The delta(15)N composition of the dominant form of dissolved inorganic nitrogen (DIN) was determined in upland groundwater, riparian groundwater, and stream water of the Barro Branco catchment, Amazonas, Brazil. The delta(15)N composition of organic nitrogen in riparian and upland leaf litter was also determined. The data for these waters could be divided into three groups: upland groundwater DIN predominately composed of NO3- with delta(15)N values averaging 6.25+/-0.9 parts per thousand; riparian groundwater DIN primarily composed of NH4+ with delta(15)N values averaging 9.17+/-1.0 parts per thousand, and stream water DIN predominately composed of NO3- with delta(15)N values averaging 4.52+/-0.8 parts per thousand. Nitrate samples taken from the stream source and from the stream adjacent to the groundwater transects showed a downstream increase in delta(15)N from 1.0 parts per thousand to 4.5 parts per thousand. Leaf litter samples averaged 3.5+/-1.2 parts per thousand. The observed patterns in isotopic composition, together with previously observed inorganic nitrogen species and concentration shifts between upland, riparian and stream waters, suggest that groundwater DIN is not the primary source of DIN to the stream. Instead, the isotopic data suggest that remineralization of organic nitrogen within the stream itself may be a major source of stream DIN, and that the majority of DIN entering the stream via groundwater flowpaths is removed at the riparian-stream interface.
