Stable isotopic signatures in the turbulent ecosystem exchange of CO2 and H2O

Abstract

In this thesis I investigate how terrestrial ecosystems exchange carbon dioxide (CO₂) and water vapour (H₂O) with the atmosphere, with the aim of separating the individual processes that together form the net ecosystem fluxes. While standard micrometeorological methods reliably measure net exchange over 30-minute periods, these measurements combine multiple bidirectional components. For CO₂, this includes photosynthetic uptake by plants and respiration from soils; for H₂O, transpiration and soil evaporation. Disentangling these components is essential because they respond differently to environmental conditions and are represented separately in ecosystem and climate models. To address this challenge, I combine high-frequency measurements of the stable isotopic composition of CO₂ and H₂O with the eddy covariance method. Stable isotopes act as natural tracers because physical and biological processes leave distinct fractionation signatures. This allows net ecosystem fluxes to be partitioned into their underlying sources and sinks. I also use these measurements to investigate short-timescale turbulent exchange processes that are poorly constrained by observations but increasingly important for high-resolution models. First, using data from the LIAISE field campaign over an irrigated alfalfa field, I develop and evaluate data processing and correction methods required to obtain reliable isotopic fluxes. I show that high-frequency signal loss differs between isotopologues due to inlet-related phase changes and propose a best-practice workflow for calibration, time alignment, spectral correction, and quality control. I then apply these methods to measurements from the Amazon rainforest during the CloudRoots-Amazon22 campaign. I analyse intermittent turbulent events, particularly understory ejections, and assess their contribution to ecosystem exchange and their potential links to cloud passages. Finally, I demonstrate isotope-based partitioning of ecosystem fluxes above the Amazon. I find that evapotranspiration is dominated by transpiration (~95.5%), while net CO₂ exchange reflects large opposing fluxes of photosynthesis and soil respiration. Overall, this work advances the observational and methodological foundation for using stable isotopes to disentangle complex ecosystem–atmosphere exchange processes.