1. We present data on the gas exchange of 32 Beech–Spruce model ecosystems including understorey species, which were planted on acidic or calcareous soil in 16 (half‐split) open‐top chambers in 1994 and were continuously exposed to the combinations of two CO₂concentrations and two wet nitrogen deposition rates for 3 years.
2. Instantaneous mid‐season net ecosystem carbon‐exchange rates (NEC) were determined during the day (11.00–16.00 h, NEC_d) and at night (22.00–02.00 h, NEC_n) with a mobile chamber in July 1996 and August 1997 for both soil types and, in addition, in August and September 1996 on acidic soil only.
3. Elevated CO₂ increased NEC_d at all sampling dates on both soil types and under both N deposition rates. Increased nitrogen deposition stimulated NEC_d on acidic soil but not on calcareous soil, irrespective of the CO₂ level. NEC_n was increased in elevated CO₂ on both soil types (but more on the calcareous soil), but did not respond to increased nitrogen deposition in both years. The daytime instantaneous ecosystem evapotranspiration (ET_d) was not significantly affected by the CO₂ and nitrogen treatments at any sampling date. Year to year differences in NEC_n and ET_d were small, but a doubling in NEC_d occurred throughout the observation period.
4. Elevated CO₂ caused a small but significant increase in stem dry mass on calcareous soil only and increased nitrogen deposition stimulated stem biomass production on acidic soil only. Leaf area index was not affected by CO₂, but was increased under high nitrogen deposition.
5. Our results suggest that nitrogen deposition and soil type are major co‐determinants of the carbon balance of forests in a future CO₂‐rich world.