The aim of this research was to characterize the biogeochemical functioning of isolated first order watersheds of forested, agricultural and mixed land uses and to locate the compartments where nitrogen retention may occur at this scale. Three first order sub-catchments were studied. An agricultural watershed of 1.3 km², a forested watershed of 1.1 km² and a mixed land use watershed of 1.7 km² containing the agricultural watershed and a forested area downstream. The F forested watershed served as a reference for the nitrate contribution of a forested area in order to understand the transfer and maybe the retention of nitrogen in the forested area of the mixed land use watershed. Flow rates and nitrate concentrations have been monitored at the outlets of the catchments. A first scheme of the hydrological behavior of the mixed land use watershed has been established. Moreover, the dry winter season 2004-2005 allowed to find and estimate deep contributions, which most likely interfere with the general hydrological functioning of the watershed, and hide possible retention processes. In an effort to identify the origin and processes undergone by nitrate molecules, 15N and 18O isotopic analyses will be performed in the future.