Impact of the Invasive Shrub Autumn Olive on Soil Nitrogen and Cohabiting Plant Growth.

Autumn olive (Elaeagnus umbellata Thunb.) is a non-native, invasive, nitrogen-fixing shrub introduced to stabilize mine and roadside soils, and now thrives in abandoned farm fields, roadsides, and some forest understories.  Research indicated that autumn olive (AO) grew faster than native competitors by accumulating higher leaf chlorophyll content and producing higher net primary productivity than co-habiting plants.  The objective of this research was to determine if AO modified the soil environment, altering the growth capacity of cohabiting native species.  Paired transplant seedlings were grown in 4L pots, either two plants of the same species (black cherry, black walnut, dogwood, hawthorn, or AO) or one AO plant and one plant of the native species.  Plants of most native species growing with a young AO produced higher leaf chlorophyll, greater numbers of new leaves, taller plants, and had faster net photosynthesis rates during the growing season.  Fewer potted native plants survived winter, however, when grown with an AO plant compared with another plant of the same native species.  Ion chromatography of field soil extracts from the AO rhizosphere in late fall contained significantly higher levels of both ammonium and nitrate compared with soils without AO.  In contrast, soils collected during spring and summer contained similar or lower concentrations of ammonium, nitrate, and other plant nutrients.  While the source of the growth advantage enjoyed by native plants in pots with AO remains unclear, germination testing with AO leaf and root extracts demonstrate that frozen AO leaf material is inhibitory to both seed germination and seedling growth of some species.  These data suggest that AO may enrich its rhizosphere with biologically available nitrogen under in the fall season, potentially enriching the actively growing plants, but not during the growing season when high AO transpiration rates may have withdrawn many of the available nutrients.