Autumn-olive (Elaeagnus umbellata Thunb.) is an exotic species that was introduced in the U. S. in the 1800's and widely promoted as an ideal plant for erosion control, wildlife habitat and soil remediation. N-fixation by autumn-olive through a symbiotic relationship with actinomycete Frankia, can alter nitrogen cycling and potentially impair water quality through nitrate leaching. Furthermore, legacy effects of accumulated N following vegetative management and restoration efforts are unknown in areas invaded by autumn-olive. The first objective of this research was to determine if there was a relationship between autumn-olive cover and stream nitrate-N (NO3--N) concentration in twelve forested headwater watersheds in southern Illinois. Secondly, changes in autumn-olive cover and stream nitrogen concentrations were assessed by comparing current results to data collected 6 years prior. Nine of the study watersheds had significantly greater autumn-olive percent cover in 2012 compared to 2006 and mean stream NO3--N concentration significantly increased from 2006 to 2012 in all watersheds. Also, a significant exponential relationship was found between stream NO3--N concentration and autumn-olive percent cover. The long term effects of autumn-olive management on N cycling were also investigated by implementing three different vegetation treatments on invaded areas: cutting of autumn-olive trees (CU), cutting and stump herbicide application (CH) and a no treatment application or control (CO). Treated plots showed that soil NO3--N increased temporarily due to the disturbance. However, after the initial post treatment period of one year, soil water NO3--N concentrations on treated plots shifted, so that CH plots had significantly lower levels of NO3--N than in CO plots. Soil water NO3--N in CU plots also decreased, but it was not significantly different from CH plots. Subsequently, soil water NO3--N declined on CH plots, reaching the lowest levels in the third and fourth year after treatment. Reduction in soil water NO3--N in CH plots was not accompanied by differences among treatments in soil N mineralization rates or soil C:N ratios indicating persistent high nitrification rates contributing to mineralization under treated and control plots. These results demonstrate that encroachment of autumn-olive can transform N cycles in natural areas and affect water quality by saturating the soil with mobile forms of N that are leached out of the system. Significant rates of soil N cycling can persist for years after removal of autumn-olive due to the cycling of legacy N in soils and litter. Nevertheless, the cut and stump herbicide application treatment produced positive vegetation management results by halting N-fixation, controlling autumn-olive re-sprouting and by gradually decreasing N availability and NO3--N flushing in treated areas. Treatment of autumn-olive should have an impact on stream water quality in headwater watersheds, given the significant observed relationship between autumn-olive cover and stream nitrogen levels.