Study on carbon cycle in forest ecosystem using stable oxygen isotopic measurement

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Systematic measurements of atmospheric CO_2 concentration and its stable oxygen isotopic ratio (δ^<18>O) in and above the forest together with δ^<18>O of CO_2 in soil an, precipitation, soil water, water vapor and leaf water were made in a cool-temperate deciduous forest at Takayama site in central Japan. Using the observed data, temporal variations of δ^<18>O discriminations during photosynthesis and ecosystem respiration were estimated, and possibility of separating these carbon budget components from net ecosystem CO_2 exchange obtained from the eddy covariance flux measurement was examined. δ^<18>O of atmospheric CO_2 showed more complicated diurnal variation than CO_2 concentration, but prominent δ^<18>O increase appeared in the daytime under sunny and dry conditions, reflecting increase of photosynthetic ^<18>O discrimination. Seasonal variation of daytime δ^<18>O of CO_2 above the forest showed scattered but low values in winter and high values in spring. From late spring to early fall, significant year-to-year difference of the variation was seen with low and high values under respective rainy and sunny summer conditions, which may reflect difference of biological and hydrological processes due to that of meteorological conditions associated with the Asian summer monsoon. Comparison with the data at northern mid latitude sites suggested that the rapid decrease of δ^<18>O seen at Takayama in early summer of the rainy summer year is characteristic of the monsoon Asian region. From comparison of the CO^<18>O fluxes associated with photosynthesis and respiration calculated using a simple one-box canopy air model with the variation of the observed δ^<18>O of CO_2, it was found that the model cannot adequately simulate the δ^<18>O variation. The result suggested that analyses of the δ^<18>O data using a multi-layered canopy process model are necessary to precisely separate photosynthetic and respiratory CO_2 fluxes.