The boreal biome comprises vast areas of coniferous forests, dotted with millions of peatlands. Plants harbouring these ecosystems fix CO₂ from the atmosphere, which is later incorporated into the vegetation biomass and subsequently buried in soils. Over the course of millennia, this process has led to the formation of a large repository of organic C, currently stored in boreal soils. Streams draining this landscape are typically enriched with carbon dioxide (CO₂), methane (CH₄) and dissolved organic carbon (DOC). As a consequence, streams tend to emit CO₂ and CH₄ to the atmosphere, two potent greenhouse gases, and thus contribute positively to radiative climate forcing. The sources fuelling C to boreal streams are not well understood. This thesis aims to unravel these sources, and promote a better consolidation of terrestrial and aquatic C biogeochemical processes. The work is largely based on stable and radiogenic C isotope characterization of various dissolved C forms in stream and groundwater, within contrasting ecosystem types across Sweden.

This thesis identifies boreal soils as the main source of CO₂ in streams. Soil respiration (i.e. biogenic sources) overwhelmingly supply CO₂ to streams, leaving only a few exceptions where geogenic CO₂ sources were present. An array of biological processes also transform CO₂ during its transport from soils to streams. These include; methanogenesis, aquatic DOC mineralization and primary production. The majority of C in boreal streams is sustained by the decomposition of recent photosynthates, with ancient C substrates holding a negligible share of the total C export. While these results suggest that the repository of ancient soil organic C is currently stable, within boreal forests and peatlands, the close connection with recently occurring photosynthesis suggest that forecasted alterations in plant C allocation patterns, driven by climate and land-use changes, will produce a rapid response in stream CO₂ emissions. Isotopic characterization of C in stream and groundwater can help reveal these sources and transformation processes, but its interpretation must be made with care.