Intercomparison of soil pore water extraction methods for stable isotope analysis and interpretation of hillslope runoff sources

Abstract

Intercomparison of soil pore water extraction methods for stable isotope analysis has been a focus of recent studies in relation to plant source waters, which found a wide isotopic variance depending on the extraction method. Few studies have yet explored extraction effects for mobile pore waters that relate to hillslope runoff. This is because it is extremely difficult in natural systems to control the boundary conditions in order to assess and compare impacts of pore water extraction on resulting hillslope flow. With our new semicontrolled experiments on outdoor mini‐hillslopes, we studied mixing and runoff processes by means of stable isotopes of water and quantified relations between pore water extraction methods. We tested the null hypothesis that nondestructive and destructive pore water sampling methods sample the same soil water pool. Three hillslopes were mounted on load cells, filled with loamy sand textured soils from the Landscape Evolution Observatoryat Biosphere 2, equipped with soil moisture and temperature sensors, a bottom outflow, and a surface runoff gauge for isotope sampling. We followed the precipitation isotopic composition over and through the soil profile. One hillslope was instrumented with suction cups, on the second we installed sampling ports for in‐situ soil water vapour measurements, and the third hillslope was sampled destructively for applying the centrifugation and vapour equilibrium methods. All hillslopes were sampled at four depths (0–10, 10–20, 20–30, and 30–40 cm) at three different downslope positions. 2H and 18O analyses were performed via laser spectroscopy. We found no isotopic differences between rainfall, surface runoff, and bottom outflow. The in situ vapour ports' soil isotope data showed the widest spread over all hillslope positions and depths. Centrifugation's and suction cups' isotope results plotted closest to the local meteoric water line and within the range of hillslope runoff and bottom outflow data. Hillslope position did not influence the soil isotope results. These results suggest caution be used in the field when selecting an extraction technique for matching soil waters to runoff waters. Soil suction lysimeters and centrifugation appeared to be the most appropriate tools in this regard.

Water Stable Isotopes in Ecohydrological Field Research: Comparison Between In Situ and Destructive Monitoring Methods to Determine Soil Water Isotopic Signatures

“…The fraction of soil water sampled by different extraction methods is currently one of the main questions faced by the ecohydrological research community (Berry et al, 2018;Penna et al, 2018;Sprenger et al, 2018;Orlowski et al, 2019). In comparison to the direct water vapor equilibration method, cryogenic vacuum extraction can likely also access hygroscopic and biologically bound water (Koeniger et al, 2011;Sprenger et al, 2015;Orlowski et al, 2016b).…”

“…Berry et al (2018) addressed the methodological state of the art in stable isotope ecohydrology as "shotgun" or "snapshot methods", referring to the lack of continuous measurements. Laboratory based water extraction methods are currently highly debated due to their inaccuracy and non-comparability of the obtained isotope results (Orlowski et al, 2019). Issues observed with laboratory based methods are mainly due to interferences with soil texture, water contents, interactions with cations, and the different pore spaces that may or may not be extracted via the different approaches (Meißner et al, 2014;Oerter et al, 2014;Orlowski et al, 2016bOrlowski et al, , 2018.…”

On the Spatio-Temporal Under-Representation of Isotopic Data in Ecohydrological Studies

“…(2) Artificial effects (ART in Figure 1) mainly include soil and xylem water field sampling and extraction methods as well as instrumental measurement uncertainty in laboratory analysis, as follows: a) Different methods for soil water sampling and extraction, such as tension lysimeters and cryogenic vacuum distillation returning different isotopic composition from the same sample (Geris et al, 2015;Orlowski et al, 2016Orlowski et al, , 2018Orlowski et al, , 2019. b) Different methods for plant water sampling and extraction such as cryogenic vacuum distillation, Scholander-type pressure chamber, and other destructive methods possibly producing artifacts and returning different isotope values (Thoma et al, 2018;Fischer et al, 2019;Zuecco et al, 2020), preferentially affecting 2 H rather than 18 O values (Chen et al, 2020).…”

Differences in soil evaporation between row and interrow positions in furrowed agricultural fields

“…The effective failure rate is at or below 5% when soil samples are double bagged (Wassenaar et al, 2008), which also avoids interlaboratory isotope contamination (Mueller et al, 2014;Orlowski, Pratt, & McDonnell, 2016) and allows the soil samples to be processed with less potential to evaporate compared with isotope-ratio mass spectrometry methods (Wassenaar et al, 2008). Work by Orlowski et al (2016) found that conventional methods, such as cryogenic distillation, may be prone to large offsets between measurements and reference waters, and assessments of the errors within our data show our approach is consistent with a recent methodological assessment study (Wang, Si, Pratt, Li, & Ma, 2020), which reported measurement errors on spiked samples of known isotopic composition. Sample handling and analysis was maintained consistent across all treatments and locations; thus, differences between samples are expected to be caused by field conditions and not normal laboratory procedures.…”