Disputation: Carbon in Boreal Streams - Isotopic Tracing of Terrestrial Sources

  • Datum: –13.00
  • Plats: Geocentrum hambergsalen
  • Doktorand: Audrey Campeau
  • Arrangör: LUVAL
  • Kontaktperson: Audrey Campeau
  • Disputation

Dissertation presented at Uppsala University to be publicly examined in Hambergsalen,Villavägen 16B, Uppsala, Friday, 14 June 2019 at 10:00 for the degree of Doctor of Philosophy. The examination will be conducted in English. Faculty examiner: Emily Stanley (Center for Limnology and Department of Integrative Biology, University of Wisconsin).

Fulltext available: http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-381847

Abstract:

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 incorporatedinto 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 inboreal 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 toradiative 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 isotopecharacterization of various dissolved C forms in stream and groundwater, within contrastingecosystem 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 duringits transport from soils to streams. These include; methanogenesis, aquatic DOC mineralizationand primary production. The majority of C in boreal streams is sustained by the decomposition ofrecent 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-usechanges, will produce a rapid response in stream CO2 emissions. Isotopic characterization of Cin stream and groundwater can help reveal these sources and transformation processes, but itsinterpretation must be made with care.