Origins and processing of organic matter in the Amazon River as indicated by carbohydrates and amino acids

Abstract

Aldose, amino acid, and elemental compositions were determined for flux‐weighted samples of coarse (>63 µm) and fine (<63 µm) particulate organic material and ultrafiltered (>1,000 Daltons) dissolved organic matter collected at three sites along the Brazilian Amazon River and six of its major tributaries. Concentrations of total organic C (TOC) were relatively uniform (550±100 µM) at all sites, with DOC comprising the major (50–100%) component. An average of 77% of the total DOC was isolated by ultrafiltration.The greatest compositional differences observed in the Amazon River system were among the coarse, fine, and dissolved organic fractions. All coarse particulate fractions were nitrogen‐poor (atomic C : N = 21) and exhibited stable carbon isotope, aldose, and amino acid compositions similar to those of angiosperm tree leaves. Coarse particulate organic materials, although the least degraded of the three fractions, had lost appreciable carbohydrate and had immobilized excess nitrogen of apparent bacterial origin. Fine particulate materials were more nitrogen‐rich (C : N = 9) than coarse counterparts and had lower total aldose yields and glucose percentages. Fine particles gave greater total yields of amino acids, characterized by high ratios of basic vs. acidic components. Coexisting dissolved organic materials recovered by ultrafiltration were nitrogen‐poor (C : N = 27–52) and yielded the lowest amounts of aldoses, among which deoxy sugars were concentrated. Dissolved fractions gave extremely low yields of amino acids in mixtures that were enriched in nonprotein components and in acidic vs. basic molecules. These yield and composition patterns are consistent with a “regional chromatography” model in which highly degraded leaf material is solubilized and then partitioned between soil minerals and water during transport to the river, resulting in suspended fine particulate organic materials of soil origin that are nitrogen‐rich and coexisting dissolved organic substances that are nitrogen‐poor.