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

Abstract. We conducted high frequency measurements of the δ¹⁸O value of atmospheric CO₂ from a juniper (Juniperus monosperma) woodland in New Mexico, USA, over a four-year period to investigate climatic and physiological regulation of the δ¹⁸O 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 (E_T) was significantly related to post-pulse δ_R enrichment because the leaf water δ¹⁸O value showed strong enrichment with increasing vapor pressure deficit that occurs following rain. Post-pulse δ_R enrichment was correlated with both E_T and the ratio of E_T to soil evaporation (E_T/E_S). In contrast, the soil water δ¹⁸O value was relatively stable and δ_R enrichment was not correlated with E_S. Model simulations captured the large post-pulse δ_R enrichments only when the offset between xylem and leaf water δ¹⁸O value was modeled explicitly and when a gross flux model for CO₂ 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 E_T. The net result, observed throughout all four years of our study, was a negative correlation of post-precipitation δ_R enrichment with increasing drought.