A new method for sampling water in the soil-plant-atmosphere continuum for stable isotope analyses

<p>Stable isotopes (&#948;²H, &#948;¹⁷O and &#948;¹⁸O) are powerful tools to trace the dynamics of water within the Soil-Plant-Atmosphere Continuum (i.e., SPAC). However, the recovery of water from the SPAC requires logistical arrangements in the field and implementation of different time and cost consuming-techniques either in the field or the laboratory. We developed a passive method to extract water from the three compartments of this continuum using a hygroscopic salt (anhydrous CaCl₂) of a high-water absorbance capacity (i.e., up to 300% of its dry weight). This method operates under two different modes which are (1) <em>infinite water reservoir</em> when the water fraction is much larger than the absorbance capacity of CaCl₂ and (2) <em>finite water reservoir</em> when the water fraction is lower than or equal to the absorbance capacity of CaCl₂. The first mode facilitates the isotope equilibration between H₂O_g (vapor) and H₂O_L (i.e., brine). Hence, the isotope composition of vapor (&#948;²H, &#948;¹⁷O and &#948;¹⁸O) can be back-calculated from that of the brine based upon the temperature-dependence isotope fractionation factor and the salt isotope effect (Horita et al., 1993; Horita and Wesolowski, 1994). By contrast, the second mode allows a quantitative absorbance of the whole water reservoir by the CaCl₂.The resulting isotope composition of the brine is similar to that of the finite water reservoir without significant isotope fractionation. The two modes were applied to different matrices including atmospheric water vapor, plant water, insect body water (see Herwartz et al. this conference) and soil water.</p> <p>&#160;</p> <p>The reproducibility (1&#963;) of our method under the two modes is better than 1.5 &#8240;, 0.32 &#8240;, 0.17 &#8240; and 6 per meg for &#948;²H, &#948;¹⁸O, &#948;¹⁷O and ¹⁷O-excess that is similar to the analytical uncertainty associated with the IRMS techniques. Our method facilitates vapour and soil water sampling while requiring only very limited logistics in the field. It also allows high sample throughput for the extraction of soil and biological tissue water in the laboratory. Moreover, the laboratory extraction is performed at low extraction temperature under ambient pressure minimizing the release of organic volatiles in extracted water. The details of the methodology and its applications will be discussed in this meeting.</p> <p><strong>References: </strong></p> <p>Horita J, Cole D R, Wesolowski DJ. The activity composition relationship of oxygen and hydrogen isotopes in aqueous salt solutions: II. Vapor-liquid water equilibration of mixed salt solutions from 50 to 100 ^oC and geochemical implications. Geochim. Cosmochim. Act. 1993; 57: 4703&#8211;4711.</p> <p>Horita J, Wesolowski DJ. Liquid-vapor fractionation of oxygen and hydrogen isotopes of water from the freezing to the critical temperature. Geochim. Cosmochim. Acta. 1994; 58: 3425&#8211;3437</p>