Amazonian river system. We find that respiration of contemporary organic 2 matter (less than 5 years old) originating on land and near rivers is the dominant source of excess carbon dioxide that drives outgassing in mid-size to large rivers, although we find that bulk organic carbon fractions transported by these rivers range from tens to thousands of years in age. We therefore suggest that a small, rapidly cycling pool of organic carbon is responsible for the large carbon fluxes from land to water to atmosphere in the humid tropics.Riverine CO 2 concentrations in Amazonian lowlands are 5-30 times supersaturated with respect to atmospheric equilibrium 1 ; such conditions may be prevalent throughout the humid tropics. In situ respiration is the primary source of CO 2 sustaining supersaturation in rivers, although inputs from groundwater supersaturated by soil respiration can be important in small systems and from submerged riparian root respiration in floodplain influenced systems [1][2][3][6][7][8] . While air-water gas exchange is a bidirectional process, atmospheric CO 2 invasion has a negligible role compared to the large CO 2 evasion fluxes, except at low supersaturation 2,3,6,7 . 13 C and 14 C isotopes can provide constraints on sources and turnover times of organic carbon (OC) fuelling river respiration, yet no previous tropical study has used a dual-isotope approach to address these questions. Studies in temperate eastern USA provide contrasting findings. In the Hudson River, up to 70% of the centuries-old terrestrial OC entering the river is respired in transit, and the average age of riverine OC decreases downstream 2 .However, the youngest components of dissolved OC (DOC) are preferentially respired in the York River 5 , and modern dissolved inorganic carbon (DIC) in the Parker River may be explained by respiration of young DOC produced within the estuary 4 .Documenting key patterns and controls on CO 2 sources in diverse ecosystems is critical to advance our understanding of CO 2 outgassing from rivers and its contribution to regional net carbon budgets. 3To identify dominant sources and turnover times of riverine carbon throughout the Amazon basin, we analysed 14 C and 13 C of DIC, DOC, and suspended fine and coarse particulate OC fractions (FPOC and CPOC), grouping sites topographically (Fig. 1).This survey represents the most extensive dual carbon isotope inventory to date in a large, diverse basin, and the first 14 C analysis of DIC in Amazonian rivers. It complements but greatly exceeds previous carbon isotope surveys 5,7,9 , enabling an integrated assessment of carbon cycling.DIC is composed of dissolved CO 2 and bicarbonate and carbonate ions in pHdependent chemical and isotopic equilibrium 10 . In studies of marine and homogeneous river systems, where pH is nearly uniform, it has been the convention to report the isotopic composition of total DIC, which is directly measured. However, when the turnover of DIC by CO 2 fluxes is as rapid as in many of these tropical rivers, a quasisteady-state co...

Lignin, elemental, and stable carbon isotope compositions are reported for local plants and for coarse (>63 pm) and fine (~63 pm) suspended particulate materials collected along a 1,950-km reach of the lower Amazon River during four contrasting stages of the 1982-1983 hydrograph.

Aldose, amino acid, and elemental compositions were determined for flux-weighted samples of coarse (> 63 pm) and fine (< 63 pm) 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 (55Ok 100 PM) 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. manuscript.

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