The feedlot system (FL) generates a high input of sewage causing imbalances in the cycling process of nitrogen (N) as well as a potential negative impact on the environment. The objective of this study was to deepen into the dynamics of N in a FL's soil so as to provide tools to reduce its environmental impact. The study was conducted in a FL, located in Balcarce County (Buenos Aires, Argentina). Soil samples were taken between November 2006 and May 2007 in a control Pasture, in the FL-High (Upper Slope position) and FL-Low (Lower Slope position) of the FL, and in the runoff area (RA). Urease activity and concentrations of Ammonium-N, Nitrate-N and urea-N were evaluated in the soil up to 60 cm depth. Urea-N values were always low (6 mg kg-1), due to the high urease activity measured (438 mg kg-1). Ammonium-N concentration was highest in FL-Low, reaching a value of 65.5 mg kg-1 at 40-60 cm soil depth. Nitrate-N concentration was highest in FL-High for all depths, with a maximum concentration of 97.4 mg kg-1 in the 0-10 cm layer; at 40-60 cm depth, Nitrate-N concentration in FL was 31.98 mg kg-1, higher than that in Pasture (1.21 mg kg-1). There were no differences in Nitrate-N concentration between Pasture and RA; hence, it can be inferred that there was no transport of nitrate by surface water flow from the FL to RA. However, the potential for groundwater contamination via nitrate, a mobile soil constituent, should be studied.