Dissolved inorganic carbon export across the soil/stream interface and its fate in a boreal headwater stream

The aim of this investigation was to determine the lateral exportof dissolved inorganic carbon (DIC) from soils of a Swedish boreal forest to a first order stream and to estimate the partitioning of this DIC into CO2 evasion from the stream surface and the DIC pool exported down through the catchment by streamwater. The groundwater entering the stream was supersaturated with CO2 with values as high as 17 times equilibrium with the atmosphere. Up to 90% of the estimated daily soil DIC export to the stream was emitted to the atmosphere as CO2 within 200 m of the water entering the stream. The annual DIC export from the soil to the stream was estimated to be 3.2 (+/- 0.1) g C m(-2) yr(-1) (normalized to catchment size). Ninety percent of the variation in soil DIC export could be explained by the variation in groundwater discharge and the DIC concentrations per se, were of minor importance. A significant correlation (R(l) = 0.74, P < 0.01) between soil DIC export and CO2 emission from the stream surface suggests that emission dynamics were primarily driven by the export of terrestrial DIC and that in-stream processes were less important. Our results reveal that current budget estimates of lateral DIC export from soils to aquatic conduits need to be revised because they do not account for conditions prevailing in headwater streams. Any quantification of lateral stream C export and CO2 emissions from freshwater systems must include headwater streams as well as the lower parts of the aquatic conduit.