Current forest carbon fixation fuels stream CO(2) emissions

Stream CO(2) emissions contribute significantly to atmospheric climate forcing. While there are strong indications that groundwater inputs sustain these emissions, the specific biogeochemical pathways and timescales involved in this lateral CO(2) export are still obscure. Here, via an extensive radiocarbon ((14)C) characterisation of CO(2) and DOC in stream water and its groundwater sources in an old-growth boreal forest, we demonstrate that the (14)C-CO(2) is consistently in tune with the current atmospheric (14)C-CO(2) level and shows little association with the (14)C-DOC in the same waters. Our findings thus indicate that stream CO(2) emissions act as a shortcut that returns CO(2) recently fixed by the forest vegetation to the atmosphere. Our results expose a positive feedback mechanism within the C budget of forested catchments, where stream CO(2) emissions will be highly sensitive to changes in forest C allocation patterns associated with climate and land-use changes.