Ecosystem carbon exchange in two terrestrial ecosystem mesocosms under changing atmospheric CO₂ concentrations

The ecosystem-level carbon uptake and respiration were measured under different CO₂ concentrations in the tropical rainforest and the coastal desert of Biosphere 2, a large enclosed facility. When the mesocosms were sealed and subjected to step-wise changes in atmospheric CO₂ between daily means of 450 and 900 μmol mol^-1, net ecosystem exchange (NEE) of CO₂ was derived using the diurnal changes in atmospheric CO₂ concentrations. The step-wise CO₂ treatment was effectively replicated as indicated by the high repeatability of NEE measurements under similar CO₂ concentrations over a 12-week period. In the rainforest mesocosm, daily NEE was increased significantly by the high CO₂ treatments because of much higher enhancement of canopy CO₂ assimilation relative to the increase in the nighttime ecosystem respiration under high CO₂. Furthermore, the response of daytime NEE to increasing atmospheric CO₂ in this mesocosm was not linear, with a saturation concentration of 750 μmol mol^-1. In the desert mesocosm, a combination of a reduction in ecosystem respiration and a small increase in canopy CO₂ assimilation in the high CO₂ treatments also enhanced daily NEE. Although soil respiration was not affected by the short-term change in atmospheric CO₂ in either mesocosm, plant dark respiration was increased significantly by the high CO₂ treatments in the rainforest mesocosm while the opposite was found in the desert mesocosm. The high CO₂ treatments increased the ecosystem light compensation points in both mesocosms. High CO₂ significantly increased ecosystem radiation use efficiency in the rainforest mesocosm, but had a much smaller effect in the desert mesocosm. The desert mesocosm showed much lower absolute response in NEE to atmospheric CO₂ than the rainforest mesocosm, probably because of the presence of C₄ plants. This study illustrates the importance of large-scale experimental research in the study of complex global change issues.