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Large Amazonian rivers are known to emit substantial amounts of CO2 to the atmosphere, while the magnitude of CO2 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 CO2 dissolved within soils, over 90% of which evaded to the atmosphere within headwater reaches of streams. Hydrologic transport of dissolved CO2 was equivalent to nearly half the gaseous CO2 contributions from deep soil (> 2 m) to respiration at the soil surface. Dissolved CO2 in emergent groundwater was isotopically consistent with soil respiration, and demonstrated strong agreement with deep soil CO2 concentrations and seasonal dynamics. During wet seasons, deep soil (2-8 m) CO2 concentration profiles indicated gaseous diffusion to deeper layers, thereby enhancing CO2 drainage to streams. Groundwater discharge of CO2 and its subsequent evasion is a significant conduit for terrestrially-respired carbon in tropical headwater catchments

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