Russian olive is an invasive riparian tree that is widespread throughout the western USA.  Unlike the native vegetation it replaces, Russian olive fixes nitrogen (N) through a mychorrizal symbiosis, and consequently may provide a novel subsidy of N to riparian zones and adjacent streams.  We hypothesized that Russian olive leaches N into riparian groundwater, and that this N input alters streambed biofilm nutrient limitation and N uptake velocity.  We tested these hypotheses during 2008 by comparing a non-invaded reference reach and a downstream invaded reach at Deep Creek, southeast Idaho. Nitrate and ammonium concentrations in groundwater were doubled in shallow well transects of riparian areas populated by Russian olive compared to those with native vegetation.  Biofilm chlorophyll-a was limited by N or suppressed by phosphorus (P) at the reference reach.  However, at the invaded site, we observed increasing P stimulation along the reach, indicating that biofilm nutrient limitation switched from N to P limitation with Russian olive presence.  Nitrate uptake velocity was an order of magnitude greater in the reference reach (5.5 mm/min) compared to the invaded reach (0.2-0.7mm/min).  Russian olive alteration of N processing in streams represents an important and previously undocumented effect of this riparian plant invasion.