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 δ13C and δ18O in atmospheric CO2, as well as its concentration, were measured in a forest in the central part of the main island of Japan during the period from June 1994 to June 1996 to examine the biospheric contribution to their temporal variations. δ13C shows a prominent diurnal variation with high values in the daytime and low values in the nighttime, especially during the warm season. δ13C also vary seasonally, showing a maximum in summer and a minimum in spring. The diurnal and seasonal variations of δ13C are opposite in phase with those of the CO2 concentration. The rate of change in δ13C with respect to the CO2 concentration is found to be approximately -0.05‰/ppmv. This suggests that the diurnal and seasonal variations of the CO2concentration are produced primarily by diurnally- and seasonally-dependent photosynthetic-respiratory processes of the biosphere near the observation site, respectively. In the warm season, δ18O also increases in the daytime and decreased in the nighttime, which is similar to the diurnal variation of δ13C, but opposite to that of the CO2 concentration. The diurnal δ18O variation is thought to be caused by the release of isotopically heavy CO2during photosynthesis, and light CO2 during respiration. However, an interpretation of the seasonal δ18O variation is found to be much more difficult than those of δ13C and the CO2 concentration. This is likely due to complicated combinations of different seasonally varying fluxes of biospheric CO2 into the atmosphere, as well as to various weather-dependent factors governing the δ18O composition in CO2.DOI: 10.1034/j.1600-0889.49.issue4.3.x