Small headwater streams are recognized for intense outgassing to the atmosphere of climate-relevant carbon dioxide (CO2) and methane (CH4). Though these headwaters are markedly oversaturated for both CO2 and CH4, the origins and controls over the fate of these two carbon-gases are still poorly constrained, especially for the stronger greenhouse gas CH4. Here, by measuring stream-based production of CO2 and CH4, concurrently with their rates of outgassing to the atmosphere, we identify distinct biophysical control mechanisms for each gas. We show that while CO2 is largely imported from the catchment in proportion to discharge, CO2 outgassing can be modulated by in-stream metabolism to offset outgassing by up to 30% in spring and summer. In contrast, CH4 shows a non-linear response to seasonal changes in discharge and is predominantly produced in the streambed in relation to sediment type. Further, once released from the streambed, outgassing of CH4 at the surface and flow-driven dilution occur far more rapidly than biological methane oxidation and CH4 leaves the water largely unaltered by biology. Incorporating the intense carbon cycling of headwater streams into the global carbon cycle will require distinct parameterizations for each carbon gas in Earth system models.
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