Spatio‐temporal heterogeneity in soil water stable isotopic composition and its ecohydrologic implications in semiarid ecosystems

Abstract

Spatio‐temporal heterogeneity in soil water content is recognized as a common phenomenon, but heterogeneity in the hydrogen and oxygen isotope composition of soil water, which can reveal processes of water cycling within soils, has not been well studied. New advances are being driven by measurement approaches allowing sampling with high density in both space and time. Using in situ soil water vapour probe techniques, combined with conventional soil and plant water vacuum distillation extraction, we monitored the hydrogen and oxygen stable isotopic composition of soil and plant waters at paired sites dominated by grasses and Gambel's oak (Quercus gambelii) within a semiarid montane ecosystem over the course of a growing season. We found that sites spaced only 20 m apart had profoundly different soil water isotopic and volumetric conditions. We document patterns of depth‐ and time‐explicit variation in soil water isotopic conditions at these sites and consider mechanisms for the observed heterogeneity. We found that soil water content and isotopic variability were damped under Q. gambelii, perhaps due in part to hydraulic redistribution of deep soil water or groundwater by Q. gambelii in these soils relative to the grass‐dominated site. We also found some support for H isotope discrimination effects during water uptake by Q. gambelii. In this ecosystem, the soil water content was higher than that at the neighbouring Grass site, and thus, 25% more water was available for transpiration by Q. gambelii compared with the Grass site. This work highlights the role of plants in governing soil water variation and demonstrates that they can also strongly influence the isotope ratios of soil water. The resulting fine‐scale heterogeneity has implications for the use of isotope tracers to study soil hydrology and evaporation and transpiration fluxes to improve understanding of water cycling through the soil–plant–atmosphere continuum.

Understanding Catchment‐Scale Forest Root Water Uptake Strategies Across the Continental United States Through Inverse Ecohydrological Modeling

“…Understanding forest‐scale RWU strategies is critical for developing a process understanding of hydrologic responses to forest cover change. While individual‐ and stand‐level estimates of RWU strategies facilitated by stable isotopes in water and sap flux have become ubiquitous (e.g., Brinkmann et al, ; De Deurwaerder et al, ; Evaristo et al, ; Knighton, Conneely, et al, ), uncertainties in field methods and “scaling‐up” these findings to the catchment remain intractable problems (e.g., Oerter & Bowen, ; Penna et al, ). The metaanalysis of Evaristo and McDonnell () reviewed 531 studies of tree RWU derived from isotopes in water, of which 354 (~67%) used less than 10 xylem samples, possibly missing infrequent locations or periods of deep RWU.…”

Unexplained hydrogen isotope offsets complicate the identification and quantification of tree water sources in a riparian forest

“…Moreover, the water stored in soil rock fragments can have an isotopic composition distinct to that of soil water or groundwater, being either relatively more depleted (in the case of δ 2 H in Oshun et al, 2015) or more enriched (Palacio et al, 2014;Rong et al, 2011). Such variable and contrasted isotopic effects of lithology are to be expected for differing minerals and can even cause fractionations of opposite signs for the hydrogen and oxygen isotopes (Meißner et al, 2014;Oerter et al, 2014). Thus, wherever weathered rocks constitute a large fraction of the soil volume, the isotopic composition of rock moisture should be measured as rock moisture could constitute a significant alternative plant water source.…”

“…Although it cannot be ruled out that rock water in the carbonate-rich soil of Bowling et al (2017) was a significant source of water for trees or caused any unexpected isotope effects, the very clayey soil texture reported by Brooks et al (2010) seems less likely to contain a large rock water component. Oerter et al (2014) showed that cations adsorbed to clay minerals create isotopically organized hydration spheres of water around them and thereby sequester these water molecules away from the bulk water. However, even if the majority of the water contained in small pores is adsorbed water that does not interact with the more mobile water (the TWW hypothesis), in summer, when only water in small pores is accessible to the trees, there should be an isotopic match between soil pore and xylem water, unless H1 is not true and isotopic fractionation occurs during root uptake.…”

“…The mismatch between xylem water and source water isotopes may be caused by three non-exclusive processes: (1) a water isotope separation between bound and mobile soil water (Tang and Feng, 2001;Brooks et al, 2010), (2) a water isotope fractionation occurring at the soil-root interface (Ellsworth and Stenberg, 2007;Vargas et al, 2017), or (3) a water isotope compartmentalization between vessel water and other stem water pools (Zhao et al, 2016). In particular, surface-water interactions operating at the pore level (Oerter and Bowen, 2017) and varying as a function of particle size (Gaj et al, 2017) or cation content (Oerter et al, 2014) may create isotopic heterogeneity within the soil matrix. The soil in our study site is sandy; thus the effect of interactions with clay-absorbed cations are likely to be small (Fig.…”

Disentangling temporal and population variability in plant root water uptake from stable isotopic analysis: a labeling study

“…Condition (ii) is often neglected but is required due to the instantaneous nature of the sap flow 80 samples. As illustrated by Oerter and Bowen (2019), the lateral variability of the soil water isotopic composition profiles can become significant in the field and could have great implications on the representability and meaningfulness of isotopicderived estimate of RWU profiles.…”