FEEDBACKS of NITROGEN CYCLING and INVASION with the NON-NATIVE PLANT, <italic>MICROSTEGIUM VIMINEUM</Italic>, in RIPARIAN WETLANDS

Abstract

<p><p>Invasive species are rapidly expanding in riparian wetlands while concurrently anthropogenic causes are increasing nitrogen (N) into these ecosystems. <italic>Microstegium vimineum (Microstegium) </italic> is a particularly abundant invasive grass in the Southeast United States. To evaluate impacts of <italic>Microstegium</italic> on both plant diversity and N cycling in a riparian floodplain, paired plots of <italic>Microstegium</italic> hand-weeded and unweeded were established for three years. Plots without <italic>Microstegium</italic> increased from 4 to 15 species m<super>-2</super> and 90% of the newly establishing species were native. The <italic>Microstegium</italic> community accumulated approximately half the annual N in biomass of the diverse community, 5.04 versus 9.36 g-N m<super>-2</super> year<super>-1</super>, respectively (p=0.05). Decomposition and release of N from <italic>Microstegium</italic> detritus was much less than in the diverse community, 1.19 versus 5.24 g-N m<super>-2</super> year<super>-1</super>. Rates of soil N mineralization estimated by in-situ incubations were relatively similar in all plots. While <italic>Microstegium</italic> invasion appears to greatly diminish within-ecosystem circulation of N through the under-story plants, it might increase ecosystem N losses through enhanced denitrification (due to lower redox potentials under Microstegium plots). Microstegium removal ceased in the fourth growing season and formerly weeded plots increased to 59% (&plusmn; 11% SE) Microstegium cover and species richness decreased to <8 species m<super>-2</super>. </p></p><p><p>To learn how <italic>Microstegium</italic> responds to increased N, we conducted a greenhouse competition experiment between <italic>Microstegium</italic> and four native plants across an N gradient. There was a unique competition outcome in each species combination, yet <italic>Microstegium</italic> was most dominant in the high levels of N. </p></p><p><p>Last, we disturbed a floodplain similar to wetland restoration disturbance and tracked available N. We also established a native community of plants with and without <italic>Microstegium</italic> in three levels of N. Disturbance to the floodplain dramatically increased inorganic N, especially in the form of NO₃ which was five times higher in the disturbed floodplain than the undisturbed floodplain. N levels remained elevated for over a year. <italic>Microstegium</italic> was N responsive, but did not show negative effects to the planted vegetation until the second year. Ironically, restoration activities are increasing available N, and favoring invasive species which in turn detracts from restoration success.</p>