Isotopic composition of transpiration

USING STABLE ISOTOPES TO EXPORE THE WATER CYCLE

(Collaboration with Margaret Barbour and Kevin Simonin at University of Sydney)

Changes in the Earth’s climate will have large impacts on the exchange of water between the biosphere and atmosphere and biosphere. At the ecosystem scale, ET is tightly linked to gross primary productivity, particularly in areas where water is limiting.

ET is composed of two fluxes: soil evaporation (E) and plant transpiration (T), and partitioning ET is important because carbon dioxide uptake by plants is linked to water loss through transpiration and not evaporation. While many ecosystem-scale models attempt to partition ET, independent measurements of evaporation and transpiration are difficult in natural, complex ecosystems.

In recent years, stable isotopes of oxygen and hydrogen have been used to partition ET because the isotopic composition of T and E are different; however, the theories underlying this approach have been questioned. One potential shortcoming with the isotope approach is that good measurements or predictions of the isotope composition of the two distinct end members, E and T, are needed.

However, this can often be difficult to measure if there are large diurnal or seasonal fluctuations in response to climate variables. However, recent advances in isotope laser spectroscopy allow us to make high frequency measurements of both oxygen and hydrogen isotopes of water vapor simultaneously, and may offer an solution for ET partitioning.

We are currently using the Picarro water vapor isotope analyzer, coupled with the Li-6400 to make real-time, online measurements of the isotopic composition of transpiration on different varieties of wheat. These values will be compared to both steady state and non-steady state isotope models to assess under which climatic conditions the plants are transpiring at isotopic steady state. These estimates will then be scaled to the ecosystem to begin partitioning ET.