Understanding Terrestrial carbon and water exchanges through carbonyl sulfide (OCS) and carbon-13 isotope
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Carbon-water coupling within terrestrial plants regulates carbon uptake and water loss in natural and agricultural systems. However, current estimations of carbon-water interactions at regional and global scales show remarkable divergence, e.g., on the magnitude and spatial patterns of terrestrial carbon sink and on the historical trajectories of global water use efficiency (WUE). This estimation uncertainty persists because carbon-water fluxes are not directly measurable beyond canopy scales. I address this uncertainty by integrating two innovative tracers consumed by plants via a pathway coupled with CO$_2$ and H$_2$O diffusion—carbonyl sulfide (OCS) and carbon-13 isotope ($^{13}$C). Through a unified framework to constrain large-scale terrestrial carbon-water fluxes with both tracers, my dissertation enhances our predictive understanding of carbon-water interactions.The introduction identifies knowledge gaps and evaluates current approaches for quantifying carbon-water fluxes. Chapter 2 presents a case study in Northwestern China, a dryland ecosystem exposing vulnerabilities of coupled carbon-water systems. With satellite products, climate reanalysis, and census statistics, I detect a significant two-decade freshwater depletion caused by escalating irrigation demands from agricultural expansion. Subsequent chapters present discoveries from my modeling-based framework to integrate OCS and $^{13}$C, using Community Land Model 5 (CLM5). Critical to this is the explicit treatment of mesophyll diffusion—an important barrier for OCS and CO$_2$ inside leaves previously ignored in models. Chapter 3 introduces an OCS-based breakthrough in terrestrial gross primary production (GPP) estimation. My global estimate (157 ± 8.5 Pg C/yr) is consistent with independent estimates from oxygen-18 isotope and soil respiration, but higher than current data-driven estimates. I reveal more productive tropics than previously thought, altering the understanding of tropical productivity. Chapters 4 and 5 address debates on ecosystem WUE (eWUE) trajectories—whether the CO$_2$-driven eWUE benefit has been compensated for by the adverse effect from vapor pressure deficit (VPD)—through $^{13}$C applications. I developed a method to estimate eWUE from intrinsic WUE constrained by atmospheric $^{13}$C signals. Chapter 4 refines the carbon isotopic discrimination ($\Delta$ 13) model in CLM5—essential to infer intrinsic WUE, reproducing the $\Delta$ 13 increase deduced from atmospheric $^{13}$C measurements. Chapter 5 uses this advanced model and $^{13}$C measurements to estimate global eWUE, revealing that CO$_2$-driven increases are reduced but not overshadowed by VPD-driven losses. Finally, Chapter 6 summarizes these discoveries and offers future research perspectives.