Stable Isotope Comparison of Soil and Saprolite Waters Extracted via Centrifugation and Cryogenic Vacuum Distillation

Abstract

Water stable isotope characterization is a central tool in ecohydrology. Recent research highlights that selecting water extraction methods from soil samples substantially impacts data interpretation. Here, we compare waters in soil and saprolite extracted via centrifugation at different water potentials to the composition of residual water (post centrifugation) extracted via cryogenic vacuum distillation. Five pits were sampled at 10 cm increments to 1.5 m across an experimental hillslope in the Northern California Coast Ranges. The site is steep, north-facing, with a mixed hardwood and conifer canopy dominated by old-growth Douglas Fir. The soil varies from about 30-80 cm thickness and is stony, highly porous, with low organic content and is derived from underlying mudstone and sandstone. We sampled soon after a snow event to use the snow’s light isotopic signal as a natural tracer. Extracting soil water began with centrifugation at a tension of -2.5MPa, representing the plant available pool. All water at tension -2.5MPa was removed before samples were sequentially centrifuged at -5MPa, representing a “transitional” pool between plant-accessible and residual pools. The most tightly-held, residual pool was cryogenically extracted post-centrifugation. The water stable isotope composition of extracted soil waters and precipitation samples collected at the site were measured via IRMS. The isotopically depleted signal of the snow event is seen in both centrifuged and cryogenic extractions of all five sampling locations, extending about 30cm into the soil. However, centrifuged extractions are isotopically heavier than cryogenic extractions at all depths in the five profiles. Centrifuged extractions from -2.5MPa and -5MPa are relatively indistinguishable. Together, this suggests that infiltrating water mixes readily between plant available and less-available waters. Centrifuge and cryogenic extracted waters both plot along the local meteoric water line. Further work is needed to explore the mechanism underlying differences in water stable isotope composition of waters extracted via different methods to better understand in-situ mixing processes in the critical zone.