Hydrologic control of the oxygen isotope ratio of ecosystem respiration in a semi-arid woodland

  • Shim J
  • Powers H
  • Meyer C
  • et al.
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<p><strong>Abstract.</strong> We conducted high frequency measurements of the δ<sup>18</sup>O value of atmospheric CO<sub>2</sub> from a juniper (<i>Juniperus monosperma</i>) woodland in New Mexico, USA, over a four-year period to investigate climatic and physiological regulation of the δ<sup>18</sup>O value of ecosystem respiration (&amp;delta;<sub>R</sub>). Rain pulses reset &amp;delta;<sub>R</sub> with the dominant water source isotope composition, followed by progressive enrichment of &amp;delta;<sub>R</sub>. Transpiration (<i>E</i><sub>T</sub>) was significantly related to post-pulse &amp;delta;<sub>R</sub> enrichment because the leaf water δ<sup>18</sup>O value showed strong enrichment with increasing vapor pressure deficit that occurs following rain. Post-pulse &amp;delta;<sub>R</sub> enrichment was correlated with both <i>E</i><sub>T</sub> and the ratio of <i>E</i><sub>T</sub> to soil evaporation (<i>E</i><sub>T</sub>/<i>E</i><sub>S</sub>). In contrast, the soil water δ<sup>18</sup>O value was relatively stable and &amp;delta;<sub>R</sub> enrichment was not correlated with <i>E</i><sub>S</sub>. Model simulations captured the large post-pulse &amp;delta;<sub>R</sub> enrichments only when the offset between xylem and leaf water δ<sup>18</sup>O value was modeled explicitly and when a gross flux model for CO<sub>2</sub> retro-diffusion was included. Drought impacts &amp;delta;<sub>R</sub> through the balance between evaporative demand, which enriches &amp;delta;<sub>R</sub>, and low soil moisture availability, which attenuates &amp;delta;<sub>R</sub> enrichment through reduced <i>E</i><sub>T</sub>. The net result, observed throughout all four years of our study, was a negative correlation of post-precipitation &amp;delta;<sub>R</sub> enrichment with increasing drought.</p>




Shim, J. H., Powers, H. H., Meyer, C. W., Knohl, A., Dawson, T. E., Riley, W. J., … McDowell, N. (2013). Hydrologic control of the oxygen isotope ratio of ecosystem respiration in a semi-arid woodland. Biogeosciences, 10(7), 4937–4956. https://doi.org/10.5194/bg-10-4937-2013

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