15N evidence for the origin and cycling of inorganic nitrogen in a small Amazonian catchment

Abstract

The δ15N 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, Amazônas, Brazil. The δ15N 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 δ15N values averaging 6.25 ± 0.9‰; riparian groundwater DIN primarily composed of NH4+ with δ15N values averaging 9.17 ± 1.0‰, and stream water DIN predominately composed of NO3- with δ15N values averaging 4.52 ± 0.8‰. Nitrate samples taken from the stream source and from the stream adjacent to the groundwater transects showed a downstream increase in δ15N from 1.0‰ to 4.5‰. Leaf litter samples averaged 3.5 ± 1.2‰. 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. © 1990 Kluwer Academic Publishers.