Partitioning ecosystem net water fluxes using stable oxygen isotopes and the link to productivity in a Mediterranean oak woodland

Maren Dubbert1, Arndt Piayda2, Filipe Costa e Silva3, Alexandra C. Correia3, Joao S. Pereira3, Matthias Cuntz4, Christiane Werner5
1 Agrarökosystemforschung, Uni Bayreuth
2 UFZ - Leipzig
3 University of Lisbon
4 UFZ-Leipzig
5 Agrarökosystemforschung Uni Bayreuth

O 3.6 in Fluxes between the atmosphere and ecosystems

17.07.2014, 15:30-15:50, H17

Water is one of the key factors driving ecosystem productivity, especially in water-limited ecosystems. Thus a separation of these component fluxes is needed to gain a functional understanding on the development of net ecosystem water and carbon fluxes. Oxygen isotope signatures are valuable tracers for such water movements within the ecosystem because of the distinct isotopic compositions of water in the soil and vegetation. In the past, determination of isotopic signatures of evaporative or transpirational fluxes has been challenging since measurements of water vapor isotopes were difficult to obtain using cold-trap methods. Recent developments in laser spectroscopy now enable direct high frequency measurements of the isotopic composition of atmospheric water vapor (δv), evapotranspiration (δET), and its components and allow validations of common modeling approaches for estimating δE and δT based on Craig and Gordon (1965).
Here, a novel approach was used, combining a custom build flow-through gas-exchange branch chamber with a Cavity Ring-Down Spectrometer in a Mediterranean cork-oak woodland where two vegetation layers respond differently to drought: oak-trees (Quercus suber L.) avoid drought due to their access to ground water while herbaceous plants survive the summer as seeds. We used this approach to quantify the impact of the understory herbaceous vegetation on ecosystem carbon and water fluxes throughout the year and disentangle how ET components of the ecosystem relate to carbon dioxide exchange.
We present one year data set comparing modeled and measured stable oxygen isotope signatures (δ18O) of soil evaporation, confirming that the Craig and Gordon equation leads to good agreement with measured δ18O of evaporation (Dubbert et al., 2013). Moreover, we found continuously strong deviations from isotopic steady-state in plant transpiration. This implies that assuming plant transpiration to be in the steady-state can have a huge impact for studies that distinguish relatively short time intervals (hours, e.g. partitioning studies). Finally. partitioning ecosystem ET and NEE into its three sources revealed that understory vegetation contributed markedly to ecosystem ET and gross primary production (GPP; max. 43 and 51%, respectively). It reached similar water-use efficiencies (WUE) as cork-oak trees and significantly contributed to the ecosystem sink-strength in spring and fall. The understory vegetation layer further strongly inhibited soil evaporation (E) and, although E was large during wet periods, it did not diminish ecosystem WUE during water-limited times.

Craig H, Gordon, LI. 1965. Deuterium and oxygen-18 variations in the ocean and the marine atmosphere. Paper presented at the Stable Isotopes in Oceanographic Studies and Paleotemperatures, Spoleto, Italy.
Dubbert M, Cuntz M, Piayda A, Maguas C, Werner C, 2013: Partitioning evapotranspiration - Testing the Craig and Gordon model with field measurements of oxygen isotope ratios of evaporative fluxes. J Hydrol.

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last modified 2014-06-19