Abstract:Nitrogen isotope and traditional water sampling, together with microcosm incubations were used to investigate processes of nitrogen downstream transports and transformations within the Wuchuan stream. Results showed NO3-N concentrations varied downstream ranging from 0.5 to 4.5 mg L-1 for most of sampling months with a mean of 1.26 mg L-1 in the stream. However, nitrate isotope δ15N decreased downstream, comparing to increased trend in NO3-N concentration. The relationship between downstream NO3-N concentrations and nitrate δ15N with a R2 exceeding 0.8 can be characterized as a typical process of nitrification. The relationships between NO3-N concentrations and NO2-N, dissolved oxygen, dissolved organic carbon concentrations for the sampling sites also indicated strong nitrification processes and consequent nitrogen transformations between nitrogen forms in the stream. Microcosm study of surface water from the stream showed that no significant change in NH4-N and NO3-N concentrations occurred during the five-day incubations. All the information and relationships showed that the downstream increases in NH4-N and NO3-N concentrations, and the downstream decreases in nitrate δ15N within the stream obviously resulted from the nitrogen input from surface runoff flowing into the channel, and also nitrogen transformations by nitrification occurred between stream sediment and overlying water within the channel. Therefore, it is essential to take the sources from nitrogen transformations within streams into account and to evaluate their contributions in pollution assessments at catchment scales.