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...

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Outgassing of carbon dioxide (CO2) from rivers and streams to the atmosphere is a major loss term in the coupled terrestrial‐aquatic carbon cycle of major low‐gradient river systems (the term “river system” encompasses the rivers and streams of all sizes that compose the drainage network in a river basin). However, the magnitude and controls on this important carbon flux are not well quantified. We measured carbon dioxide flux rates (FCO2), gas transfer velocity (k), and partial pressures (pCO2) in rivers and streams of the Amazon and Mekong river systems in South America and Southeast Asia, respectively. FCO2 and k values were significantly higher in small rivers and streams (channels <100 m wide) than in large rivers (channels >100 m wide). Small rivers and streams also had substantially higher variability in k values than large rivers. Observed FCO2 and k values suggest that previous estimates of basinwide CO2 evasion from tropical rivers and wetlands have been conservative and are likely to be revised upward substantially in the future. Data from the present study combined with data compiled from the literature collectively suggest that the physical control of gas exchange velocities and fluxes in low‐gradient river systems makes a transition from the dominance of wind control at the largest spatial scales (in estuaries and river mainstems) toward increasing importance of water current velocity and depth at progressively smaller channel dimensions upstream. These results highlight the importance of incorporating scale‐appropriate k values into basinwide models of whole ecosystem carbon balance.

[1] Large Amazonian rivers are known to emit substantial amounts of CO 2 to the atmosphere, while the magnitude of CO 2 degassing from small streams remains a major unknown in regional carbon budgets. We found that 77% of carbon transported by water from the landscape was as terrestrially-respired CO 2 dissolved within soils, over 90% of which evaded to the atmosphere within headwater reaches of streams. Hydrologic transport of dissolved CO 2 was equivalent to nearly half the gaseous CO 2 contributions from deep soil (>2 m) to respiration at the soil surface. Dissolved CO 2 in emergent groundwater was isotopically consistent with soil respiration, and demonstrated strong agreement with deep soil CO 2 concentrations and seasonal dynamics. During wet seasons, deep soil (2 -8 m) CO 2 concentration profiles indicated gaseous diffusion to deeper layers, thereby enhancing CO 2 drainage to streams. Groundwater discharge of CO 2 and its subsequent evasion is a significant conduit for terrestrially-respired carbon in tropical headwater catchments. Citation: Johnson, M. S.,

Constraining the fate of dissolved organic matter (DOM) delivered by rivers is a key to understand the global carbon cycle, since DOM mineralization directly influences air-sea CO 2 exchange and multiple biogeochemical processes. The Amazon River exports large amounts of DOM, and yet the fate of this material in the ocean remains unclear. Here we investigate the molecular composition and transformations of DOM in the Amazon River-ocean continuum using ultrahigh resolution mass spectrometry and geochemical and biological tracers. We show that there is a strong gradient in source and composition of DOM along the continuum, and that dilution of riverine DOM in the ocean is the dominant pattern of variability in the system. Alterations in DOM composition are observed in the plume associated with the addition of new organic compounds by phytoplankton and with bacterial and photochemical transformations. The relative importance of each of these drivers varies spatially and is modulated by seasonal variations in river discharge and ocean circulation. We further show that a large fraction (50-76%) of the Amazon River DOM is surprisingly stable in the coastal ocean. This results in a globally significant river plume with a strong terrigenous signature and in substantial export of terrestrially derived organic carbon from the continental margin, where it can be entrained in the large-scale circulation and potentially contribute to the long-term storage of terrigenous production and to the recalcitrant carbon pool found in the deep ocean.

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Methane (CH 4 ) fluxes from world rivers are still poorly constrained, with measurements restricted mainly to temperate climates. Additional river flux measurements, including spatio-temporal studies, are important to refine extrapolations. Here we assess the spatiotemporal variability of CH 4 fluxes from the Amazon and its main tributaries, the Negro, Solimões, Madeira, Tapajós, Xingu, and Pará Rivers, based on direct measurements using floating chambers. Sixteen out of 34 sites were measured during low and high water seasons. Significant differences were observed within sites in the same river and among different rivers, types of rivers, and seasons. Ebullition contributed to more than 50% of total emissions for some rivers. Considering only river channels, our data indicate that large rivers in the Amazon Basin release between 0.40 and 0.58 Tg CH 4 yr -1 . Thus, our estimates of CH 4 flux from all tropical rivers and rivers globally were, respectively, 19-51% to 31-84% higher than previous estimates, with large rivers of the Amazon accounting for 22-28% of global river CH 4 emissions.3

A consistent observation of river waters in the Amazon Basin and elsewhere is that suspended fine particulate organic matter (FPOM) is compositionally distinct from coexisting dissolved organic matter (DOM). The present article presents experimental results that show that at least some of these compositional patterns are the outcome of selective partitioning of nitrogen-rich DOM components onto mineral surfaces. Nine laboratory experiments were conducted in which natural DOM from two rivers, one wetland, and two leachates from the Peruvian Amazon were mixed with natural suspended riverine minerals or organic-free kaolinite. Concentrations of organic carbon, organic nitrogen, and hydrolyzable amino acids were measured in both dissolved and particulate phases before and after mixing. In each of these trials, nitrogen was preferentially taken into the particulate fraction relative to the ''parent'' DOM, as were total hydrolyzable amino acids with respect to total organic carbon and nitrogen. Amino acid compositional patterns also indicated preferential sorption of basic amino acids, with positively charged nitrogen side chains, to the negatively charged aluminosilicate clay minerals. In short, sorption of natural DOM to minerals reproduced all contrasting organic nitrogen compositional patterns observed in the Amazon Basin. Although previously conjectured from FPOM-DOM compositional trends from river samples, this is the first direct evidence for preferential uptake of naturally occurring nitrogenous DOM by suspended riverine minerals. Last, nonprotein amino acids, which are commonly used as diagenetic indicators in sediments, preferentially remained dissolved, which suggests that sorptive fractionation may significantly complicate comparisons of FPOM and DOM diagenesis on the basis of interpretation of organic composition.