STABLE ISOTOPIC TRACERS OF NITROGEN AND CARBON CYCLING IN RIVERS AND STREAMS IN THE ANDEAN AMAZON BASIN OF PERU

The rivers and streams of the Peruvian Andes are a major source of sediments and dissolved organic and inorganic matter to the Amazon; however, few studies have been published of nutrient cycling in these regions. In July of 2002 an expedition along the remote upland reaches of the Huancabama River in the Pachitea basin of Peru collected soil, plant, water, and suspended particulates from the river and its tributaries for the analysis of the natural abundance of carbon and nitrogen isotopes. Nutrient and dissolved organic carbon concentrations were also determined for the water samples. River sampling started at an elevation of about 4500 meters and dropped to about 1500 meters over a distance of about 150 kilometers. Analyses of these water samples for oxygen and hydrogen isotopes revealed the relative contributions of snow, rain, groundwater and tributary streams to the main channel of the river. This study should provide novel and interesting information about the upland sources of water, nutrients, and particulates to the lower Amazon basin.