Ecologists have studied nitrogen extensively, but its evolutionary effects are rarely considered. High nitrogen availability favors plant species with certain traits over others. For example, plant species that grow in high-nitrogen environments tend to have faster growth rates, while species that engage in close mutualistic relationships with nitrogen-fixing bacteria are commonly favored in low-nitrogen environments. Similar selection may occur within species, as genotypes with different resource use and growth traits or traits related to mutualism may be favored in high vs. low environments. To investigate how nitrogen can affect the genetic differentiation of both growth and mutualism-related traits, Amphicarpea bracteata (an understory legume) seeds from 12 populations across a natural nitrogen gradient were grown in the greenhouse with and without soil microbial communities from high or low nitrogen sites; plants were also grown in three different levels of nitrogen. Soil inocula were included to allow for investigation of traits related to mutualism with nitrogen-fixing Bradyrhizobium bacteria and because soil microbes can have a large effect on plant fitness and the strength and direction of selection.
Results/Conclusions
Preliminary results show that nitrogen history has a significant effect on growth rate (measured as number of leaves 5 weeks after planting; p = 0.0065) and phenology (days until the first fruit was produced; p = 0.0085). Plants from the highest nitrogen sites produced fewer leaves than plants from the lowest nitrogen sites, particularly when grown in live soil inoculates. Plants from higher nitrogen sites also fruited earlier than plants from lower nitrogen sites, with an average decrease of approximately five days between the lowest and highest site nitrogen levels. These results suggest that plants evolving in different nitrogen environments differ in early growth and phenology traits. Additional data from this experiment (biomass, fruit/seed number, nodule measures) will address traits related to the resource mutualism with rhizobia and whether observed differences are adaptive by comparing plant fitness in varying nitrogen environments.