Contributions of carbon and nitrogen from the Andes Mountains to the Amazon River: Evidence from an elevational gradient of soils, plants, and river material

We determined the carbon (C) and nitrogen (N) elemental and stable isotopic composition of riverine and terrestrial organic matter (OM), as well as the concentration of dissolved organic C (DOC), delta(NO3-)-N-11 and delta O-18 of river water along an altitudinal (4,043-720 In above sea level [masl]) transect in the Andes of Peru. Plant delta C-13 increased with increasing elevation, but unlike previous studies, foliar C-13 and %N were negatively correlated. Soil delta C-13 values did not exhibit similar trends and were enriched by 1-3 parts per thousand over plants. Isotopically, riverine fine particulate OM (FPOM, < 60 mu m) resembled soils, and coarse particulate OM (CPOM, > 60 mu m) resembled leaves. Both FPOM and CPOM exhibited OM levels beyond those attributable to sorption. Percentage OC and N of soils and FPOM were positively correlated with altitude and highlight a trend of sequential downstream dilution of OM with inorganic material. FPOM began to resemble plant OM isotopically at lower altitudes, perhaps due to increased plant and surface soil inputs to lower rivers. The compositional similarity of particulate organic matter to terrestrial plants and soils indicates that the dominant processes affecting riverine OM are occurring on the landscape, not within the river. Dissolved OC (< 0.2 mu m) concentration, delta(NO3-)-N-15, and delta O-18 of H2O are variable in high-altitude tributaries but approach constant values downstream. Elemental and isotopic analyses of riverine OM suggest compositional differences between size fractions, similar to the lower Amazon; however, unlike previous studies, we have found significant within-stream changes with altitude in OM composition.