Mutualisms between plants, such as legumes, and nitrogen-fixing bacteria are highly specialized interactions between roots and microbes. To engage in such a mutualism, legumes basally translocate photoassimilates to root nodules, fueling biological nitrogen fixation in beneficial microbes. Alfalfa (Medicago sativa) and rhizobia (Sinorhizobium meliloti) are one such study system for these interactions. Aboveground herbivory of alfalfa, such phloem feeding of potato leafhoppers (Empoasca fabae), can alter rates of photosynthesis and disrupt basal translocation. The effect of aboveground leafhopper herbivory on nitrogen fixation in alfalfa, however, is unknown. Additionally, when faced with leafhopper herbivory, alfalfa roots may divert nitrogen uptake from the atmosphere (via nitrogen fixation of microbes) to available soil nitrogen. Here, our objectives were 1) to determine the reduction in nitrogen content of alfalfa due to leafhopper herbivory, and 2) to quantify the source of nitrogen assimilation (soil vs. atmosphere) using nitrogen isotope ratios when alfalfa is fed on by leafhoppers. To do this, we are conducting greenhouse experiments to test the effect of leafhopper herbivory on nitrogen fixation, as well as applying additional sources of heavy nitrogen to determine assimilation source. We also took advantage of a multistate, multiyear field experiment comparing varied leafhopper densities.
Results/Conclusions
Our results to date from August 2016 show the experimental field experienced 20-27% reductions in nitrogen content between plots of highest leafhopper density and plots of lower leafhopper densities. We also found a significant inverse relationship between leafhopper density and percentage nitrogen. The research is part of a larger research project to document the impact of leafhopper feeding on nitrogen fixation, furthering our understanding of the connection between aboveground and belowground processes in plants.