Soil and groundwater nitrogen response to invasion by an exotic nitrogen-fixing shrub.
Autumn-olive (Elaeagnus umbellata Thunb.) is an invasive, exotic shrub that has become naturalized in the eastern United States and can fix nitrogen (N) via a symbiotic relationship with the actinomycete Frankia. Fixed N could potentially influence nutrient cycling rates and N leaching into soil water and groundwater. In situ net N mineralization, net nitrification, and net ammonification rates, as well as soil water and groundwater nitrate N (NO3-N) and ammonium N (NH4-N) concentrations, were measured under autumn-olive-dominated and herbaceous open field areas in southern Illinois. Soil net N mineralization and net nitrification rates were higher under autumn-olive compared with open field (p<0.05) and could be driven, in part, by the relatively low C/N ratio (11.41±0.29) of autumn-olive foliage and subsequent litter. Autumn-olive stands also had greater soil water NO3-N (p=0.003), but soil water NH4-N concentrations were similar between autumn-olive and open field. Groundwater NO3-N and NH4-N concentrations were similar beneath both types of vegetation. Groundwater NO3-N concentrations did not reflect patterns in soil N mineralization and soil water NO3-N most likely due to a weak hydrologic connection between soil water and groundwater. The increased N levels in soil and soil water indicate that abandoned agroecosystems invaded by autumn-olive may be net sources of N to adjacent terrestrial and aquatic systems rather than net sinks.