In a mature mixed forest (NW Switzerland), the nutrients in the soil solution were monitored over the first year of canopy enrichment with CO₂ (+140 μmol.mol^-1). For this purpose, ceramic suction cups and resin bags were installed along three transeccs through the experimental area under control and CO₂-treated trees: one transect under beech (Fagus sylvatica), one under oak (Quercus perraea. Q. robur) and one under hornbeam (Carpinus betulus). There was an indication of enhanced nitrate availability under beech and oak trees exposed to elevated CO₂, but not under hornbeam trees. There was no δ¹³C-signal in the dissolved organic carbon (DOC) of the soil solution and only a small depletion of ¹³C in the roots of CO₂-enriched trees at the end of the first growing season. Therefore it is probably too early to decide whether the observed nutrient effects were due to the CO₂-treatment or to the natural variability of the soil.