COS 57-2 - The influence of legume plant traits and abiotic factors on rates of symbiotic nitrogen fixation in longleaf pine savannas

Background/Question/Methods

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 nodule formation and N-fixation, 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 (plant volume, aboveground and belowground biomass, specific leaf area, and N-fixation rates) and abiotic factors (light availability, soil moisture, soil temperature, and soil nutrient levels) influence rates of legume nodulation (i.e., number of nodules) in LLP savannas. Ten legume individuals of four species were collected from three field sites (n=120) varying in fire history. Nodule and plant trait data, and abiotic data relating to the location of each legume was collected. Generalized linear mixed model ANOVAs were used to determine how nodule number is influenced by legume individual, legume species, fire frequency, soil moisture and nutrient availability, plant volume, specific leaf area, and above- and belowground biomass.

Results/Conclusions

Preliminary data suggests that soil moisture does not influence nodule number for any species, and neither do our other abiotic factors (total soil nitrogen, soil temperature and light availability). Plant volume and aboveground biomass influence nodule number for two of our four legume species (Chamaecrista nictitans and Robinia nana), leaf percent nitrogen influences nodule number for one species (Galactia regularis), as does leaf percent carbon for another species (Chamaecrista nictitans). Although some variables had a significant effect on nodule number, the correlations are relatively weak, indicating that a lot of variation is still left to be explained. Therefore, the effects of specific leaf area and N-fixation rates occurring within nodules will be explored to look for effects on nodule number. Additionally, the effect of all abiotic and biotic factors on other important nodule characteristics such as nodule weight and size will also be explored.