Temporal Dynamics of Oxygen Isotope Compositions of Soil and Canopy CO2 Fluxes in a Temperate Deciduous Forest

Abstract:

The stable isotopomer of CO₂, C¹⁸OO, is a valuable tracer and have been used to study the CO₂ exchange in different spatial scales. The objectives of this study were: to quantify the magnitude of isotopic desiquilibrium (D_eq) in a temperate deciduous forest throughout the growing season, and to determine the sensitivity of D_eq to CO₂ hydration efficiency. In this study, C¹⁸OO and H₂¹⁸O composition of the air were measured quasi-continuously and simultaneously above a temperate deciduous forest using tunable diode laser trace gas analyzers. In addition, detailed measurements of H₂¹⁸O composition of ecosystem water pools were performed throughout the experimental period. These field measurements were used with existing formulations to estimate the C¹⁸OO of soil (δ_R) and canopy (δ_R) CO₂ fluxes. Values of δ_A also showed large day to day variation in our site. The dynamics of δ_R was mainly driven by changes in soil water isotope composition, caused by precipitation events. The magnitude of D_eq was variable throughout the season and very sensitive to the extent of the CO₂ hydration in the canopy. In this ecosystem, for most of the growing season, the magnitude of D_eq was inversely proportional to θ_eq. This can be explained by the very negative δ_R signal in our ecosystem.