The study presents the first simultaneous in-situ eddy covariance measurements of CO₂ and H₂O isotopologue fluxes above an Amazonian rainforest canopy. Using δ¹⁸O as a shared tracer, the authors reconstruct a coherent isotopic cascade linking soil, xylem, leaf, canopy, and atmosphere for both water and carbon dioxide, and demonstrate that the two isotopic states are physically consistent at the leaf evaporative sites where they intersect. This allows the elusive gross components of net exchange to be resolved: transpiration accounts for 95.5% of evapotranspiration, while photosynthesis (144% of NEE) and soil respiration (-44%) are separated. As the Amazon shifts from carbon sink toward source, such methods offer a valuable route to disentangle carbon–water coupling.