This work studies the effect of increased atmospheric carbon dioxide concentration on carbon and water budgets of model ecosystems. In three long-term investigations, mixed plant stands of Trifolium pratense Huds. and Festuca pratensis L. were exposed to CO₂ concentrations of 350, 450, 600, and 800 ppm over a period of up to 1007 days.
A specially constructed measuring system, placed outdoors, enabled the exposure of four model ecosystems—consisting of these species in a soil block of 80 × 80 × 60 cm³—to natural environmental conditions over several vegetation periods.
Atmospheric CO₂ Trends and Biomass Accumulation
Continuous measurements from 1984 to 1991 showed a significant yearly increase in atmospheric CO₂ concentration of 2.8 ppm. Dry-matter accumulation (aboveground and roots) of the herbaceous plant stands was significantly enhanced under elevated CO₂ concentrations.
Using all phytomass data and a saturation model, a fertilization factor was determined, revealing an average increase in dry-matter accumulation of 18% (450 ppm), 50% (600 ppm), and 52% (800 ppm). This effect was particularly evident at the start of the vegetation period and after mowing.
Photosynthesis and CO₂ Exchange
Aboveground phytomass accumulation peaked at approximately 600 ppm, while roots acted as storage organs, incorporating additional carbon beyond this level. The reproductive organs, including seed and caryopsis weights, increased significantly under higher CO₂ concentrations.
The leaf area index (LAI) increased exponentially with CO₂ levels, with an average rise of 14% at 600 ppm and 35% at 800 ppm. Net CO₂ exchange rates (NCER) also increased, with enhancements of 21-31% (450 ppm), 48-58% (600 ppm), and 59% (800 ppm) compared to 350 ppm.
Acclimatization and Seasonal Effects
Long-term exposure led to acclimatization, as shown in a "switch experiment": NCER values were higher in ecosystems grown under high CO₂ compared to those adapted to lower CO₂ levels. Across all treatments, NCER saturation was observed at high LAI.
A linear correlation between daily photon flux densities and NCER was found. On high-light days, ecosystems at high CO₂ showed greater CO₂ exchange, whereas on cloudy days, CO₂ loss was higher compared to the 350 ppm control.
Annual Carbon Budget
The study (1984-1989) found that during April-August, carbon gain at 600 ppm was up to 40% higher than at 350 ppm. In the remaining months, net CO₂ gain decreased or CO₂ loss increased. Overall, the annual carbon budget of the 600 ppm ecosystem was 16% higher than that of the 350 ppm control.
Water Budget and Efficiency
Higher CO₂ concentrations affected the water budget. Evapotranspiration decreased at 450 ppm, remained stable at 600 ppm, and increased at 800 ppm, leading to reduced soil water storage in the highest CO₂ treatment. Water use efficiency improved significantly, enhancing productivity.
Grassland ecosystems function as "sinks" for anthropogenic CO₂ sources, but their sink capacity is considerably lower than expected.
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