Do increased CO2 concentrations affect the availability of N in a mature temperate forest?

Schleppi Patrick 1, Hagedorn Frank 1, Bucher-Wallin Inga 1, Körner Christian 2

1 Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), Zürcherstr. 111, CH-8903 Birmensdorf
2 Institute of Botany, University of Basel, Schönbeinstr. 6, CH-4056 Basel

NitroEurope Open Science Conference Reactive nitrogen and the European greenhouse gas balance, Gent, BE, 20-21/02/2008


Abstract

The interaction between atmospheric carbon dioxide (CO2) concentration and soil nitrogen (N) availability is critical in predicting the future effects of CO2 on forests. A currently debated hypothesis is that CO2-induced effects are limited due to the sequestration of N together with carbon and hence to a progressive N limitation in the ecosystem.
In a mature mixed forest in northern Switzerland, nutrients in the soil solution were monitored during five years of canopy enrichment with CO2 (+140 ppm). Ceramic suction cups and resin bags were installed along three transects through the experimental area under control and CO2-treated trees: one transect under beech (Fagus sylvatica), one under oak (Quercus petraea, Q. robur) and one under hornbeam (Carpinus betulus). Because the CO2 used for the treatment is depleted in 13C, it was possible to obtain a measure of how much each soil sampling point is affected by this treatment. In spite of a strong spatial and temporal variability, there was an indication that the nitrogen availability (nitrate, ammonium) increases at locations with more 13C depletion compared to locations with less depletion. The mature trees of this experiment do not express a sustained growth enhancement when exposed to more CO2, but they do have reduced foliar N concentrations. Their N demand is thus diminished and this probably explains why more N is left available in the soil.