1077 Autumn-olive (Elaeagnus umbellata Tunb.) 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 (NO 3 –N) and ammonium N (NH 4 –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 NO 3 –N (p = 0.003), but soil water NH 4 –N concentrations were similar between autumn-olive and open field. Groundwater NO 3 –N and NH 4 –N concentrations were similar beneath both types of vegetation. Groundwater NO 3 –N concentrations did not reflect patterns in soil N mineralization and soil water NO 3 –N most likely due to a weak hydrologic connection between soil water and groundwater. Te 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. Soil and Groundwater Nitrogen Response to Invasion by an Exotic Nitrogen-Fixing Shrub Christine L. Goldstein, Karl W. J. Williard,* Jon E. Schoonover, Sara G. Baer, and John W. Groninger Southern Illinois University Jennie M. Snyder Frost Valley YMCA I nvasive species reduce biodiversity (Williamson, 1996; Rout and Callaway, 2009) and affect biogeochemical cycling (Gordon, 1998), particularly if they are associated with nitrogen (N) fixation (Vitousek and Walker, 1989) or alter fluxes of N during decomposition (Ehrenfeld et al., 2001; Ehrenfeld, 2003). For example, changes in biomass productivity, plant allocation pat- terns of N, C/N ratios, and lignin/N ratios have been implicated in driving N mineralization dynamics during the early stages of invasion (Witkowski, 1991; Ehrenfeld, 2003). Secondary succes- sional systems in particular may be highly vulnerable to invasion because they exist in highly fragmented landscapes and regenerate under altered conditions where past legacies of disturbance per- sist (Baer et al., 2009). In southern Illinois, invasion by volunteer plant species drives site and watershed-level recovery after agricul- tural abandonment. Tese successional communities can be dom- inated by invasive plants of native and nonnative origin, including autumn-olive (Bazzaz, 1968; Baer and Groninger, 2004). Autumn-olive is an invasive, exotic shrub native to Asia (Munger, 2003; USDA, 2007) that fixes N via a symbiotic rela- tionship with the actinomycete Frankia (Wang et al., 2005). It was first introduced in the eastern United States in the 1830s for its wildlife value and to help remediate disturbed areas and has become naturalized throughout much of temperate North America (VDCR, 1994; Munger, 2003). In southern Illinois, autumn-olive reaches a height of 3 to 5 m on formerly pastured and row cropped land, often forming closed-canopy, nearly monospecific stands. Te impacts of N-fixers and other invasive species on soil N cycling rates and soil water N concentrations have been estab- lished for several species (Rout and Callaway, 2009). However, to our knowledge, this is the first study to investigate the impact of autumn-olive invasion on in situ N cycling and groundwater qual- ity in addition to soil and soil water quality (Baer et al., 2006). We compared in situ net N mineralization rates beneath autumn-olive stands and open-field herbaceous vegetation. Furthermore, we quantified soil water and groundwater N concentrations beneath the two vegetation types to determine if an invasion by autumn- Abbreviations: MT, Minetree; PH, Pleasant Hill; PVC, polyvinylchloride; TIC, Tree Improvement Center. C.L. Goldstein, Dep. of Forestry, Southern Illinois Univ. Carbondale, currently at 1040 2nd St., Columbus, IN 47201; K.W.J. Williard, J.E. Schoonover, and J.W. Groninger, Dep. of Forestry and Center for Ecology, MC 4411, Southern Illinois Univ. Carbondale, Carbondale, IL 62901; S.G. Baer, Dep. of Plant Biology and Center for Ecology, MC 6509, Southern Illinois Univ. Carbondale, Carbondale, IL 62901; J.M. Snyder, Frost Valley YMCA, 2000 Frost Valley Rd., Claryville, NY 12725. Assigned to Associate Editor Christopher Green. Copyright © 2010 by the American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America. All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including pho- tocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. J. Environ. Qual. 39:1077–1084 (2010) doi:10.2134/jeq2009.0155 Published online 24 Mar. 2010. Received 29 Apr. 2009. *Corresponding author (williard@siu.edu). © ASA, CSSA, SSSA 5585 Guilford Rd., Madison, WI 53711 USA TECHNICAL REPORTS: VADOSE ZONE PROCESSES AND CHEMICAL TRANSPORT Published May, 2010