Legume-rhizobia symbiosis: How do plant traits influence number of nodules in the longleaf pine

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Sarah L. Gora and Alyssa L. Young, Department of Biology, University of North Carolina Greensboro, Greensboro, NC, Sally E. Koerner, Department of Biology, University of North Carolina Greensboro, Greensboro, NC

Background/Question/Methods
Historically, the longleaf pine (Pinus palustris) ecosystem was dominant in the Southeastern United States. Today, this gorgeously biodiverse ecosystem is now endangered with a mere 3% of its original habitat remaining. Members of the legume family or Fabaceae, comprise up to 10% of the vascular plants in the longleaf pine ecosystem. The legume-rhizobia symbiosis is a unique, biological interaction between legumes and nitrogen-fixing bacteria. I hypothesized that plant traits may play a role in the number of nodules (plant organs that house rhizobia) and the amount of nitrogen fixed for the legume host. For this study, plant trait data was collected from two dominant legumes, Lespedeza hirta and Galactia regularis. Aboveground measurements of plant traits were taken in the field while belowground traits were collected in-situ. Bacterial nodules were carefully harvested from legume roots. Calculations of root area and nodule percent pink were recorded using Photoshop software. Regression analyses were used to identify the strength of the effect that plant traits have on nitrogen fixation rate. Understanding the legume-rhizobia symbiosis is of particular importance in fire-dependent ecosystems like the longleaf pine, where frequent burning can result in a decrease of significant amounts of organic nitrogen in the soil.
Results/Conclusions
There was no effect of species in predicting nodule number/fixation rates or biomass allocation. Leaf number and plant volume were found to predict ANPP with plant volume found to predict nodule number. There was a significant interaction with species and nodule number (p=0.04) on the biomass allocation ratio (ANPP:BNPP). Demonstrating impressive ecological value due to their diversity, abundance, and importance to wildlife, native legumes of the Longleaf pine may be of particular interest in conservation and restoration efforts of this recovering ecosystem.