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

Abstract

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 CO₂ during 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.