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

[摘要] We conducted high frequency measurements of the δ¹⁸O value ofatmospheric CO₂ from a juniper (Juniperus monosperma) woodland in New Mexico, USA,over a four-year period to investigate climatic and physiologicalregulation of the δ¹⁸O value of ecosystem respiration (δ_R). Rain pulses reset δ_R with the dominant water sourceisotope 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 strongenrichment with increasing vapor pressure deficit that occurs followingrain. Post-pulse δ_R enrichment was correlated with bothE_T and the ratio of E_T to soil evaporation (E_T/E_S). Incontrast, the soil water δ¹⁸O value was relatively stable andδ_R enrichment was not correlated with E_S. Modelsimulations captured the large post-pulse δ_R enrichments onlywhen the offset between xylem and leaf water δ¹⁸O value wasmodeled explicitly and when a gross flux model for CO₂ retro-diffusionwas included. Drought impacts δ_R through the balance betweenevaporative demand, which enriches δ_R, and low soil moistureavailability, which attenuates δ_R enrichment through reducedE_T. The net result, observed throughout all four years of our study, wasa negative correlation of post-precipitation δ_R enrichment withincreasing drought.

[学科分类] 地球化学与岩石