Fluxes and concentrations of CO₂ isotope compositions provide information about not only carbon balance but also ecosystem physiological processes and their response to environmental variation. Continuous measurement of ¹³CO2 over various time-scales (diurnal, daily, seasonal..) using a tunable diode laser spectrometer (TDLS) help us to assess the impact of environmental variability on plant metabolism and water relations.

For testing the performance of a TDLS (G2101-i, Picarro Inc.), we measured the air from two different CO₂ concentration cylinders and then analyze Allan's variance. The Allan standard deviations of δ¹³C for integration time of 500 seconds were 0.22‰ (389ppm CO₂) and 0.09‰ (715ppm CO₂). Using this TDLS, we continuously measured the concentrations of ¹²CO₂ and ¹³CO₂ inside the automated closed dynamic soil chambers and the surrounding atmosphere of a cool-temperate coniferous forest on the northern slope of Mount Fuji in Japan (AsiaFlux site code:FJY). The δ¹³C values of the atmospheric CO₂ were fluctuated according to the changes of CO₂ concentration. Fluctuations of the carbon isotope signal from soil respired CO₂(δ¹³CR_s) was not detected, although Keeling plots to estimate δ¹³CR_s were very sensitive and the results were scattered.