Towards continuous, long-term eddy covariance measurements of CO2 isotopologues

Eddy covariance measurements are a standard practice when measuring fluxes of CO₂ between the surface and the atmosphere. However, they may be limited in their use for disentangling and quantifying the different contributors to these fluxes (i.e. sources and sinks). Measuring the stable isotopologue composition of the CO₂ flux, using ¹³C and ¹⁸O signatures, allows for a more direct flux partitioning, based on differences in the isotopologue composition of CO₂ sources in ecosystems. Yet, studies investigating isotopologue fluxes remain scarce and mostly limited to short time periods on weekly or monthly scales.

Here we present a new setup for continuous, long-term stable isotopologue eddy covariance measurements of CO₂ using a quantum cascade laser absorption spectrometer and evaluate ongoing data collection over a two-year period (2025-2027) in a managed beech forest in central Germany. Furthermore, we discuss the calibration strategy and performance requirements necessary to conduct high-frequency isotopologue measurements suitable for eddy covariance applications and present first flux calculation results.

First results show that frequent instrument calibration of the isotope raw reading is critical and must be performed regularly. We identify pressure and temperature fluctuations as major sources of instrumental drift. To address this, we developed an automated calibration system that performs hourly drift corrections and daily concentration-dependence corrections to reach the precision needed for eddy covariance measurements and resolve the subtle differences in the environmental signal.

Our results highlight important methodological requirements, for continuous, long-term, isotopologue eddy covariance measurements. This work can act as a stepping stone toward the implementation of similar measurements into existing flux observation networks such as ICOS or FLUXNET. Furthermore, this represents an important step toward using stable isotopologues to better understand ecosystem-atmosphere exchange processes by characterizing greenhouse gas sources and sinks in ecosystems.