Evaluation of CO2 Exchange Rates in a Wetland Ecosystem Using the Closed Geosphere Experiment Facility

Abstract

o evaluate annual CO2 exchange rates in a wetland ecosystem, ecosystem respiration rate (Re), net ecosystem productivity (NEP), and gross primary productivity (GPP) were investigated using the Closed Geosphere Experiment Facility (CGEF) located in northeastern Japan. The CGEF is highly airtight and equipped with a Geosphere Module (GM). The GM has a ground area of 5.8 ? 8.7 m2 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 CO2 concentration in the GM were controlled automatically. The hourly nighttime Re 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 CO2 exchange rates (Re, NEP, and GPP) were influenced by above-ground plant biomass. The annual NEP was found to be 64.2 ± 19.2 g C m?2 yr?1 and it resulted from the annual GPP of 555.8 ± 17.0 g C m?2 yr?1 and annual Re of ?491.6 ± 15.6 g C m?2 yr?1. Therefore, the wetland ecosystem behaved as a CO2 sink for the entire year. The annual CO2 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.