances across the landscape. In contrast, mutualism theory suggests that specialized legumes can demand more effective partnerships from their rhizobia to maximize individual legume SNF rates, while generalist legumes have lower SNF rates. It is unknown how these opposing processes balance each other to drive ecosystem scale N inputs via SNF in nature. In a greenhouse experiment, we have demonstrated that generalist and specialist legume species differ in the benefit they derive from the diversity of their rhizobial partners and the ultimate consequences for their SNF rates (Taylor and Komatsu 2024). We are working to unpack the diversity within the legume-rhizobia symbiosis to understand the consequences of these symbiotic interactions for legume species coexistence, community dynamics, and contributions to SNF. |
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Legume InvasionsMutualisms with rhizobial bacteria can be highly beneficial for legumes, allowing them to differentiate their nitrogen niche from non-legumes. This symbiotic association may be a key factor underlying legume invasions around the world. Exotic legumes may only be successful in a novel range if they are able to (a) form novel associations in their new range or (b) bring their familiar rhizobial associates with them from their native range. Through observational field collections of legume root nodules, we have found that invasive legumes often utilize very distinct communities of rhizobia from co-occurring native legumes (La Pierre et al 2017).
As rhizobia are not maternally transmitted through seeds, our results are surprising and inspire new questions about rhizobial dispersal. For example, how are rhizobia dispersing? Do invading legumes affect soil rhizobial communities in their exotic range? And if so, do commonly used restoration practices effectively restore the soil rhizobial community to its native state (Komatsu and Simms 2020)? The principles we learn from the model legume-rhizobia system can inform future work investigating the role of mutualisms, and symbioses more broadly, in species invasions. |
Soybean AgroecologyLegumes are not only found within natural ecosystems, but are also a critical component of agroecosystems. Many economically important crops, such as soybean, are legumes! Soybeans are a leading component of global sustainable agriculture and food security because of their high protein content and relatively low fertilizer demands, due to their nitrogen-fixing rhizobial mutualists. In the United States, soybeans are the largest crop by acreage and the second largest cash crop at $41 billion annually. However, soybean farming is environmentally costly due to intensive pesticide use, and economically risky due to nearly complete dependence on rainfall.
Although soybeans can simultaneously associate with multiple rhizobial strains, and different strains are differentially tolerant of drought and other stressors, nearly all research and agricultural applications emphasize single-strain inoculations. We are borrowing from Biodiversity-Ecosystem Function theory to predict that rhizobial strain diversity may buffer soybeans against crashes by providing complementary benefits during drought and herbivore stress, a phenomenon termed the “ecological insurance effect”. We are testing this hypothesis with experimental greenhouse assays, distributed surveys, and field experiments in the extensive Maryland Soybean Varietal Trial Farm Network. Specifically, by altering the structure of soybean rhizobial communities, we posit improved crop performance and stability and lower costs for farmers through reduced reliance on pesticides, irrigation, and fertilizer (Komatsu et al 2023). The end goal of this research is to inform the development of innovative and sustainable solutions to address the threat posed by an increasingly variable climate, which is projected to limit the productivity of the soybean production system in coming decades. |
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Longleaf Pine Ecosystem
The longleaf pine (LLP) savanna ecosystem has great potential to serve as a multifunctional landscape, contributing to both economic and environmental sustainability in the southeastern US. Yet less than 4% of this once expansive ecosystem remains; thus, methods for restoration and management of LLP savanna are critical. Extant LLP savannas are nutrient poor, limiting productivity of both the LLP timber trees as well as the understory, hindering successful restoration of the plant-fire feedback loop. We aim to begin to understand the role of two key microbial symbioses in promoting nutrient availability and ultimately improving functioning of the LLP ecosystem: (1) rhizobial bacteria critical for nitrogen fixation in legumes and (2) mycorrhizal fungi critical for phosphorous uptake in many plant lineages. Through observational data collection, establishment of an experimental platform manipulating the frequency of nutrient-based mutualisms, and formation of a network of sites for future LLP research, we aim to increase understanding of nutrient-based mutualisms in LLP ecosystems to develop innovative and environmentally-sound management strategies for this system.