PS 26-100
The invasive nitrogen fixing shrub autumn olive (Elaeagnus umbellata) alters soil nitrogen fixing microbial communities
Autumn olive (Elaeagnus umbellata) is an invasive nitrogen fixing plant commonly found throughout the U.S. Midwest region. Autumn olive often invades sandy, nutrient poor soils; it is typically assumed that it has a competitive advantage in those soils, because of its capability to form nitrogen fixing nodules with bacteria of the genus Frankia. We sampled three populations of autumn olive located in central Michigan. In each population, we constructed 2 transects, such that half of each transect was located in an autumn olive population, and half in an adjacent non-invaded area. Soils were sampled from 10 points along those transects, 2.5-5 meters apart (depending on the population). In addition, we excavated 10 plants from each population and collected nodules from them. In order to study the nitrogen-fixing communities in soils, we extracted DNA and amplified the nitrogenase gene with primers specific for Frankiae or for Rhizobia. We then used DGGE to separate amplicons and generate fingerprints for each soil sample. In addition, we isolated Frankia DNA from individual nodules and performed rep-PCR. Different strains of Frankia have different rep-PCR profiles, which allowed us to determine the diversity of Frankia that actually nodulate autumn olive plants.
Results/Conclusions
We found a significant impact of autumn olive on nitrogen fixing communities in soils under autumn olive, compared to non-invaded soils. DGGE fingerprints of autumn olive soils appear to be dominated by Frankia species/strains, which were not prominent in noinvaded soils. Interestingly, fingerprints based on the nitrogenase gene of Rhizobia indicated that autumn olive presence affects rhizobial communities, as well. We found a great diversity of Frankia in the nodules of excavated plants. Individual plants rarely shared their Frankia symbionts (as determined by rep-PCR fingerprints). In addition, for most of the plants, different nodules on the same plant appeared to contain different strains of Frankia. These results clearly indicate that autumn olive modifies the nitrogen fixing community of soils, perhaps by selecting for and enriching certain Frankia species and/or strains in the soil. The effect of autumn olive possibly extends to rhizobial nitrogen fixers as well, indicating that this might be an important, yet overlooked, mechanism that facilitates this plant’s invasion.