ABSTRACT
We conducted continuous, high time-resolution measurements of CO2 and water vapour isotopologues (16O12C16O, 16O13C16O and 18O12C16O for CO2, and H218O for water vapour) in a red pine forest at the foot of Mt. Fuji for 9 days from the end of July 2010 using in situ absorption laser spectroscopy. The δ18O values in water vapour were estimated using the δ2H–δ18O relationship. At a scale of several days, the temporal variations in δ18O-CO2 and δ18O-H2O are similar. The orders of the daily Keeling plots are almost identical. A possible reason for the similar behaviour of δ18O-CO2 and δ18O-H2O is considered to be that the air masses with different water vapour isotopic ratios moved into the forest, and changed the atmosphere of the forest. A significant correlation was observed between δ18O-CO2 and δ13C-CO2 values at nighttime (r2≈0.9) due to mixing between soil (and/or leaf) respiration and tropospheric CO2. The ratios of the discrimination coefficients (Δa/Δ) for oxygen (Δa) and carbon (Δ) isotopes during photosynthesis were estimated in the range of 0.7–1.2 from the daytime correlations between δ18O-CO2 and δ13C-CO2 values.
Acknowledgements
We thank Dr A. Shimono of Shoreline Science Research Inc. for valuable suggestions on the development of the instrumentation used in this study. We also thank Dr Y. Takahashi at NIES for measuring δ13C-CO2 and δ18O-CO2 levels in standard gases by IRMS for calibration of the instrument.
Disclosure statement
No potential conflict of interest was reported by the authors.
ORCID
Yutaka Matsumi http://orcid.org/0000-0002-5881-1899