Carbon and water fluxes in Beech-Spruce model ecosystems in response to long-term exposure to atmospheric CO2 enrichment and increased nitrogen deposition

1. We present data on the gas exchange of 32 Beech-Spruce model ecosystems including understorey species, which were planted on acidic or calcareous s oil in 16 (half-split) open-top chambers in 1994 and were continuously expo sed to the combinations of two CO2 concentrations and two wet nitrogen depo sition rates for 3 years. 2. Instantaneous mid-season net ecosystem carbon-exchange rates (NEC) were determined during the day (11.00-16.00 h, NECd) and at night (22.00-02.00 h , NECn) with a mobile chamber in July 1996 and August 1997 for both soil ty pes and, in addition, in August and September 1996 on acidic soil only. 3. Elevated CO2 increased NECd at all sampling dates on both soil types and under both N deposition rates. Increased nitrogen deposition stimulated NE Cd on acidic soil but not on calcareous soil, irrespective of the CO2 level . NECn was increased in elevated CO2 on both soil types (but more on the ca lcareous soil), but did not respond to increased nitrogen deposition in bot h years. The daytime instantaneous ecosystem evapotranspiration (ETd) was n ot significantly affected by the CO2 and nitrogen treatments at any samplin g date. Year to year differences in NECn and ETd were small, but a doubling in NECd 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 bio mass production on acidic soil only. Leaf area index was not affected by CO 2, 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.