Critical issues with cryogenic extraction of soil water for stable isotope analysis

Abstract

Cryogenic water extraction is the most widely used method to facilitate the laboratory removal of soil pore water for isotopic analysis. However, recent studies have suggested that cryogenic extraction conditions (extraction time, temperature, vacuum threshold) and physicochemical soil properties can influence extracted water isotopic signatures. Here, we argue that new work is needed to analyse the full extent of these effects on the extracted water isotopic composition. We illustrate this need with a simple lab experiment and show that in addition to extraction times, soil organic matter and its exchangeable bonded hydrogen fraction influence the resulting isotope composition. We hope these comments stimulate discussion on the assumptions and limitations of cryogenic extraction for soil water and lead ultimately to a standardization of testing approaches.

Soil water stable isotopes reveal evaporation dynamics at the soil–plant–atmosphere interface of the critical zone

“…However, recent experimental studies indicated potential for fractionation during the cryogenic extraction (Orlowski et al, 2016), probably induced by watermineral interactions (Oerter et al, 2014;Gaj et al, 2017a, b). The direct-equilibration method -as a potential alternative analysis method -we applied in our study was so far usually limited to study percolation processes (e.g., Garvelmann et al, 2012;Mueller et al, 2014;Sprenger et al, 2016a).…”

“…Including the isotopes of the bulk soil water, could allow for an improved conceptualization of the soil-plant-atmosphere interface, which is crucial for an adequate representation of evaporation and transpiration. Especially the marked differences of the evaporation signal within the first few centimeters of the soil depth will significantly affect the age distribution of transpiration (Sprenger et al, 2016c), evaporation (Soulsby et al, 2016a) or evapotranspiration (Harman, 2015;Queloz et al, 2015;van Huijgevoort et al, 2016).…”

“…For example, it has been shown that evapotranspiration dynamics can affect travel times of percolating water in the unsaturated zone (Sprenger et al, 2016c;Heße et al, 2017) and catchment outflows (van der Velde et al, 2015;Rinaldo et al, 2011). Additionally, understanding the age distributions of evapotranspiration water has recently gained interest in the literature (Harman, 2015;Soulsby et al, 2016a;van Huijgevoort et al, 2016;Queloz et al, 2015). However, estimates of these evapotranspiration ages and catchment travel times require a sound understanding of the storage and mixing dynamics of the subsurface and surface water pools, which form the sources of evapotranspiration within catchments.…”

Non-destructive estimates of soil carbonic anhydrase activity and associated soil water oxygen isotope composition

“…The accuracy of cryogenic vacuum distillation techniques has been questioned as the δ 18 O of extracted waters tends to be depleted in 18 O relative to the irrigation water when oven-dried soils that have been re-wetted are considered (Orlowski et al, 2016a;Sprenger et al, 2015). The extent of this depletion depends on soil properties.…”

Soil water migration in the unsaturated zone of semiarid region in China from isotope evidence

“…The naturally occurring vertical gradients of δD and δ 18 O in soil water provide similar information about plant water uptake depth from soils. A number of studies have used isotopes to characterize soil water movement in one location and thus one climatic regime (Orlowski et al, 2016b;Manzoni et al, 2013;Liu et al, 2011;Stumpp et al, 2009).…”