COS 265-4 - Rhizobia communities as drivers of nitrogen fixation in longleaf pine savanna legumes

Abstract: Symbiotic nitrogen fixation (SNF), where N-fixing microorganisms live in symbiosis with host plants, is common in legumes. Understanding the role of SNF, and in particular, rhizobia N-fixation, in nutrient cycling is critical, particularly in fire prone ecosystems, such as longleaf pine (LLP) savannas, that can potentially benefit from the additional N inputs courtesy of these symbioses. If we understand the drivers of SNF in LLP savannas, we will be able to manipulate these drivers and potentially enhance effectiveness of legume-rhizobia symbioses in restoration. Here, I investigate whether legume plant traits (aboveground and belowground biomass, specific leaf area), abiotic factors (light availability, soil moisture, soil temperature, and soil nutrient levels), surrounding plant community characteristics (richness, dominant species cover, and legume cover), and mutualism characteristics (identity of rhizobia within nodules, legume root nodule characteristics like number, percent pink, size, and weight, and mycorrhizae colonization) influence rates of SNF in LLP savannas. Ten legume individuals of four species (Chamaecrista nictitans, Galactia regularis, Lespedeza hirta, and Robinia nana) were collected from three field sites (n=120) varying in understory plant community composition. Plant traits and mutualism characteristics were collected from each individual and abiotic and surrounding community data relating to the location of each legume were collected. Generalized linear mixed model ANOVAs were used to determine how SNF rates are influenced by all these factors for each legume species.

Preliminary data suggests that most of the factors that were measured in this study were not influencing rates of N-fixation. None of the abiotic factors nor plant community characteristics influenced rates of N-fixation. All legumes that were stained for mycorrhizae showed colonization ranging from 6-35%, but this did not significantly influence N-fixation rates. Across all legume species studied, aboveground biomass was associated with greater N-fixation while belowground biomass was not significantly associated with fixation. Larger nodules were associated with significantly lower rates of N-fixation, while a greater nodule weight per plant was associated with significantly higher rates of fixation. Although most of the measured variables were not found to be drivers of N-fixation rates, the identity of the rhizobia within the nodules was a significant driver. Bradyrhizobium was the dominant rhizobial genus present in nodules of C. nictitans, G. regularis, and L. hirta, and was associated with greater rates of N-fixation, while Mesorhizobium fixed less N and was the dominant rhizobial genus in R. nana.