Ecology of symbiotic nitrogen fixation in a fire-dependent ecosystem

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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 and Kimberly Komatsu, Smithsonian Environmental Research Center, Edgewater, MD

Background/Question/Methods
The largest pool of nitrogen (N), atmospheric N, is unusable by plants, with N frequently limiting plant growth. In order to utilize atmospheric N, legumes evolved a symbiosis with N-fixing bacteria, typically rhizobia, to aid in, and even induce, N-fixation. In return, the microorganisms receive shelter and nutrition from the host plant. These mutualisms are particularly important in fire-prone ecosystems such as the legume-rich longleaf pine (LLP) savanna ecosystem where they can facilitate the return of N after losses from fire. To explore the ecology of legume-rhizobia symbioses in LLP savannas, I asked three questions. (1) How abundant are legumes in LLP savannas, and what controls this abundance? (2) What are the rates of nodulation in common legume species? and (3) do microbial species differ with legume host species? To address this, two field studies were conducted in North Carolina (NC) LLP savannas. To address Q1, legume presence and abundance were recorded within twenty-five 1m2 plots at each of 15 study sites across NC. To address Q2 and Q3, plant traits and nodule characteristics were measured at a single site on 82 legume individuals belonging to 11 different species and microbial DNA from whole nodules was extracted and sequenced.
Results/Conclusions
Legume cover ranged from 0-27% across the 13 of 15 sites that contained legumes, with legume communities grouping into three distinct community types. Additionally, within a single site, we found a wide range in nodule number (0-25) and % pink (10.5-27.5%), an estimate of N-fixation, on individual plants; however, the range in nodules and % pink did not vary significantly by species even though species growth form and resource allocation differed. All legume species had root nodules (although some individuals within a species did not) suggesting all eleven species can form the symbiosis. Interestingly, none of the plant traits (e.g., plant volume, leaf number, ANPP, BNPP, root area) were correlated with either nodule number or average % pink. For example, larger plants (higher ANPP) did not have more nodules. Finally, I found that different legume species associate with different rhizobia species, indicating that there is specificity in this mutualism. Overall, this study shows that N-fixation in the LLP savanna ecosystem can vary due to legume abundance and identity, the formation and frequency of the symbiosis, and the rhizobia species present – all aspects that require further study to understand the impacts that legume-rhizobia symbioses have on the global nitrogen cycle.