Swiss Canopy Project: Dissolved organic matter dynamics in a mature deciduous forest under elevated CO2

CO2 enrichment of the 100y old deciduous forest with the help of a crane. Foto: Frank Hagedorn.

Dissolved organic matter = liquid gold. Foto: Frank Hagedorn.

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.
Partners: Institute of Botany, University of Basel (Ch. Körner), Patrick Schleppi (WSL)

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).

Hagedorn, F., Machwitz M. (2007): Controls on dissolved organic matter leaching from forest litter grown under elevated atmospheric CO₂. Soil Biology & Biochemistry 39, 1759-1769.