TEMPORAL VARIATIONS OF THE CO2 CONCENTRATION AND ITS CARBON AND OXYGEN ISOTOPIC-RATIOS IN A TEMPERATE FOREST IN THE CENTRAL PART OF THE MAIN ISLAND OF JAPAN

Using discrete air sampling, values of delta(13)C and delta(18)O in at mospheric CO2, as well as its concentration, were measured in a forest in the central part of the main island of Japan during the period fro m June 1994 to June 1996 to examine the biospheric contribution to the ir temporal variations. delta(13)C shows a prominent diurnal variation with high values in the daytime and low values in the nighttime, espe cially during the warm season. delta(13)C also vary seasonally, showin g a maximum in summer and a minimum in spring. The diurnal and seasona l variations of delta(13)C are opposite in phase with those of the CO2 concentration. The rate of change in delta(13)C with respect to the C O2 concentration is found to be approximately -0.05 parts per thousand /ppmv. This suggests that the diurnal and seasonal variations of the C O2 concentration are produced primarily by diurnally-and seasonally-de pendent photosynthetic-respiratory processes of the biosphere near the observation site, respectively. In the warm season, delta(18)O also i ncreases in the daytime and decreased in the nighttime, which is simil ar to the diurnal variation of delta(13)C, but opposite to that of the CO2 concentration. The diurnal delta(18)O variation is thought to be caused by the release of isotopically heavy CO2 during photosynthesis, and light CO2 during respiration. However, an interpretation of the s easonal delta(18)O variation is found to be much more difficult than t hose of delta(13)C and the CO2 concentration. This is likely due to co mplicated combinations of different seasonally varying fluxes of biosp heric CO2 into the atmosphere, as well as to various weather-dependent factors governing the delta(18)O composition in CO2.