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