Isotopic partitioning of evapotranspiration in a mesic grassland during two wetting–drying episodes

Abstract

Evapotranspiration (ET) is the dominant water loss flux in mesic tallgrass prairie. Partitioning of ET into its two components—soil evaporation (E) and plant transpiration (T)—is challenging but critical for unraveling biophysical processes underlying ecosystem functioning and sustainability in a changing environment. Because of the pulsed nature of ecophysiological processes in this water-limited ecosystem, we carried out two field campaigns during wetting–drying episodes following precipitation pulses. We applied a two-source isotopic mixing model for ET partitioning. The isotopic compositions of ET, E, and T (δET, δE, and δT) were determined by the Keeling-plot method, the Craig–Gordon model, and midday plant xylem water, respectively. We found that the ET partitioning results (T/ET) could be more accurately quantified with 2H than with 18O, because of (1) the better performance of 2H in Keeling-plot regressions of high-temporal-frequency isotopic measurements of water vapor, and (2) the stronger sensitivity of 2H to the equilibrium fractionation. Using 2H values, we found that the mean ± standard deviation of T/ET was 0.84 ± 0.05 and 0.92 ± 0.06 during two field campaigns. Soil water near the surface (especially the top 10 cm) responded actively during these two wetting–drying episodes and was the major source for the total ET flux during the initial drying periods. Only after shallow soil moisture had become substantially exhausted did deeper soil layers (up to 1 m) increasingly become the major source for the T flux, while the E flux declined progressively to a negligible level.