Stable isotope tracer studies have revealed much about nitrogen dynamics in streams. However, the majority of these studies have focused on systems with limited anthropogenic disturbance. In summer 2006, we conducted a 21-day tracer addition (>500 ‰) of ¹⁵NH₄Cl in Mimm’s Creek, a first-order urban stream in Jacksonville, Florida. Our goal was to investigate how catchment urbanization affects NH₄ dynamics. The catchment has urban/residential land use and is an important conveyance for stormwater. A 250-meter, incised channel with sandy substrata was used for the experiment. Discharge was uniform and low (~3 L sec^-1) and stream NH₄ concentrations averaged ~250 μg L^-1 during the release. Uptake rates and lengths of NH₄ were determined from NH₄ diffusions from eight sites located downstream from the addition on days 1 and 21. Preliminary results from days 1 and 21 showed NH₄ uptake lengths were 85 and 300 m, uptake velocities were 0.03 and 0.01 mm sec^-1, and whole stream uptake were 0.24 and 0.13 g m^-2 d^-1 respectively. When compared to other NH₄ tracer studies, our preliminary results indicate that increased NH₄ loading associated with urbanization can lead to longer uptake lengths, slower uptake velocities, but increased total uptake.