Abstract. The stable isotope compositions of soil water (δ 2 H and δ 18 O) carry important information about the prevailing soil hydrological conditions and for constraining ecosystem water budgets. However, they are highly dynamic, especially during and after precipitation events. In this study, we present an application of a method based on gas-permeable tubing and isotope-specific infrared laser absorption spectroscopy for in situ determination of soil water δ 2 H and δ 18 O. We conducted a laboratory experiment where a sand column was initially saturated, exposed to evaporation for a period of 290 days, and finally rewatered. Soil water vapor δ 2 H and δ 18 O were measured daily at each of eight available depths. Soil liquid water δ 2 H and δ 18 O were inferred from those of the vapor considering thermodynamic equilibrium between liquid and vapor phases in the soil. The experimental setup allowed for following the evolution of soil water δ 2 H and δ 18 O profiles with a daily temporal resolution. As the soil dried, we could also show for the first time the increasing influence of the isotopically depleted ambient water vapor on the isotopically enriched liquid water close to the soil surface (i.e., atmospheric invasion). Rewatering at the end of the experiment led to instantaneous resetting of the stable isotope profiles, which could be closely followed with the new method. From simple soil δ 2 H and δ 18 O gradients calculations, we showed that the gathered data allowed one to determinate the depth of the evaporation front (EF) and how it receded into the soil over time. It was inferred that after 290 days under the prevailing experimental conditions, the EF had moved down to an approximate depth of −0.06 m. Finally, data were used to calculate the slopes of the evaporation lines and test the formulation for kinetic isotope effects. A very good agreement was found between measured and simulated values (Nash and Sutcliffe efficiency, NSE = 0.92) during the first half of the experiment, i.e., until the EF reached a depth of −0.04 m. From this point, calculated kinetic effects associated with the transport of isotopologues in the soil surface air layer above the EF provided slopes lower than observed. Finally, values of kinetic isotope effects that provided the best model-to-data fit (NSE > 0.9) were obtained from inverse modeling, highlighting uncertainties associated with the determinations of isotope kinetic fractionation and soil relative humidity at the EF.

Abstract. The stable isotope compositions of soil water (δ2H and δ18O) carry important information about the prevailing soil hydrological conditions and for constraining ecosystem water budgets. However, they are highly dynamic, especially during and after precipitation events. The classical method of determining soil water δ2H and δ18O at different depths, i.e., soil sampling and cryogenic extraction of the soil water, followed by isotope-ratio mass spectrometer analysis is destructive and laborious with limited temporal resolution. In this study, we present a new non-destructive method based on gas-permeable tubing and isotope-specific infrared laser absorption spectroscopy. We conducted a laboratory experiment with an acrylic glass column filled with medium sand equipped with gas-permeable tubing at eight different soil depths. The soil column was initially saturated from the bottom, exposed to evaporation for a period of 290 days, and finally rewatered. Soil water vapor δ2H and δ18O were measured daily, sequentially for each depth. Soil liquid water δ2H and δ18O were inferred from the isotopic values of the vapor assuming thermodynamic equilibrium between liquid and vapor phases in the soil. The experimental setup allowed following the evolution of typical exponential-shaped soil water δ2H and δ18O profiles with unprecedentedly high temporal resolution. As the soil dried out, we could also show for the first time the increasing influence of the isotopically depleted ambient water vapor on the isotopically enriched liquid water close to the soil surface (i.e., atmospheric invasion). Rewatering at the end of the experiment led to instantaneous resetting of the stable isotope profiles, which could be closely followed with the new method.

