The effect of the invasive Elaeagnus umbellata on soil microbial communities depends on proximity of soils to plants

Invasive plants are often associated with dense monocultures, which reduce native plant diversity. Multiple studies have shown significant impacts of dense invasive plant populations on soil microbial communities. In many cases, however, invasive plants are present in lower, non-uniform densities. This is especially true of some woody/shrubby invaders. We studied two such populations of autumn olive: a highly invasive, nitrogen-fixing shrub, commonly found throughout the Midwestern United States. In each population, we collected soil samples in a rectangular grid, noting the distance of each sample point to the nearest autumn olive plant, as well as the local density of autumn olive. Soils were assessed for pH, moisture, nitrogen availability, microbial community composition, and enzyme activity. We found that autumn olive significantly affected the dominant taxa of bacterial and archaeal communities, particularly ammonia-oxidizing microorganisms, as determined by denaturing gradient gel electrophoresis of functional genes characteristic of these microbial groups. The changes in microbial communities were related to the proximity and local density of autumn olive shrubs. In most cases, the effect of autumn olive was greater than that of either pH or moisture. In contrast, microbial function (measured as enzyme activities), while non-uniform, did not seem to be as strongly related to the presence of the invasive plant. Our findings suggest that autumn olive can affect soil microbial community composition even when it is present in relatively low densities. This should be considered when assessing remediation strategies for autumn olive-invaded areas.