Invasive plants cause substantial environmental and economic harm, worldwide. Even when an invasive plant species can be successfully removed from a site, it is very difficult to prevent re-invasion or colonization by other invasive species. Such ?legacy effects? of invasive plants are thought to arise from the way in which those plants change microbes in the soil responsible for nutrients, diseases, and other factors that determine which plants can grow in a given area. This study will examine how an invasive grass, Japanese stiltgrass, may affect soil microbes and increase the intensity of forest fires in the eastern North America. One focus will be on a group of bacteria responsible for making nitrogen (in the form of nitrate) available in a way that benefits stiltgrass over native grass species. Another focus will be on how stiltgrass invasion results in hotter fires, which in turn may impact soil bacteria differently than fires in areas without stiltgrass.
By revealing how invasive species change soil microbes and fire intensity, this study will assist land managers in developing effective strategies for control of stiltgrass and similar invasive species. Results will be distributed to local land managers via a pamphlet created for the Department of Parks and Recreation. In collaboration with a local high school teacher, the project is also creating materials on invasive plants for the high school biology curriculum. Additionally, material on invasive plants and soil microbes will be distributed as part of Bloomington, Indiana's ?Naturalist at Market? program.
Invasive plants cause substantial environmental and economic harm worldwide. Even when an invasive plant species can be successfully removed from a site, it is very difficult to prevent re-invasion or colonization by another invasive species. Such "legacy effects" of invasive plants could arise from changes in the structure of soil microbial community (i.e., changes in species diversity, richness, or abundance) and associated changes in ecosystem functions (e.g., nitrogen cycling). We studied how Japanese stiltgrass (Microstegium vimineum) indirectly affects the nitrogen cycle by affecting key soil microbes. Stiltgrass is an invasive grass that has a variety of negative impacts on native plants and wildlife. In addition to other effects, it can speed up ammonia oxidation, a key step in the nitrogen cycle. To test the effects of stiltgrass, we made use of experimental invasion plots that had been created in 2005, half of which were seeded with stiltgrass. We measured changes in the abundance of stiltgrass, native plant species, and ammonia-oxidation microbes in these plots from 2008 to 2012. We found that stiltgrass increases the abundance of ammonia-oxidizing microbes (specifically, ammonia-oxidizing archaea) in the soil. These microbes are responsible for turning ammonia into nitrate. Nitrate-nitrogen is water soluble and more likely to leach from the soil, thus reducing nitrogen levels in the soil. Previous studies have suggested that stiltgrass prefers nitrate as a soil nitrogen source, and have shown that stiltgrass performs better in conditions with high soil nitrate levels. Our findings suggest that stiltgrass’s association with high soil nitrate levels is caused by an ability to promote the abundance of soil nitrifying archaea. Previous studies also determined that stiltgrass increases the intensity of forest fires in the Eastern Deciduous Forest. Since stiltgrass is able to recover quickly after a fire, increased fire intensity increases the abundance of stiltgrass at a site and decreases the abundance of native plants. One quadrant within each of the experimental plots was burned in 2011 and 2012. Similar to the previous studies, stiltgrass became more abundant in the burned quadrants of the invaded plots compared to the unburned quadrants. We hypothesized that these intense fires in the invaded plots would affect the abundance of ammonia-oxidizing archaea in the soil. We found that burning tends to decrease the abundance of ammonia-oxidizing archaea in these invaded plots. The abundance of these microbes in burned invaded quadrants became more similar to their abundance in uninvaded plots. Thus, fire promotes stiltgrass abundance while at the same time reducing the abundance of ammonia-oxidizing archaea. We determined that the impacts of stiltgrass are seasonal and change over time. Impacts on soil ammonia-oxidation microbes were strongest in the spring when stiltgrass is active. In addition to these seasonal fluctuations, the impacts of stiltgrass also fluctuated over the five years of our study. These long-term changes could be due to a variety of variables not tested in our study (e.g., recent droughts, changes in the native plant community). Our findings argue in favor of more long-term studies. Most research on invasive plants looks at a single time point, showing only a snapshot of what is occurring. It would be more accurate to look at these species over time and take into account seasonal and annual variation. We found evidence of soil legacy effects: ammonia-oxidizing microbes in the unburned invaded plots remained high in 2012, even though stiltgrass started dying off in these plots in 2010. By revealing invasive plant-soil-microbial linkages, our study will assist land managers in developing effective strategies, such as treatment with ammonia-oxidation inhibitors, for control of stiltgrass and similar invasive species. Our results were presented to faculty and the general public as part of Sarah Shannon’s thesis defense at Indiana University. We are preparing a manuscript with these results for publication in a peer-reviewed scientific journal. Our findings were also explained to the general public as part of the City of Bloomington’s Roving Naturalist program.
