Insights into tension-mediated and antecedent water effects on soil water isotopic composition

Using isotopic spike experiments, we investigated the existence and magnitude of soil-mediated isotopic effects and of the interaction between isotopically distinct soil water pools, both associated in isotopic mismatches between water extracted from soil and soil water taken up by the roots. For this, we applied and compared four established techniques commonly used for the extraction of water (vapor) from soil, three of them relying on destructive soil sampling (cryogenic vacuum distillation, centrifugation, and direct water vapor equilibration), and one being a nondestructive in situ online technique. We observed an almost complete mixing of sequentially added, isotopically distinct water samples to a pure quartz sand (memory effect). The isotopic composition of water held at high soil tension in the pure quartz sand (pF = 2) as well as in a sandy soil (pF = 1.8 and 3) deviated considerably from that of the added water (tension effect). However, we could attribute this deviation not exclusively to a soil-mediated effect but also to methodological shortcomings during our experiments. Finally, we found the following decreasing trend in precision as well as in accuracy of the used water extraction methods: in situ online > centrifugation > direct water vapor equilibration > cryogenic vacuum distillation. The investigation of isotopic fractionation of soil water due to physicochemical processes in soil can be facilitated if the experimental techniques used do not involve isotopic fractionation. In addition, methodological uncertainties and inaccuracies can be minimized by method standardization, increasing the potential of water stable isotopic monitoring in ecohydrological studies.