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

Townsend-Small, Amy, Michael E. McClain, Jay A. Brandes

Limnol. Oceanogr., 50(2), 2005, 672-685 | DOI: 10.4319/lo.2005.50.2.0672

ABSTRACT: 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), δ15NO3- and δ18O of river water along an altitudinal (4,043-720 m above sea level [masl]) transect in the Andes of Peru. Plant δ13C increased with increasing elevation, but unlike previous studies, foliar 13C and %N were negatively correlated. Soil δ13C values did not exhibit similar trends and were enriched by 1-3‰ over plants. Isotopically, riverine fine particulate OM (FPOM, <60 µm) resembled soils, and coarse particulate OM (CPOM, >60 µm) 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 µm) concentration, δ15NO3-, and δ18O 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.

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