Using stable isotopes of water to trace plant water uptake

The stable isotopes of water (2H, 18O) are apparently not fractionated as plants take up water, as has been shown in laboratory and glasshouse studies. The authors verify this for a field situation in a semiarid area of southeast Australia in which Eucalyptus spp. are growing on a sand dune. No fractionation implies that the stable isotopes of water in the conducting tissue of plants can be considered to be the sum of stable isotopes from the various soil water reservoirs from which the plants may be extracting. If there are large enough natural variations in the isotopic composition of the soil and groundwater, the stable isotopes in the plant conducting tissue can be used to assess the source of the water used by the plant. The authors have developed a method for doing this. For the vegetation investigated there was no significant difference between the isotopic compositions of heartwood and sapwood. Nor was there a trend in isotopic composition of sapwater from the base of the trunk to the extremities of the twig. This meant that twigs could be used for sampling. Moreover, the isotopic composition of the soil, in which the total suction was not high, matched well with the isotopic composition of the twigs for a range of soil conditions and a range of isotopic values (50 per mille for 2H and 9 per mille for 18O). On a 2H-18O plot, the twig data lay near the best fit soil line. These results showed that the isotope method was valid for plant water uptake studies in a semiarid field situation. (author). 11 refs, 2 figs