devices and electric vehicles. [1][2][3][4] To achieve even higher energy densities, the use of lithium metal as the negative electrode is considered the next big step. However, the continuous electrolyte decomposition at the electrode|electrolyte interface, owing to the lack of a stable solid electrolyte interphase (SEI), results in low Coulombic efficiency (CE) and, potentially, dendritic lithium deposition. Thus eventually cause rapid cell failure and, in a worst case, accidental short-circuiting, posing severe safety issues and hindering commercialization. [5][6][7] Nonetheless, there has been a revitalized interest in lithiummetal anodes, encouraged by recent advances towards the stabilization of the anode|electrolyte interface. These advances were achieved by different strategies, including the formulation of beneficial electrolyte compositions, [8] the application of artificial interphases, [9] the use of 3D host matrices, [10] and the replacement of conventional liquid electrolytes by solidstate electrolytes. [11] Among these strategies, the utilization of solid-state electrolytes -inorganic and/or polymeric -potentially provides great advantages concerning the safe operation of lithium-metal anodes. [12,13] The first report on polymer electrolytes, characterized by high flexibility and light weight, dated back to the late 1970s with poly(ethylene oxide) (PEO) serving as the lithium salt dissolving medium. [14,15] Later, gel-type polymer electrolytes were developed by swelling a polymer matrix, such as poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP), polyacrylonitrile (PAN), or poly(vinyl alcohol) (PVA) with a lithium salt-containing liquid electrolyte. [16] In such systems, the polymer essentially takes over the role of the separator and is not actively involved in the charge transport. Differently, the lithium salt anions substantially contribute to the charge transport, resulting in a lithium transference number (t Li + ) well below 0.5. This leads to a large concentration gradient and reversed electric field in the cell, which in turn results in large overpotentials, limited dis-/charge rates, and fast dendrite growth. [17][18][19][20] Accordingly, increasing the t Li + , ideally to a value close to unity, provides a solution to overcome the above mentioned challenges. The most straightforward approach to realize this is the covalent tethering of the anionic function to the polymer to immobilize the negative charge, yielding single-ion Single-ion conducting polymer electrolytes are considered particularly attractive for realizing high-performance solid-state lithium-metal batteries. Herein, a polysiloxane-based single-ion conductor (PSiO) is investigated. The synthesis is performed via a simple thiol-ene reaction, yielding flexible and self-standing polymer electrolyte membranes (PSiOM) when blended with poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP). When incorporating 57 wt% of organic carbonates, these polymer membranes provide a Li + conductivity of >0.4 mS cm −1 at 20 °C ...

Summary Root water uptake is a key ecohydrological process for which a physically based understanding has been developed in the past decades. However, due to methodological constraints, knowledge gaps remain about the plastic response of whole plant root systems to a rapidly changing environment. We designed a laboratory system for nondestructive monitoring of stable isotopic composition in plant transpiration of a herbaceous species (Centaurea jacea) and of soil water across depths, taking advantage of newly developed in situ methods. Daily root water uptake profiles were obtained using a statistical Bayesian multisource mixing model. Fast shifts in the isotopic composition of both soil and transpiration water could be observed with the setup and translated into dynamic and pronounced shifts of the root water uptake profile, even in well watered conditions. The incorporation of plant physiological and soil physical information into statistical modelling improved the model output. A simple exercise of water balance closure underlined the nonunique relationship between root water uptake profile on the one hand, and water content and root distribution profiles on the other, illustrating the continuous adaption of the plant water uptake as a function of its root hydraulic architecture and soil water availability during the experiment.

Near-surface soil moisture profiles contain important information about the evaporation process from a bare soil. In this study, we demonstrated that such profiles could be monitored noninvasively and with high spatial resolution using Nuclear Magnetic Resonance (NMR). Soil moisture profiles were measured in a column exposed to evaporation for a period of 67 days using a stationary Magnetic Resonance Imaging (MRI) high field scanner and a unilateral NMR sensor. The column was packed with medium sand and initially saturated. Two distinct shapes of soil moisture profiles that are characteristic for stage I (evaporation rate is controlled by atmospheric demand) and stage II (evaporation rate is controlled by the porous medium) of the evaporation process were followed by both MRI and unilateral NMR. During stage I, an approximately uniform decrease of soil moisture over time was monitored, whereas during stage II, Sshaped moisture profiles developed which receded progressively into the soil column. These promising results and the specific design of the unilateral NMR system make it very well suited for determining soil moisture profiles in the field.