Das Projekt "Swiss Canopy Project: Dissolved organic matter dynamics in a mature deciduous forest under elevated CO2" wird vom Umweltbundesamt gefördert und von Eidgenössische Forschungsanstalt für Wald, Schnee und Landschaft durchgeführt. The exposure of a 100 year-old mixed broadleaf forest to elevated CO2 has little effects on dissolved organic matter. Our results suggest that CO2 enrichment can stimulate initial mineralization and leaching of C from litter by altering its quality, but these effects will be short-term and much smaller than any change in species composition. Rationale: The continued rise in atmospheric CO2 changes ecosystem functioning. Most CO2 enrichment experiments were conducted in young expanding ecosystem, much less is known on the response of older ecosystems. We are measuring DOM cycling within a unique CO2 enrichment study experiment - the Swiss Canopy Project - led by the Institute of Botany, University of Basel (Prof. Körner). Here, a 100 year-old forest with beech, oak, hornbeam, cherry, maple, and linden trees is exposed to elevated CO2 with a so-called Web-Face which was woven into the tree crowns with the help of a crane. The added CO2 is depleted in 13C as compared to normal air, allowing to trace assimilated CO2 through the tree and soil system and to gain insight into soil C cycling. Aims: The objectives of our contribution are (1) to quantify the response of DOM leaching to increasing atmospheric CO2 and (2) to trace the input of recent photosynthates (root exudates, litter) into dissolved organic carbon (DOC) of mature forests. Methods: We are collecting soil solutions under the litter layer and at 5 and 15 cm depth along three transects from un-treated trees to CO2-erniched trees (at 30 locations). We measure concentrations DOC, DON, hydrophobic/hydrophilics, phenolics and d13C signatures. In a lab study, we measured initial C mineralization and leaching rates of DOC, DON and biodegradable DOC from litter of eight tree species from the CO2 enrichment experiments in the deciduous forest and at the treeline. Results: In the field, DOC and DON concentrations were largely unaffected by elevated CO2. The laboratory study indicated that CO2 enrichment of forests can stimulate initial mineralization and leaching of C from litter by altering its quality, but these effects will be short-term and much smaller than any change in species composition (Hagedorn & Machwitz, 2007).