Terrestrial ecosystems in the humid tropics play a potentially important but presently ambiguous role in the global carbon cycle. Whereas global estimates of atmospheric CO2 exchange indicate that the tropics are near equilibrium or are a source with respect to carbon, ground-based estimates indicate that the amount of carbon that is being absorbed by mature rainforests is similar to or greater than that being released by tropical deforestation (about 1.6 Gt C yr-1). Estimates of the magnitude of carbon sequestration are uncertain, however, depending on whether they are derived from measurements of gas fluxes above forests or of biomass accumulation in vegetation and soils. It is also possible that methodological errors may overestimate rates of carbon uptake or that other loss processes have yet to be identified. Here we demonstrate that outgassing (evasion) of CO2 from rivers and wetlands of the central Amazon basin constitutes an important carbon loss process, equal to 1.2 +/- 0.3 Mg C ha-1 yr-1. This carbon probably originates from organic matter transported from upland and flooded forests, which is then respired and outgassed downstream. Extrapolated across the entire basin, this flux-at 0.5 Gt C yr-1-is an order of magnitude greater than fluvial export of organic carbon to the ocean. From these findings, we suggest that the overall carbon budget of rainforests, summed across terrestrial and aquatic environments, appears closer to being in balance than would be inferred from studies of uplands alone.

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.