Should Aquatic CO2 Evasion be Included in Contemporary Carbon Budgets for Peatland Ecosystems?

Quantifying the sink strength of northern hemisphere peatlands requires measurements or realistic estimates of all major C flux terms. Whilst assessments of the net ecosystem carbon balance (NECB) routinely include annual measurements of net ecosystem exchange and lateral fluxes of dissolved organic carbon (DOC), they rarely include estimates of evasion (degassing) of CO2 and CH4 from the water surface to the atmosphere, despite supersaturation being a consistent feature of peatland streams. Instantaneous gas exchange measurements from temperate UK peatland streams suggest that the CO2 evasion fluxes scaled to the whole catchment are a significant component of the aquatic C flux (23.3 ± 6.9 g C m−2 catchment y−1) and comparable in magnitude to the downstream DOC flux (29.1 ± 12.9 g C m−2 catchment y−1). Inclusion of the evasion flux term in the NECB would be justified if evaded CO2 and CH4 were isotopically “young” and derived from a “within-ecosystem” source, such as peat or in-stream processing of DOC. Derivation from “old” biogenic or geogenic sources would indicate a separate origin and age of C fixation, disconnected from the ecosystem accumulation rate that the NECB definition implies. Dual isotope analysis (δ13C and 14C) of evasion CO2 and DOC strongly suggest that the source and age of both are different and that evasion CO2 is largely derived from allochthonous (non-stream) sources. Whilst evasion is an important flux term relative to the other components of the NECB, isotopic data suggest that its source and age are peatland-specific. Evidence suggests that a component of the CO2-C evading from stream surfaces was originally fixed from the atmosphere at a significantly earlier time (pre-AD1955) than modern (post-AD1955) C fixation by photosynthesis.