Oxygen isotope analysis
Root crown and leaf water oxygen isotope ratios
(δ18OLW) were analyzed by isotope
ratio mass spectrometer (IRMS). A 0.3% CO2/He gas
mixture was flushed through the vials and equilibrated with water for 48
h on a ThermoFinnigan GasBench II (Bremen). The equilibrated
CO2 gas was separated at 40 °C with a 25 m x 0.32 mm ID
GC column (Varian, poraplot Q) and analyzed with a continuous flow
isotope ratio mass spectrometer (Delta PlusXP, Thermofinnigan, Bremen)
(Brenna et al. 1997;
Qi et al. 2003) to derive the
oxygen isotope ratios of the water. Standards (SLAP and Puerto Rico
water) were analyzed alongside the samples. Final delta values were the
mean of 5 sample peaks reported relative to Vienna Standard Mean Ocean
Water (V-SMOW).
Irrigation water and water vapor were measured by isotope ratio-cavity
ringdown spectroscopy using a water analyzer (model L1102-i, Picarro,
Sunnydale, CA, USA). Each sample was analyzed six consecutive times, and
the mean of the last three analyses was used. Three laboratory
standards, calibrated to the V-SMOW scale, were interspersed among the
samples and used to correct the sample δ18O values to
the V-SMOW scale. Water vapor was introduced into the water analyzer
through the inlet valve and analyzed for at least 15 minutes, and the
δ18O of water vapor
(δ18Ov) was calculated as the mean
δ18O over the last five minutes. The
δ18Ov was corrected to the V-SMOW
scale in the same way that irrigated water samples were corrected.
Further δ18Ov was corrected for water
concentration dependency. The stable isotope composition of oxygen
(δ18O) was reported in δ notation (Eqn. 12),