Isotopic tracers of nutrient and carbon sources in Andean river systems

Tropical river systems draining high relief mountain systems are important, but poorly studied, components of global carbon and nutrient cycles. In the Amazon river it is estimated that a majority of the suspended material and a significant amount of the carbon and nutrients originate in Andean rivers. These estimates are based upon a handful of studies scattered throughout the basin, and a detailed examination of carbon and nutrient cycling across a range of environments has yet to be done. We have conducted a sampling campaign in the Pachetea basin, Peruvian Andes aimed at determining the distribution of carbon and nutrient sources as a function of altitude, ecotone and soil type. Preliminary results corroborate the previously reported existence of strong13C isotopic gradients with drainage basin altitude of riverine particulate organic matter. These gradients can be used to constrain the percentages of particulate carbon and associated nutrients entering the system as a function of altitude. Comparisons between plant, soil and river carbon and nitrogen isotopic values indicate the presence of significant shifts in isotopic values of C and N as diagenesis proceeds. Particulate and dissolved nitrogen isotopic values appear to be controlled by small basin processes occurring at stream edge and not in the rivers as a whole. Overall, Andean sources of nutrients and carbon to the lower Amazon appear to be skewed towards the high rainfall/ high erosion region below 3000 m in elevation.