Current forest carbon fixation fuels stream CO₂ emissions.

Abstract

Stream CO₂ 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 CO₂ export are still obscure. Here, via an extensive radiocarbon (¹⁴C) characterisation of CO₂ and DOC in stream water and its groundwater sources in an old-growth boreal forest, we demonstrate that the ¹⁴C-CO₂ is consistently in tune with the current atmospheric ¹⁴C-CO₂ level and shows little association with the ¹⁴C-DOC in the same waters. Our findings thus indicate that stream CO₂ emissions act as a shortcut that returns CO₂ 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 CO₂ emissions will be highly sensitive to changes in forest C allocation patterns associated with climate and land-use changes.