Young organic matter as a source of carbon dioxide outgassing from Amazonian rivers

Rivers are generally supersaturated with respect to carbon dioxide, resulting in large gas evasion fluxes that can be a significant component of regional net carbon budgets 1,2 . Amazonian rivers were recently shown to outgas more than ten times the amount of carbon exported to the ocean in the form of total organic carbon or dissolved inorganic carbon 1 . High carbon dioxide concentrations in rivers originate largely from in situ respiration of organic carbon 1‐3 , but little agreement exists about the sources or turnover times of this carbon 2,4,5 . Here we present results of an extensive survey of the carbon isotope composition ( 13 C and 14 C) of dissolved inorganic carbon and three size-fractions of organic carbon across the Amazonian river system. We find that respiration of contemporary organic matter (less than five years old) originating on land and near rivers is the dominant source of excess carbon dioxide that drives outgassing in medium 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 CO2 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 CO2 sustaining supersaturation in rivers, although inputs from groundwater supersaturated by soil respiration can be important in small systems and from submerged root respiration in floodplain-influenced systems 1–3,6–8 . Although air–water gas exchange is a bi-directional process, atmospheric CO2 invasion has a negligible role compared to the large CO2 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 fuelling river respiration, but 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 organic carbon entering the river is respired in transit, and the average age of riverine organic carbon decreases downstream 2 . However, the youngest components of dissolved organic carbon (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 CO2 sources in diverse ecosystems is critical to advance our understanding of CO2 outgassing from rivers and its contribution to regional net carbon budgets. To identify dominant sources and turnover times of riverine carbon throughout the Amazon basin, we analysed 14 C and 13 Co f