The capacity of streambeds to process nitrogen (N) is of great interest, particularly where high NO₃ concentrations occur from agricultural or urban runoff.  A key question is: how does human disturbance affect NO₃ removal and retention in hyporheic sediments?  A second question is: how does microbial diversity differ among different types of streams?  Bromide(Br) and ¹⁵N-NO₃ were injected for 24-hr in nine streams (3 each in native, urban, and agricultural) in the southwestern U.S.  Four transects of 3–4 wells were placed along a longitudinal gradient below the15N addition.  Water and gas samples were taken during and after the injections from groundwater wells.  Gas samples were analyzed for O₂, ¹⁵N₂O and ¹⁵N₂.  Hyporheic water samples were analyzed for all major cations and anions, DOC, ¹⁵N-NO₃, ¹⁵N-NH₄ and microbial diversity.  Microbial diversity was determined using Denaturing Gradient Gel Electrophoresis (DGGE).  Results showed between 0-90% surface water in the wells and most had labeled water(¹⁵N-NO₃ and ¹⁵N-NH₄).  There was more ¹⁵N-NH₄ in agricultural subsurface systems than reference streams.  This suggests that dissimilatory nitrate reduction to ammonium (DNRA) is an important process in these hyporheic sediments.  This study shows that hyporheic sediments are significant in uptake and processing of NO₃ in these streams.