In situ measurement of CO₂ and water vapour isotopic compositions at a forest site using mid-infrared laser absorption spectroscopy

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 (Δ_a/Δ) for oxygen (Δ_a) 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.

KEYWORDS:

• Carbon-13
• forest
• discrimination coefficient; hydrogen-2
• isotope measurement methods and equipment
• isotope ecology
• isotopologues
• laser spectroscopy
• oxygen-18
• temporal variations
• water vapour

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

Additional information

Funding

This work was supported by the Grant-in-Aid for Scientific Research [KAKENHI 24310012] and the joint research programme of the Institute for Space–Earth Environmental Research, Nagoya University.