Evaluation of CO ₂ exchange rates in a wetland ecosystem using the closed geosphere experiment facility

To evaluate annual CO ₂ exchange rates in a wetland ecosystem, ecosystem respiration rate (R _e), net ecosystem productivity (NEP), and gross primary productivity (GPP) were investigated using the Closed Geosphere Experiment Facility (CGEF) located in northeastern Japan. The CGEFis highly airtight and equipped with a Geosphere Module (GM). TheGMhas a ground area of 5.8 3 8.7 m ² and an average height of 11.9 m, including a soil depth of 3.1 m. A wetland ecosystem dominated by Phragmites australis was introduced into the CGEF. Air temperature and CO ₂ concentration in the GM were controlled automatically. The hourly nighttime R _e increased exponentially with the hourly average air temperature. Both hourly NEP and GPP depended on hourly photosynthetic photon flux density (PPFD). In addition, daily ecosystem CO ₂ exchange rates (R _e, NEP, and GPP) were influenced by above-ground plant biomass. The annual NEP was found to be 64.2 6 19.2 g C m ^-2 yr ^-2 and it resulted from the annual GPP of 555.8 6 17.0 g C m ^-2 yr ^-2 and annual Re of 2491.6 6 15.6 g C m ^-2 yr ^-2. Therefore, the wetland ecosystem behaved as a CO ₂ sink for the entire year. The annual CO ₂ exchange rates obtained were reasonable values compared to the findings of published studies in P. australis-dominated wild wetlands using the eddy covariance technique and the combined method of internal gas pressures and flow measurements and harvesting.